Carbohydrate functionalized hybrid latex particles

Carbohydrate Polymers 173 (2017) 233–252

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Review

Carbohydrate functionalized hybrid latex particles Niels M.B. Smeets a,b,∗ , Spencer Imbrogno a,2 , Steven Bloembergen a,1 a b

EcoSynthetix Inc., 3365 Mainway, Burlington, Ontario, L7 M 1A6, Canada Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4L8, Canada

a r t i c l e

i n f o

Article history: Received 24 February 2017 Received in revised form 5 May 2017 Accepted 24 May 2017 Available online 31 May 2017 Keywords: Carbohydrate Starch Cellulose Nanoparticles Nanocrystals Latex Polymer Emulsion polymerization

a b s t r a c t In this review we highlight the progress in the synthesis of carbohydrate functionalized hybrid latex particles, focusing on different synthetic approaches which use carbohydrates as a surfactant/stabilizer, initiator, grafting site and/or as a macromonomer. These nanocomposites are receiving increasing attention in academia as well as in industry, due to increasingly stringent societal demands for biobased, biodegradable, and biocompatible materials. Furthermore, we will report on the use of nanostructured carbohydrate materials, such as cellulose nanocrystals, starch nanocrystals, and starch nanoparticles. These novel materials represent an interesting emerging field, and examples of latex nanocomposites have only recently been reported. It is the authors’ opinion that using carbohydrate materials for the synthesis and production of latex polymers will become of increasing importance as we move towards a more sustainable future. © 2017 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 Brief overview of carbohydrate chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 2.1. Carbohydrate modification using small molecule chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 2.2. Carbohydrate modification using polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 Carbohydrate functionalized hybrid latex particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 3.1. Carbohydrates as surfactant/stabilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 3.2. Carbohydrate macromers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 3.3. Grafting from the carbohydrate using peroxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 3.4. Grafting from the carbohydrate using cerium(IV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 3.5. “Grafting from” using living/controlled radical polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 3.6. Other synthetic approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 Carbohydrate granules, crystals and nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 4.1. Starch granules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 4.2. Carbohydrate nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 4.3. Starch nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

∗ Corresponding author at: EcoSynthetix Inc. 3365 Mainway, Burlington, Ontario, L7 M 1A6, Canada. E-mail address: [email protected] (N.M.B. Smeets). 1 Current affiliation: 325 E. Grand River Avenue, Suite 314, East Lansing, MI 48823-4375, USA. 2 Current affiliation: Bioproducts Research Lab, Department of Chemical Engineering and Applied Chemistry, University of Toronto,200 College Street, Toronto, Ontario, M5S 3E5, Canada http://dx.doi.org/10.1016/j.carbpol.2017.05.075 0144-8617/© 2017 Elsevier Ltd. All rights reserved.

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1. Introduction Amongst the biopolymers available to polymer chemists, carbohydrates in particular stand out as they are biodegradable, encompass a broad diversity in structure and functionality, are abundant, and are produced at sufficient scale to be of industrial importance (Poli et al., 2011). Cellulose and starch (i.e. amylose and amylopectin), are two prime examples of carbohydrates that meet these criteria, and have consequently found widespread use in academia as well as in the chemical industry (Heinze & Koschella, 2005; Whistler, BeMiller, & Paschall, 1984). Cellulose, amylose, and amylopectin are high molecular weight polymers, all consisting of glucose monosaccharide units linked through ␣(1 → 4), ␣(1 → 6) or ␤(1 → 4) glycosidic bonds. Despite this structural similarity, these carbohydrates possess remarkably different physiochemical properties. The ␤(1 → 4) glycosidic linkages in cellulose provide a highly regular helical structure responsible for introducing a high degree of crystallinity and strong inter- and intramolecular hydrogen bonds. This makes cellulose a stiff, water-insoluble polymer and consequently an important skeletal component in plants. Conversely, amylopectin is a branched polymer and an important part of the metabolism cycle of plants (Zeeman, Kossmann, & Smith, 2010) due to the presence of ␣(1 → 6) glycosidic bonds present in addition to the ␣(1 → 4) glycosidic bonds. Once isolated, amylopectin is a predominantly amorphous polymer and can be readily digested by enzymes to release the stored energy. However, in plants amylopectin is deposited in semi-crystalline, water-insoluble granules, interspersed with amorphous amylose, offering the possibility of prolonged energy storage (Zeeman et al., 2010). This diversity in physiochemical properties of cellulose and starch, which originates from minor structural differences, is one of the main reasons why carbohydrates are used in a broad range of (industrial) applications, and why much research is currently dedicated to exploring novel uses for these biopolymers and their chemically modified analogs. For industrial applications (Santana, Angela, & Meireles, 2014) such as paper manufacturing (Cimpeanu & Kern, 2014), corrugating (Vishnuvarthanan and Rajeswari, 2013), gypsum fibre board (Qiang et al., 2002), and textiles (Meshram, Patil, Mhaske, & Thorat, 2009), starch is the carbohydrate of choice as the starch granules can be readily dispersed or solubilized in water, partially or fully depolymerized through the use of acids and enzymes, and modified through functional hydroxyls that are more reactive than those in cellulose (Maurer, Kearney, & Rapids, 1998). In paper coating, for example, starch adds binding power, flexibility, thickening, higher stiffness, and film formation properties to the coating (Maurer et al., 1998). However, the use of starch also increases water sensitivity, which impairs wet strength, wet rub resistance, and can cause rheological problems due to retrogradation. Consequently, over the past decades starch faces increasing competition from synthetic binders, which have become the dominant binder technology because they outperform starch in some (or multiple) aspects (Maurer et al., 1998). With increasing environmental constraints and societal pressures, alternatives to synthetic binders are becoming increasingly relevant. Two recent developments that meet this sustainable requirement are the use of starch nanoparticles and the use of starch containing synthetic latexes. Carbohydrate (modified) nanoparticles have also generated a significant amount of interest for biomedical applications (Caro & Pozo, 2015; Liu, Jiao, Wang, Zhou, & Zhang, 2008; Oh, Lee, & Park, 2009). The hydrophilic nature of many carbohydrates provides “stealth” properties allowing the nanoparticle to effectively evade the host’s immune system and deliver a payload to a target tissue (Lemarchand et al., 2006; Lemarchand et al., 2004). Furthermore, certain carbohydrates can promote selective cell uptake due to specific receptor interactions. But perhaps the most

important properties that carbohydrates provide is that they are non-cytotoxic and biodegradable (Oh et al., 2009). The above examples demonstrate the potential of carbohydrate functionalized nanoparticles, especially in industrial applications such as paper manufacturing, where the blending of starches with synthetic latexes is often still the state-of-the-art technology (Maurer et al., 1998; Bloembergen, McLennan, Lee, & van Leeuwen, 2008). The objective of this review paper is to present an overview of the various academic and industrial routes towards carbohydrate functionalized hybrid latex particles. First, a brief summary of general carbohydrate chemistry will be provided and, subsequently, used to describe how modification of soluble carbohydrates results in functional building blocks that can be used in a range of polymerization processes. Furthermore, we will discuss the emerging field of carbohydrate nanostructures and how these colloids can be used to synthesize carbohydrate functionalized hybrid latex particles. Last, the review will be concluded with a perspective by the authors on the future directions in this field. 2. Brief overview of carbohydrate chemistry Carbohydrates have been intensely studied since the early 19th century, but it took until the beginning of the 20th century before their molecular structure was determined, and until the end of that century before these polymers were chemically synthesized (Kobayashi, Kashiwa, Shimada, Kawasaki, & Shoda, 1992). Many excellent and comprehensive books, book chapters and review papers have been written on the topics of carbohydrate structure, chemistry and properties and the interested reader is referred to these works for a more in-depth review (Cumpstey, 2013; Kaur, Ariffin, Bhat, & Karim, 2012; Robyt, 1998; Zaikov, 2005). Here, we will limit ourselves to a brief overview of carbohydrate chemistry, for the benefit of the reader, to support the following sections of this review. 2.1. Carbohydrate modification using small molecule chemistry Although some carbohydrates contain reactive carboxylic acid or amine functionalities, the predominant accessible functionality on carbohydrates are the less reactive hydroxyl groups. Consequently, much attention has been dedicated to the selective modification of carbohydrates to introduce chemical functionalities other than hydroxyl to expand their use into various industrial applications, see Scheme 1. The extent of derivatization of a carbohydrate is given by the degree of substitution (DS) and defined as the number of hydroxyl substitutions made per monosaccharide repeat unit. Carbohydrates based on glucose, such as cellulose, dextran, amylose and amylopectin, have 3 available hydroxyls per repeat unit and therefore the maximum DS equals 3. Modifications of carbohydrates are commonly performed through the formation of esters and ethers with the available hydroxyls (Cumpstey, 2013). Etherification is an important industrial process that yields carboxymethylated (1) and hydrophobically modified (2) carbohydrates. In general the etherification involves the reaction of any hydroxyl on the carbohydrate with an alkylation agent in the presence of a base. The industrially important anionic carboxymethyl cellulose, for example, is prepared by treating cellulose with mono chloroacetic acid in the presence of sodium hydroxide to form a carboxymethyl ether (Heinze & Koschella, 2005). Hydrophobically modified hydroxyethyl cellulose and hydroxypropyl cellulose, which are commonly used as viscosity modifiers or colloidal stabilizers, are synthesized through etherification of cellulose with the corresponding epoxy alkanes in the presence of base (Fox, Li, Xu, & Edgar, 2011). Esterification of carbohydrates involves the reaction of a carbohydrate with an acy-

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235

Scheme 1. Common modification strategies for carbohydrate polymers.

lation agent (3). Cellulose acetate is produced through the acylation of cellulose with acetic anhydride in the presence of a base and is an important raw material for the production of clear injectionmolded thermoplastics, films, coatings and membranes (Cerqueira, Filho, & Meireles, 2007). Vinyl functionalized carbohydrates are also obtained through esterification of a carbohydrate, for example, with (meth)acrylic acid chlorides (4) or the corresponding anhydrides (5) (Heinze, Liebert, & Koschella, 2006). Both etherification and esterification chemistries are generally non-selective for the available hydroxyls and a range of biopolymers can be produced ranging from low to high DS. Oxidation provides another commonly used synthetic pathway to prepare carboxylic acid, aldehyde and ketone functionalized carbohydrates. Moreover, oxidation can be used to regioselectively modify carbohydrates. For example, the use of a catalytic amount of (2,2,6,6-tetramethyl-piperidine-1-yl)-oxyl (TEMPO) in combination with sodium hypochlorite (6) can selectively oxidize the C6 primary hydroxyl on cellulose and starch to the corresponding aldehyde and carboxylic acid (Bragd, van Bekkum, & Besemer, 2004; de Nooy, Besemer, & van Bekkum, 1995). Alternatively, periodate can be used to selectively cleave the 1,2 diol present at the C2-C3 position of a carbohydrate (7), and oxidize the hydroxyls to the dialdehyde or to the dicarboxylate (Kristiansen, Potthast, & Christensen, 2010). However, it should be noted that depending on the level of oxidation moderate to severe depolymerization (i.e. a reduction in the degree of polymerization, DP) may occur

(Kristiansen et al., 2010). Once the carbohydrate is functionalized with better nucleophiles, conjugation chemistries can be used to introduce a broad range of additional functionalities. However, as multistep chemistries are cost prohibitive in practice, these synthetic strategies are predominantly used in small scale applications and the biomedical arena. 2.2. Carbohydrate modification using polymerization Along with small molecule modification as discussed in the previous section, carbohydrates have been modified using various polymerization processes, including free radical polymerization (Meimoun et al., 2017) and ring-opening polymerization (Carlmark, Larsson, & Malmström, 2012). Free-radical polymerization in the presence of carbohydrates proceeds through “grafting from” techniques where persulfate radicals (8) can generate a radical on the carbohydrate backbone through hydrogen abstraction from a C H bond (Berlin & Kislenko, 1992; George et al., 2001). Generated radicals can further propagate with vinyl monomers, resulting in graft copolymers with macroscopic properties that differ substantially from the native carbohydrate. A more selective grafting strategy involves the use of cerium(IV) ions (9). The mechanism by which cerium(IV) ions generate radicals is believed to involve the formation of a coordination complex between the oxidant (cerium(IV) ion) and the reductant (hydroxyl groups of the carbohydrate) (Dumitriu, 2004; McCormick

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& Park, 1981). The ceric(IV)-carbohydrate complex then disproportionates into an aldehyde or a ketone and a free radical through the C C bond cleavage of the 1,2 diol (Casinos, 1992; Mange et al., 2007; Schatz & Lecommandoux, 2010). It has been shown that grafting on carbohydrate chains occurs predominantly at the reducing-end hemiacetal group and at the C2 C3 glycol groups in anhydroglucose units (Doba, Rodehed, & Ranby, 1984). Besides cerium(IV), other heavy metal ions such as manganese(VII) and iron(II) can also be used, however, these are generally less efficient than cerium(IV). Alternatively, the hydroxyls on the carbohydrate backbone can act as initiating sites for ring-opening polymerization of cyclic monomers such as lactones (10) (Carlmark et al., 2012). The most commonly accepted mechanism for ring-opening polymerization is a “coordination-insertion” mechanism where a catalyst, e.g. tin(II) ethyl hexanoate (Sn(Oct)2 ), is converted into a tin alkoxide through the reaction with the hydroxyls and/or other protic compounds such as the lactone monomers (Dechy-Cabaret, Martin-Vaca, & Bourissou, 2004; Dubois, Coulembier, & Raquez, 2009; Jérôme & Lecomte, 2008). The molecular weight of the grafted side chains is determined by the ratio of the number of initiating sites (i.e. the hydroxyls) and the amount of monomer added. As water is an impurity, ring-opening polymerization of carbohydrates is performed in a homogeneous system (i.e. organic solvent) or a heterogeneous system (i.e. dry state). The great advantage of the latter methodology is that no chemical modification of the native carbohydrate is required to facilitate the polymerization. Note that the extent of derivatization of grafted carbohydrates through free radical polymerization or ring-opening polymerization can technically no longer be expressed in terms of a DS, as the modification is not selective to the available hydroxyl groups on the polymer backbone, and a distribution of chain lengths can be grafted from the carbohydrate backbone. 3. Carbohydrate functionalized hybrid latex particles A polymer latex is a colloidially stable dispersion of polymer nanoparticles that are produced in a heterogeneous free-radical polymerization process. Typically, these polymer nanoparticles

are electrostatically stabilized using low molecular weight anionic surfactants. Carbohydrates are hydrophilic, highly functional, water-soluble polymers, and there is a natural affinity for these polymers to reside on interfaces. Consequently, native carbohydrates such as starch and dextran, as well as hydrophobically modified carbohydrates such as hydroxy alkylated celluloses can be used to sterically stabilize latex particles. Carbohydrates have found increasing and varying use in polymerization processes to prepare “carbohydrate functionalized hybrid latex particles”, imparting unique properties to the final hybrid latex. This topic has been widely investigated (this review being the first summary of these efforts) and an overview of the different uses of carbohydrates in the different polymerization techniques resulting in latex particles is presented in Table 1. In the following sections, the different uses of carbohydrates will be illustrated and discussed with relevant examples from academic and industrial sources. 3.1. Carbohydrates as surfactant/stabilizers Native and hydrophobically modified carbohydrates have been used as steric stabilizers in dispersion polymerization for the preparation of micron-sized, monodisperse latex particles. Carbohydrates and their analogs are advantageous in dispersion polymerization as these polymers are fully soluble in the continuous phase (predominantly mixtures of low molecular weight alcohols with water or hydrocarbons) and display strong adsorption to the formed polymer particles. The majority of the reports on carbohydrate-stabilized dispersion polymerizations has been reported by Lok, Ober and Hair (Lok & Ober, 1985; Ober, van Grunsven, McGrath, & Hair, 1986; Ober & Hair, 1987; Ober, Lok, & Hair, 1985), and is based on the hydrophobically modified hydroxypropyl cellulose (Scheme 1, Strategy 2). Interestingly, the use of hydrophobically modified hydroxypropyl cellulose yields relatively large polystyrene particles in the 10 ␮m range. Extensive kinetic work by Paine (Paine, 1990a, 1990b; Paine, Deslandes, Gerroir, & Henrissat, 1990; Paine, Luymes, & McNulty, 1990) has shown that the choice of solvent is critical to control the average particle size and particle size distribution (Paine, 1990c). Furthermore, it was

Table 1 Overview of the uses of carbohydrates in different polymerization techniques for the synthesis of carbohydrate functionalized hybrid latex particles. Function

Polymzn Technique

Advantage

Disadvantage

Examples

Surfactant

Dispersion/Emulsion/ Miniemulsion

Emulsion

Grafting site (Peroxides)

Emulsion/Miniemulsion

1) High industrial relevance 2) Carbohydrate grafted to latex particle

1) Carbohydrate not grafted to latex particle 2) Particle size limited to micrometer range 1) Chemical modification required 2) Not applicable to large scale industrial processes 1) Low grafting efficiency 2) Limited to low molecular weight carbohydrates 3) Homopolymer formation

Marie et al., (2002); Deslandes et al. (1990); Rotureau et al. (2008)

Macromer

1) No chemical modification required 2) Good control over particle size distribution 1) Carbohydrate grafted to latex particle

Grafting site (Cerium)

Emulsion

Grafting site (CLRP)

Emulsion

Grafting site (Other)

Emulsion

1) Does not initiate vinyl monomers 2) Carbohydrate grafted to latex particle 1) Excellent control over polymer composition, structure and architecture 2) Carbohydrate grafted to latex particle 1) Unique structures that cannot be attained through other methods 2) Carbohydrate grafted to latex particle

1) Use of heavy metal ions 2) Colouration of the latex 3) Limited to relatively water-soluble monomers 1) Use of specialized chemicals (e.g. catalysts) 2) Modification of carbohydrate required to carry initiating moiety 3) Not industrially applicable 1) Techniques not industrially applicable

Brune and Eben-Worlée (2002) Chauvierre et al. (2003); Cimpeanu and Kern (2014); Kin Man Ho, Li, Wong et al. (2010); Möller & Glittenberg (1990); Wang et al. (2013) Gaborieau et al. (2009); Kightlinger (1983); Passirani et al. (1999) Bernard et al. (2008)

Krasznai, McKenna, Cunningham, Smeets, and Champagne (2012); Smeets et al. (2010)

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Fig. 1. TEM images from the work of Marie and co-workers. (A,B) Polystyrene latexes prepared using a low and high molecular weight chitosan, respectively. (C,D) Hollow capsules prepared by cross-linking chitosan with diepoxides. Reprinted and adapted with permission from Marie et al. (2002). Copyright 2002 American Chemical Society.

demonstrated that the resulting latex particles consisted of a coreshell morphology, with a 10–15 nm hydroxypropyl cellulose shell (∼1–2 vol% of the particle) (Paine, Deslandes et al., 1990). The use of biopolymers in dispersion polymerization, however, is not limited to hydroxypropyl cellulose as other modified cellulose derivatives such as cellulose acetate/butyrate (Horak, 1999; Horák, Boháˇcek, ˇ & Subrt, 2000; Horák & Shapoval, 2000) as well as other carbohydrates such as chitosan (Lee, Wen, & Chiu, 2003) have also been reported. In emulsion polymerization, typically low molecular weight ionic surfactants are used to create small micelles which subsequently form small polymer particles in the 50–300 nm range. High molecular weight polymers are rarely used as stabilizers in emulsion polymerization, as they cannot support the micellar nucleation mechanism. Xu and Hu (Xu & Hu, 2012a, 2012b; Xu, Long, & Hu, 2013) reported the use of a cationic starch in a soap-free emulsion polymerization approach to prepare a styrene-acrylic latex for paper sizing and paper coating applications. The emulsion polymerization was performed in the presence of 5–10 wt% cationic starch and a cationic dimethylamino ethyl methacrylate co-monomer to provide sufficient electrostatic stabilization for the formed styreneacrylic latex particles. The optimized core-shell latex was used as a binder for paper coating and performed well under the conditions studied (Xu & Hu, 2012a). An alternative approach to the use of biopolymer stabilizers has been reported by the group of Dellacherie (Fournier, Leonard, Le Coq-Leonard, & Dellacherie, 1995; Rouzes, Durand, Leonard, & Dellacherie, 2002; Rouzes, Leonard, Durand, & Dellacherie, 2003). This group has reported the synthesis and application of phenyl modified dextrans for the stabilization of polystyrene and poly(lactic acid) nanoparticles (Scheme 1, Strategy 2) hydrophobically modified dextrans (with a DS ranging from 0.07 to 0.22) were successfully used as a stabilizer for dodecane-in-water emulsions as well as in the emulsion polymerization of styrene and methyl methacrylate (Rouzes et al., 2002). Improved surface active properties were achieved by preparing amphiphilic dextran surfactants, containing both hydrophobic phenyl, hexyl, or decyl groups as well as hydrophilic sulfonate groups (Rotureau, Leonard et al., 2006; Rotureau, Marie et al., 2006; Rotureau, Marie, Dellacherie, & Durand, 2007). These type of surfactants were subsequently used in the direct and inverse mini-emulsion polymerization of styrene, lauryl methacrylate and acrylamide, respectively (Rotureau et al., 2008). Excellent control over the mini-emulsion stability, particle size, and size distribution was demonstrated as a function of the surfactant type (i.e. DS of hydrophobic and hydrophilic groups) and the reaction conditions. This approach was further extended to the preparation of poly(lactic acid) grafted dextran (Nouvel et al., 2004) and poly(lactic acid) nanoparticles (Rouzes et al., 2003). Hydrophobically modified dextran (phenyl, hexyl or decyl) was dissolved in

water and added to a solution of poly(lactic acid) in methylene chloride. After vigorous stirring and evaporation of the methylene chloride, stable poly(lactic acid) nanoparticle dispersions with a dextran corona in the range of 150–250 nm were obtained. Similarly to the stabilization of the polystyrene nanoparticles, it is dependent on the interfacial adhesion between the immiscible dextran and poly(lactic acid) phases (Rouzes et al., 2003). The biodegradable (dextran) and bio-erodible (poly(lactic acid)) nature makes these nanoparticles suitable candidates for biomedical drug delivery. A potential disadvantage of using carbohydrate stabilizers is that a dynamic equilibrium exists as not all the carbohydrate is covalently bound to the synthetic latex particle. Marie and coworkers reported an interesting approach where the chitosan stabilizer is effectively crosslinked into a permanent shell (see Fig. 1). These authors prepared emulsions, latexes and nanocapsules down to 100 nm, stabilized by a cationic chitosan layer using a mini-emulsion approach (Marie, Landfester, & Antonietti, 2002). Unlike the previous examples where the carbohydrate was modified to enhance its stabilizing properties, these authors blended the rigid chitosan with other more flexible polymers such as Jeffamine and Gluadin and exploited the reactive amine functionalities to make capsules by polyaddition using di-epoxides in the presence of an inert oil. The authors expect that the resulting capsules are biocompatible and biodegradable and could be used in biomedical applications. 3.2. Carbohydrate macromers In many applications it is favorable to chemically graft carbohydrates to a synthetic polymer to minimize the amount of carbohydrate in the continuous (water) phase of the final latex. Although some grafting of the carbohydrate in dispersion polymerization cannot be excluded, a substantial amount of biopolymer remains in the continuous phase increasing the viscosity and/or causing flocculation or coagulation. Functionalization of carbohydrates with polymerizable functional groups (Scheme 1, Strategy 4,5) converts the biopolymer into a macromer that can effectively participate in a free-radical polymerization. Despite the vast body of work reported on the acrylation of carbohydrates, the number of reports on the use of carbohydrate macromers for the synthesis of hybrid latex particles is limited. One of the few examples concerns the preparation of a starch based macromer, synthesized using a condensation reaction between hydroxyls of the biopolymer and a bifunctional monomer such as N-methylolmethacrylamide, N-methylolacrylamide, hydroxyethyl methacrylate, or hydroxypropylmethacrylate in the presence of a catalyst such as aluminum chloride (Brune & Eben-Worlée, 2002). The functionalized starch polymer is subsequently copolymerized

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with conventional vinyl monomers such as styrene in an emulsion polymerization process to form starch-grafted latex particles that can be used as printing inks or overprint varnishes (Brune & Eben-Worlée, 2002). 3.3. Grafting from the carbohydrate using peroxides Persulfates are commonly used in industrial applications as an initiator for free-radical polymerizations (Scheme 1, Strategy 8). Biopolymers such as carbohydrates can undergo hydrogen abstraction in the presence of persulfate radicals and undergo graft-copolymerization. This principle has, for example, been exploited for the formation of graft copolymers of starch (and other carbohydrates) with polystyrene, poly(methyl methacrylate), poly(methacrylic acid), poly(acrylamide) and poly(acrylic acid) for applications such as super-absorbers (Fanta, Swanson, Burr, & Doane, 1983; Shi, Reddy, Shen, Hou, & Yang, 2014; Worzakowska and Grochowicz, 2015; Wu, Zhang, Liu, & Yao, 2012; Zou et al., 2012). An alternative approach has been reported by Finkenstadt and Willett, who prepared graft copolymers of starch and poly(acrylamide) using persulfate in a reactive extrusion process (Finkenstadt & Willett, 2005; Willett & Finkenstadt, 2003). More than 90% conversion of the acrylamide monomer is achieved with a grafting efficiency of 75% of higher (Willett & Finkenstadt, 2003), significantly higher than comparable batch processes. Furthermore, an additional advantage of the reactive extrusion process is that it eliminates the need for large amounts of water and organic non-solvents for precipitation, commonly required for batch processes. As persulfates are commonly used in dispersed phase freeradical polymerization processes, grafting of the carbohydrate onto the polymer particles readily occurs. Indeed, some of the examples described earlier, where (modified) carbohydrates are used as surfactants, do report that some grafting of the carbohydrate had occurred. This principle has been exploited in many commercial applications, where starch is used to enhance the properties of conventional synthetic polymer dispersions or to partially replace the more expensive vinyl monomers. One example was reported by Möller and Glittenberg in 1990 (Möller & Glittenberg, 1990), who performed a styrene-butadiene emulsion copolymerization in the presence of different corn starches (broad and narrow molecular weight distribution, cationic and oxidized). The solids content (50%) and viscosity (150–700 mPa s) of the final hybrid latexes containing 50 pphm (parts per hundred monomer) starch proved to be similar to the incumbent latexes without the starch. The grafted morphology of the latex particles was claimed to result in performance improvements for a paper coating application. Although the exact coating recipes, performance improvements and nature of the grafting reaction between the starch and the synthetic polymer were not disclosed, some unusual latex particle morphologies were reported. It was proposed that, depending on comonomer composition (and thus the glass transition temperature of the polymer), the starch was either grafted on the surface of a synthetic particle (core-shell particle), or starch domains are formed within a synthetic particle (inclusion particles) (Möller & Glittenberg, 1990). The latter particle morphology is particularly surprising as it was commonly assumed that the starch would be exclusively on the particle’s surface (core-shell morphology). The different latex particle morphologies were inferred from transition electron microscopy following the enzymatic treatment of the latexes. It should be noted, however, that it seems unlikely that an enzyme can diffuse into a solid latex particle to degrade the starch inclusions. Consequently, the starch inclusions are likely partially located on the surface of the latex particle, which leaves a “crater” after enzymatic treatment. Further examination of this morphology was reported almost two decades later by the group of Gilbert (de

Bruyn et al., 2006; de Bruyn & Sprong, 2007) (as will be discussed in detail in the following section). Rong-Min Wang et al. (Wang, Wang, Guo, He, & Jiang, 2013) used potassium persulfate to degrade a potato starch in-situ in order to prepare latexes consisting of butyl acrylate and methyl methacrylate. Diacetone acrylamide monomer was copolymerized into the latex, providing the opportunity to cure the latex film by crosslinking the keto-carbonyl groups using adipic dihydrazide. The potato starch was gelatinized for 30 min at 85 ◦ C and degraded in-situ for 10 min prior to monomer addition. Grafting efficiencies (i.e. starch-acrylic graft co-polymer) were close to 80% as determined from soxhlet extractions using acetone to remove the synthetic homopolymer. Films prepared from the hybrid latex copolymers that were crosslinked using adipic dihydrazide displayed the ability to uptake or release moisture levels depending on the humidity. The authors proposed these hybrid polymers could be used for indoor architectural coatings. In the patent literature there are many examples of industrial latex manufacturers that disclose the use of starches in the production of latexes (Brockmeyer, Ettl, & Leman, 2010; Cimpeanu & Kern, 2014; Danley & Hurley, 2000; Degen, Hoehr, Reichel, & Riebeling, 1989; Eiffler & Fruehauf, 2004; Ettl, Hamers, & SchmidtThuemmes, 2003; Gramera & Hicks, 1972; Krückel & Werner, 2011; Kuhn & Schuhmacher, 2004; Martin, Nguyen, & Pauley, 1991; Rink, Möller, Fullert, Krause, & Koch, 1990; Samaranayake, Tomko, Ruhoff, & Rao, 2011; Wendel, Schwerzel, & Hirsch, 1994). Starch is a high molecular weight carbohydrate, which has a considerable viscosity in aqueous solution at relatively low concentration. Consequently, most reports include a molecular weight reduction step is in the polymerization process. Starch degradation is either achieved chemically through the use of peroxides, acids or persulfates (Danley & Hurley, 2000; Krückel & Werner, 2011; Martin et al., 1991), or enzymatically through the use of amylases (Brockmeyer et al., 2010; Cimpeanu & Kern, 2014; Degen et al., 1989; Kuhn & Schuhmacher, 2004). WO2012080145 A1 discloses the uses of a cationic starch that was degraded to low molecular weight by treatment with hydrogen peroxide and sulfuric acid (Krückel & Werner, 2011) to a molecular weight of approx. 15,000–20,000 g mol−1 . These starches could be successfully copolymerized up to 36 pphm in the emulsion copolymerization of styrene and butyl acrylate. Similarly, US 5,358,998 uses a sugarized starch with a molecular weight of approx. 2000–25,000 g mol−1 , that was subsequently copolymerized in an emulsion polymerization (Wendel et al., 1994). Although successful, the ex-situ degradation of the starch is not desirable in an industrial setting. Consequently, this process was improved by including an in-situ degradation step in the polymerization process. US 6,090,884 describes the in-situ degradation of hydroxyethylated starch (Penford Gum 230) through the use of persulfates (Danley & Hurley, 2000). The starch is dissolved in water at low concentration and subsequently treated with approx. 7 w/w% ammonium persulfate at an elevated temperature of 82 ◦ C for 30 min prior to the start of the vinyl monomer feed. This process can be further improved for efficiency as degradation and polymerization can occur simultaneously. Stable latexes free of particulates were obtained, however, the starch content was limited to approx. 20 pphm (Danley & Hurley, 2000). According to the invention, the resulting dispersions can provide a low-cost latex for graphic arts materials, coatings and adhesives. An alternative approach is based on the in-situ degradation of starch by amylases (Brockmeyer et al., 2010; Cimpeanu & Kern, 2014; Degen et al., 1989; Kuhn & Schuhmacher, 2004). For example, ® US 2010/0324178 A1 discloses the use of a ␣-amylase (Termamyl 120L, Novozymes) to degrade a cationic starch (DS = 0.045) at an elevated temperature of 85 ◦ C for 30 min. The enzymatic degradation of the starch is quenched by the addition of phosphoric acid, prior to the start of the monomer feed. The low molecular

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weight starches are subsequently grafted onto the synthetic polymer particles via hydrogen abstraction by the persulfate initiator used in the polymerization process. The resulting dispersion contains approx. 33 pphm starch with a mean particle size of 106 nm. Regardless of the approach chosen, either pre-degraded starch or in-situ degradation using persulfate or enzymes, the final latex particle morphology in all likelihood would consist of a synthetic core covered by a starch shell. Cheng and co-workers reported a similar approach where high molecular weight cassava starch is degraded in-situ by ␣-amylase at 80 ◦ C (Cheng, Zhang, & Wu, 2014). Interestingly, the authors related the degradation time to the grafting efficiency in an emulsion polymerization of styrene and butyl acrylate. Short degradation time (≤5 min) resulted in gelation during polymerization and no stable latex was obtained. Degradation times of 10 min or longer result in a stable latex and an increase in particle size as the degradation time increased. Although the authors did not offer an explanation for this observation, it is likely that as the average molecular weight of the dextrins decreased, their surface active properties decreased concomitantly leading to an increase in the average particle size. Low molecular weight (modified) dextrins can be used directly to avoid the necessary molecular weight reduction step. Cheng and co-workers (Cheng, Xu, & Wu, 2014) used an oxidized cassava starch in the emulsion polymerization of styrene and butyl

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acrylate in the presence of a Tween 80 surfactant. Optimum grafting efficiencies of 40–45% were obtained by optimizing the amounts of oxidized starch and ammonium persulfate. As a result, approximately 50% of the initial amount of starch added to the polymerization remained in the aqueous phase, which was also observed by TEM using a stain for the carbohydrate, and had no effect on the colloidal stability of the latex. The same authors later demonstrated that this latex can be used as a binder for paper coating (Cheng, Zhao, & Wu, 2015). Important paper properties, such as brightness, gloss, pick resistance and Cobb value were reported to be within experimental error when compared to a blend of a conventional SA latex with oxidized starch. Another approach was disclosed in US 5,147,907 (Rink, Moller, Fullert, Krause, & Koch, 1992) and EP 0.408,099 A1 (Rink et al., 1990) which describes the copolymerization of acrylic monomers in the presence of dextrins ranging from 3000 to 11,000 g mol−1 . Where the majority of processes start from a dextrin solution in the initial fill stage of the polymerization reactor, this disclosure is based on the continuous feed of dextrin as part of a pre-emulsion containing the monomers. The resulting latex has high solids content (48–50%), viscosities ranging from 200 to 1,500 cP at a biocontent of 33%. The use of persulfates (with/without the use of enzymes) is preferred over other grafting chemistries based on the use of cerium, iron, manganese, iron and copper as these heavy metal species can

Fig. 2. Schematic representation of the mechanism of amphiphilic core-shell latex particles via graft copolymerization of water-soluble amino-containing polymers and vinyl monomers. Inset: TEM images containing a poly(methyl methacrylate) core and a chitosan shell. Reprinted and adapted with permission from Ho et al. (2010). Copyright 2010 Elsevier.

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cause colouring of the final latex product (the use of cerium in particular will be discussed in more detail in the following section). These heavy metal species have the advantage that they are selective in terms of generating radicals on carbohydrates, whereas the use of persulfates results in significant amounts of homopolymer formation. However, it should be noted that for most industrial dispersed phase polymerization processes homopolymer formation is considered irrelevant. Selective hydrogen abstraction on certain biopolymers can be achieved by exploiting the redox interaction between alkyl hydroperoxides (such as tert-butyl hydroperoxide) and amino groups (see Fig. 2) (Ho, Li, Lee et al., 2010; Li, Zhu, Sunintaboon, & Harris, 2002). An amino radical on the biopolymer backbone and an alkoxy radical are generated following electron transfer and hydrogen abstraction by tert-butyl hydroperoxide. In the presence of conventional hydrophobic vinyl monomers, the amino radical can initiate a graft polymerization which results in amphiphilic polymers that subsequently self-assemble into micelle-like domains. The alkoxy radical can initiate homopolymerization or it can abstract another hydrogen from the biopolymer backbone. Ultimately this too results in the formation of amphiphilic synthetic core-biopolymer shell nanoparticles (Ho & Li, 2013; Ho, Li, Wong, & Li, 2010). The group of Pei Li has advanced this technique and prepared a vast array of core-shell hybrid latex particles from biopolymers such as chitosan (Ye et al., 2005), casein (Li et al., 2002), bovine serum albumin (Li et al., 2002), cellulase (Ho, Mao, Gu, & Li, 2008), and gelatin (Li et al., 2002). The invention was patented in 2002 under US20020143081 A1, which discloses the use of various alkyl peroxides, biopolymers, and various acrylate, acrylamide and vinyl monomers (Li, Zhu, & Harris, 2003). Poly(methyl methacrylate) core-chitosan shell nanoparticles are currently being

investigated for their use as anti-microbial coatings in textiles (Xin, Li, & Ye, 2013) and endotoxin removal (Li & Ho, 2014; Li, Zhu, Liu, & Zhang, 2008; Qian, Cui, Ding, Tang, & Yin, 2006). Another polymerization system that utilizes the pendant amine groups as an initiation moiety is the emulsion polymerization of butyl cyanoacrylate in the presence of chitosan (Chauvierre, Labarre, Couvreur, & Vauthier, 2003; Yang, Ge, Hu, Jiang, & Yang, 2000). Alkyl cyanoacrylates are highly reactive monomers that are extremely difficult to handle in their pure form and will readily polymerize in the presence of moisture or traces of basic compounds (Vauthier, Dubernet, Fattal, Pinto-Alphandary, & Couvreur, 2003). These monomers can be polymerized using a free radical polymerization mechanism, however, in practice anionic and zwitterionic mechanisms are strongly favored (Vauthier et al., 2003). In the presence of chitosan, and absence of free radical initiator, poly(butyl cyanoacrylate) core-chitosan shell latex particles are formed, in part through initiation of the butyl cyanoacrylate monomer by the tertiary amine groups of chitosan (Yang et al., 2000). Similar poly(butyl cyanoacrylate) core-biopolymer shell latex particles have been prepared using dextran (Chauvierre et al., 2003; Couvreur et al., 1979), dextran sulfate (Chauvierre et al., 2003), and amphiphilic dextran derivatives (M. Wu, Dellacherie, Durand, & Marie, 2009). 3.4. Grafting from the carbohydrate using cerium(IV) Cerium(IV) initiation for grafting from carbohydrates (Scheme 1, Strategy 9) is often preferred over the use of persulfates, as cerium(IV) does not initiate vinyl monomers directly and subsequently prevents the formation of significant amounts of homopolymer. Cerium(IV) initiation has been used in conjunc-

Fig. 3. Schematic representation of the different approaches taken by Gaborieau and co-workers towards poly(methyl methacrylate) core-carbohydrate shell latex particles. (1) grafting of debranched amylopectin onto pre-synthesized poly(methyl methacrylate) latex particles, (2) emulsion copolymerization of degraded amylopectin in the presence of methyl methacrylate (MMA), and (3) emulsion polymerization of methyl methacrylate in the presence of thermoresponsive graft copolymers of poly(Nisopropylacrylamide) and degraded amylopectin. Reprinted with permission from Gaborieau et al. (2009). Copyright 2009 Wiley Interscience.

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tion with a broad range of water-soluble vinyl monomers to make bio-synthetic graft-copolymers (Abdel-Halim, 2013; Li, Lee, & Cho, 2012; McCormick & Park, 1981, 1984; Rahman et al., 2000) and block-copolymers (Schatz & Lecommandoux, 2010), as well as in industrial processes to produce graft copolymers of starch (Tomasik & Schilling, 2004) and cellulose (Hebeish & Guthrie, 1981). Similarly, graft copolymers have also been synthesized using hydrophobic monomers which results in the formation of latex particles. Gaborieau and co-workers have used cerium(IV) initiation of hydrophobic vinyl monomers in three different ways to obtain core-shell hybrid latex particles with a cationic carbohydrate shell (Gaborieau et al., 2009): (1) grafting of debranched amylopectin onto pre-synthesized poly(methyl methacrylate) latex particles, (2) emulsion copolymerization of degraded amylopectin in the presence of methyl methacrylate, and (3) emulsion polymerization of methyl methacrylate in the presence of thermoresponsive graft copolymers of poly(N-isopropylacrylamide) and degraded amylopectin (see Fig. 3). In the first approach, a relatively high molecular weight starch (Amylofax HS, cationic potato starch modified with hydroxypropyl-trimethyl-ammonium groups, DS ∼ 0.048) was debranched using isoamylase. The low molecular weight debranched Amylofax was added to a poly(methyl methacrylate) seed latex swollen with methyl methacrylate, and grafted to the surface through the continuous addition of cerium(IV) ammonium nitrate dissolved in 0.2 mol L−1 nitric acid over 30 min. Successful grafting of the debranched starch was confirmed from the intensity average particle size of the poly(methyl methacrylate) hybrid latex which increased from 99 nm to 106 nm, and improved stability of the grafted latex particles under the electron beam in TEM. These authors previously reported a similar approach using a neutral amioca starch (Mange et al., 2007). The grafting efficiency (8.5%) for the debranched Amylofax was significantly lower than the 33% reported for the neutral amioca starch, and the specific surface coverage per chain was higher (24 nm2 compared to 103 nm2 , respectively), which explains the higher colloidal stability. In the second approach, the Amylofax is randomly degraded using ␣-amylose and subsequently used in an ab-initio emulsion polymerization of methyl methacrylate. The degraded starch was dissolved in 0.2 mol L−1 nitric acid and the required amounts of cerium(IV) ammonium nitrate and methyl methacrylate added to prepare the low solids content (2%) hybrid latex. Although the grafting efficiency was higher (24%) than the previous example (8.5%), the latex had a large average particle size and a broad particle size distribution. These undesirable properties were accounted to slow particle nucleation. In the third and final approach, degraded Amylofax was grafted with N-isopropylacrylamide using cerium(IV) ammonium nitrate as the initiator. This thermoresponsive graft-copolymer was then used to form nanoparticles (∼130 nm by dynamic light scattering) by raising the temperature above the cloud point temperature of the polymer (Tc = 29 ◦ C). Subsequently, methyl methacrylate was polymerized by using a lauryl persulfate initiator at 60 ◦ C in the presence of the nanoparticles. The resulting latex had a high graft efficiency of 63%, however, with a much larger intensity average particle size of 450 nm. Although the authors demonstrated the viability of different approaches to cationic core-shell latex particles, further optimization is required to improve the hybrid latex properties. Another ab-initio emulsion polymerization approach was reported by Vauthier and co-workers (Bertholon, Lesieur, Labarre, Besnard, & Vauthier, 2006; Chauvierre et al., 2003), who polymerized alkyl cyanoacrylates in the presence of dextran and cerium(IV) ions. Although the cyanoacrylates readily polymerize in the presence of moisture and nucleophiles, cerium(IV) was added to promote grafting between the dextran and poly(alkyl cyanoacrylate) polymer. Monodisperse nanoparticles around 200–300 nm

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were obtained, which could be controlled by changing the alkyl chain length of the cyanoacrylate monomer and the molecular weight of the dextran. Proof of successful grafting was derived from NMR, Raman spectroscopy and solubility analysis (Chauvierre et al., 2003). The amphiphilic dextran-poly(alkyl cyanoacrylate) nanoparticles have since found application as colloidal nanomedicines (Nicolas & Couvreur, 2009). A similar approach has also been reported for other carbohydrates such as hyaluronic acid (He, Zhao, Yin, Tang, & Yin, 2009). Poly(methyl methacrylate) graft-dextran/heparin was synthesized in emulsion polymerization by Passirani and co-workers (Passirani, Ferrarini, Barratt, Devissaguet, & Labarre, 1999). Dextran (70,000 g mol−1 ) or heparin (16,000–17,000 g mol−1 ) was dissolved in a 0.2 M nitric acid solution, after which a solution of cerium(IV) ammonium nitrate and monomer were added under vigorous stirring. At low methyl methacrylate concentration no nanoparticles are formed due to the relatively short length of the poly(methyl methacrylate) grafts. At very high methyl methacrylate concentration the graft-copolymer flocculated due to an imbalance between the hydrophilic and hydrophobic segments of the graft copolymer. However, under appropriate polymerization conditions stable nanoparticles in the size range of 100 nm were formed. It was speculated that the nanoparticle surface was probably “brush”-like and could thus infer “stealth” properties to the nanoparticles for in vivo applications. Kightlinger and co-workers used cerium(IV) initiation of acrylate monomers to produce high solids content latexes (Kightlinger, 1983). Similar to many of the industrial examples discussed in the previous section, the starch is pre-treated with a ␣-amylase to reduce the molecular weight of the amylose and amylopectin macromolecules. The starch solution viscosity was reduced to approximately 200 cP prior to the addition of ethyl acrylate and cerium ammonium nitrate. The final latex had a solids content of close to 45% and a high starch content. De Bruyn and co-workers have reported an approach to prepare starch-graft-poly(vinyl) latexes starting from a suspension of amylopectin, followed by an emulsion polymerization of hydrophobic monomers in the presence of cerium(IV) (Hank de Bruyn et al., 2006; H de Bruyn & Sprong, 2007). Their unique approach utilizes the highly branched, high molecular weight (107 –108 g mol−1 ; hydrodynamic radius of 300–400 nm) amylose as a scaffold for the polymerization of synthetic monomers (Hank de Bruyn et al., 2006). The amylopectin solution was treated either by ozonolysis (Hank de Bruyn et al., 2006) or cerium(IV) (H de Bruyn & Sprong, 2007) in combination with persulfate or glucose to enhance the activity to create radical grafting sites on the starch backbone. The synthetic precursor particles formed were tethered and stabilized by high molecular weight branched amylopectin polymers (indeed, substantial degradation of the amylopectin resulted in coalescence and gelation). The growth of the precursor particles within the amylopectin scaffold induced controlled coalescence and likely resulted in fragments of the amylopectin to adsorb onto the synthetic particle surface. The final latex particles that were produced using this approach contained 10–15 w/w% starch and resisted film formation due to the branched starch corona preventing coalescence above the glass transition temperature of the polymers in the latex (H de Bruyn & Sprong, 2007). Interestingly, energy-dispersive spectroscopy (EDS) performed on the latex particles suggested that starch is incorporated more or less uniformly throughout the latex particles, and not exclusively restricted to the particles’ coronas (see Fig. 4). This unexpected morphology may partly have confirmed the microscopy work of Möller and Glittenberg (Möller & Glittenberg, 1990). However, it should be noted that significant encapsulation of the starch was only observed for degraded starches of low molecular weight (de Bruyn & Sprong, 2007).

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latex particles using RAFT chemistry (Bernard, Save, Arathoon, & Charleux, 2008). A xanthate end-functionalized dextran hybrid polymer was prepared by using click chemistry between a propargyl end-functionalized dextran and a xanthate bearing an azide group. The xanthate end-functionalized dextran (“macroRAFT agent”) was subsequently used as a reactive stabilizer in the emulsion polymerization of vinyl acetate. The macroRAFT agent afforded good control over the polymerization and resulted in the formation of well-defined dextran-b-poly(vinyl acetate) block copolymers. Colloidally stable latexes (up to 27% solids with negligible amounts of coagulum) with monodisperse particles ranging from 90 nm to 150 nm could be obtained depending on the amount of the macroRAFT agent added to the polymerization (2–6 w/w%). 3.6. Other synthetic approaches

Fig. 4. (A) Energy-dispersive spectroscopy spectrum for a 1:1 starch:polystyrene latex demonstrating that the starch component is homogeneously distributed throughout the latex particle, and (B) the corresponding TEM-image of the latex particles. Reprinted with permission from de Bruyn et al., (2006). Copyright 2006 Wiley Interscience.

3.5. “Grafting from” using living/controlled radical polymerization Considerable progress has been made in the field of living/controlled radical polymerization and has given unprecedented control over the polymer architecture and functionality (Matyjaszewski and Spanswick, 2005). Many controlled radical polymerization techniques have been developed, of which atom transfer radical polymerization (ATRP) (Braunecker and Matyjaszewski, 2007; Matyjaszewski, 2012), nitroxide mediated polymerization (NMP) (Nicolas et al., 2013), and reversible addition-fragmentation chain transfer (RAFT) (Moad et al., 2012; Moad, Rizzardo, & Thang, 2008) polymerization have become dominant for the synthesis of well-defined polymeric materials. Generally, these methodologies rely on establishing a dynamic equilibrium between a low concentration of active propagating chains and a predominant concentration of dormant chains that are unable to propagate. Consequently, radical termination is suppressed, and in the ideal case virtually completely absent, and results in the formation of polymers of well-defined molecular weight and narrow molecular weight distribution that can be reinitiated to yield complex architectures such as block copolymers. These techniques are also ideally suited to prepare carbohydratebased graft copolymers as CRP initiators can be readily attached to a broad range of biopolymers (Roy, Semsarilar, Guthrie, & Perrier, 2009; Tizzotti, Charlot, Fleury, Stenzel, & Bernard, 2010). The majority of the literature on grafting-from biopolymers using controlled radical polymerization techniques concerns the synthesis of watersoluble hybrid materials. The formation of carbohydrate functionalized latex particles has been sparsely reported. The group of Charleux has reported an example where monodisperse poly(vinyl acetate)-graft-dextran

A number of other synthetic approaches to carbohydrate functionalized latex particles have been reported. A common approach is the self-assembly of biopolymer-b-synthetic blockcopolymers synthesized from controlled radical polymerization or ring-opening polymerization. Lemarchand and co-workers (Lemarchand et al., 2005, 2004) prepared dextran-g-poly(␧caprolactone) amphiphilic graft-copolymers from the covalent attachment of carboxyl end-functionalized poly(caprolactone) to the hydroxyls of dextran (Gref, Rodrigues, & Couvreur, 2002) (Scheme 1, Strategy 10). These graft copolymers self-assemble in an aqueous environment to form core-shell hybrid nanoparticles of approx. 200 nm that resist protein adsorption due to the dextran layer present on the surface of the poly(␧-carprolactone) nanoparticles (Lemarchand et al., 2005, 2004). Similar examples have also been reported for synthetic carbohydrate block-copolymers prepared using ATRP (Houga, Meins, Borsali, Taton, & Gnanou, 2007; Leonardis, Mannina, Diociaiuti, & Masci, 2010). Houga and coworkers (Houga et al., 2007) prepared a dextran macroinitiator by attaching 2-bromoisobutyrylbromide to the terminal chain end of a silylated dextran. The macroinitiator is chain extended with styrene and the resulting block copolymer treated with hydrochloric acid to obtain the amphiphilic dextran-b-polystyrene block copolymer. These block copolymers assemble in aqueous solution into various morphologies (Houga et al., 2007). De Leonardis (Leonardis et al., 2010) and co-workers used a similar approach where a pullulan macroinitiator was chain extended with methyl methacrylate. Assembly in aqueous solution led to the formation of core-shell hybrid nanoparticles consisting of a poly(methyl methacrylate) core and a pullulan shell. A somewhat different approach has been reported by Krasznai and co-workers (Krasznai, McKenna, Cunningham, Champagne, & Smeets, 2012), who prepared hyperbranched poly(methyl methacrylate)-b-dextran block-copolymers. Reductive amination was used to attach multiple low molecular weight dextran polymers to an amine functionalized hyperbranched poly(methyl methacrylate) polymer synthesized from a combination of catalytic chain transfer polymerization (Heuts & Smeets, 2011) and thiol-Michael addition. Depending on the amount of dextran, these amphiphilic polymers could be dispersed in water to form poly(methyl methacrylate) core-dextran shell nanoparticles (Krasznai, McKenna, Cunningham, Champagne et al., 2012). Smeets and co-workers reported a sonochemical route towards carbohydrate functionalized latex particles (Smeets et al., 2010). When subjected to ultrasound, polymer chains of sufficient molecular weight can undergo homolytic chain scission to form two macro-radicals. These radicals can subsequently be used to initiate a free-radical polymerization to prepare block copolymers. This principle was exploited by the authors to sonochemically degrade hydroxyethyl cellulose in the presence of butyl acrylate to form stable amphiphilic core-shell hybrid latex particles. The technique is

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Fig. 5. SEM images of cassava starch granules polymerized in the presence of styrene. (A) Polystyrene grafted starch granules and (B) polystyrene latex particles. Reprinted from Kaewtatip and Tanrattanakul (2008). Copyright 2008 Elsevier.

broadly applicable in terms of the choice of water-soluble polymer and hydrophobic monomer and can thus be used to prepare a broad range of different hybrid latex nanoparticles. 4. Carbohydrate granules, crystals and nanoparticles All the examples discussed so far have dealt with the use of soluble carbohydrates, obtained from chemical or physical modification of their native state. However, the hierarchical organization and semi-crystalline nature of carbohydrates such as cellulose, starch and chitin can be exploited to prepare carbohydrate nanostructures such nanocrystals, and nanoparticles (Le Corre, Bras, & Dufresne, 2010). The colloidal nature of these carbohydrate nanostructures makes them well suited to grafting with common vinyl monomers to prepare carbohydrate containing nanoparticles. However, the colloidal nature of these nanostructures also promotes the formation of latex particle morphologies that differ substantially from the earlier discussed examples. 4.1. Starch granules Starch naturally occurs as microscopic granules ranging from 2 to 100 ␮m in size, which can be dispersed in water. The granule itself has a complex multiscale internal structure consisting of growth rings (120–500 nm), composed of blocklets (20–50 nm), made of amorphous and crystalline lamellae (9 nm) (Gallant, Bouchet, & Baldwin, 1997) that contain the amylopectin and amylose polymer chains (0.1–1 nm) (Le Corre et al., 2010). It is because of this structure of the granule that most starches are cooked prior to use to remove the crystallinity. However, the amorphous regions of the starch granule (i.e. the amylose and amylopectin branch points) are accessible for chemical modification, which is exploited industrially to prepare chemically modified starches. Similarly, a number of examples (Cho & Lee, 2002; Kaewtatip and Tanrattanakul, 2008; Nikolic, Velickovic, Antonovic, & Popovic, 2013; Pimpan & Thothong, 2006) have been reported where starch granules have been used directly in a dispersed phase polymerization process. Cho and Lee copolymerized dried corn starch granules with styrene in an emulsion polymerization-like process (Cho & Lee, 2002). The authors concluded from the grafting efficiency that the number of potential graft sites on the granule is limited and that, consequently, the amount of styrene grafted to the surface is determined by the molecular weight of the grafts. Grafting of polystyrene is assumed to proceed through hydrogen abstraction from the surface of the starch granule via the potassium persulfate initiator. Since corn starch granules were used, the final particles are in the 10–20 ␮m size range, significantly larger than

commonly found in emulsion polymerization (0.1–0.5 ␮m). Gelatinization of the starch granules resulted in somewhat smaller particles in the range of 3–5 ␮m. Interestingly, these authors reported that no styrene grafting occurred in the absence of a surfactant or when a redox initiator was used instead of potassium persulfate. Similar approaches were reported by Kaewtatip and co-workers (Kaewtatip and Tanrattanakul, 2008) and Pimpan and co-workers (Pimpan & Thothong, 2006), who grafted polystyrene and poly(methyl methacrylate) onto cassava starch granules using a suspension polymerization approach. Contrary to the work reported by Cho and Lee (Cho & Lee, 2002), successful grafting of polystyrene and poly(methyl methacrylate) (maximum grafting percentage is 32%) onto the starch granules was achieved in the absence of surfactant. SEM analysis (see Fig. 5) shows that the final polymerization product consists of polystyrene nanoparticles (<1 ␮m), which are formed as a result of the styrene homopolymerization, deposited onto the much larger cassava starch granules (Kaewtatip and Tanrattanakul, 2008). Furthermore, these authors adjusted the polymerization temperature to 50 ◦ C to prevent gelatinization of the starch granules and used X-Ray diffraction to prove that the crystallinity did not change as a result of the polymerization. They proposed that these materials could be used as a thermoplastic starch compatibilizer for polystyrene/starch blends (Kaewtatip and Tanrattanakul, 2008). Nikolic´ and coworkers (Nikolic et al., 2013) studied biodegradation of polystyrene and poly(methacrylic acid) grafted corn starch granules. Similar to the other two examples, grafting of styrene and methacrylic acid was performed in an emulsion polymerization process using sodium dodecylsulfate as a surfactant and a potassium persulfate/amine initiation redox system. Although the corn starch granules were deformed during the polymerization process, the surface of the granules was covered with polystyrene. These starch component of these graft copolymers could be biodegraded using different microorganisms within 27 days. However, complete biodegradation of the starch could only be obtained at relatively low grafting percentages, i.e. where the starch granules were not completely covered with polystyrene. 4.2. Carbohydrate nanocrystals The hierarchical organization and semi-crystalline nature of carbohydrates such as cellulose, starch and chitin can be exploited to prepare carbohydrate nanostructures such as nanofibrils, nanocrystals, and nanoparticles. The most well-known examples of such nanostructured materials are cellulose nanocrystals (CNC) (Klemm et al., 2011) and starch nanocrystals (SNC) (Le Corre, 2011). These nanostructures are typically prepared via acid hydrolysis (typically concentrated sulfuric acid is used) by removing the amor-

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Fig. 6. The use of starch nanocrystals (SNC) as a Pickering stabilizer in the emulsion polymerization of butyl methacrylate. (A) Schematic representation of the postulated polymerization mechanism, (B,C) SEM images of the poy(butyl methacrylate) latexes stabilized by SNCs prepared from sulfuric acid and hydrochloric acid treatment, respectively, and (D) Optical appearance of composite film prepared from the latexes. Reprinted and adapted from Bel Haaj, Thielemans et al. (2014). Copyright 2014 American Chemical Society.

phous regions of native cellulose fibrils and starch granules. As a result, negatively charged CNC and SNC are produced due to the presence of anionic sulfate groups, which provide dispersibility and colloidal stability in water. These colloidal structures have dimensions that are typically in the range of 0.1–1.0 ␮m in length (L), 0.001–0.1 ␮m in width (w), providing high aspect ratios (L/w) of 10–1000 (Habibi, Lucia, & Rojas, 2010). The high aspect ratio is the key property that provides CNC and SNC with their advantageous properties in many applications. Li and co-workers (C. Li, Sun, & Yang, 2012) have used SNC (prepared from sulfuric acid treatment) in the range of L = 40–100 nm as a Pickering stabilizer for the emulsification of paraffin oil in water. These authors elegantly showed that the SNC’s are surface active and able to stabilize a paraffin emulsion. Bel Haaj and coworkers (Bel Haaj, Ben Mabrouk, Thielemans, & Boufi, 2013) used sulfuric acid hydrolyzed SNC to stabilize a butyl methacrylate miniemulsion, with the ultimate goal of producing an SNC-poly(butyl methacrylate) nanocomposite. The SNC’s only provided partial emulsion stability, due to their hydrophilic nature and/or the presence of sulfate anions on their surface, which increase water wettability and reduce binding energy onto the droplet’s surface. However, stable mini-emulsions could be prepared by exploiting the synergistic stabilization effect of adding a cationic surfactant. Bel Haaj and co-workers then built on this example by using SNC as a Pickering stabilizer in the emulsion polymerization of butyl methacrylate (see Fig. 6) (Bel Haaj, Thielemans, Magnin, & Boufi, 2014). Two types of SNC platelets were prepared from either sulfuric acid hydrolysis (SNCSA : 58 nm and zeta potential (␰) − 32 mV) or hydrochloric acid hydrolysis (SNCHCl : 51 nm and ␰ = −2 mV). Independent of the type of SNC used in the emulsion polymerization, the reciprocal particle size is linearly correlated to SNC loading which demonstrates that the hybrid latex particles are stabilized through a Pickering stabilization mechanism. From their results, however, these authors concluded that the SNCHCl are preferred as Pickering stabilizers due to the lower surface charge which promotes adsorp-

tion onto nucleated latex particles. Mechanistically, latex particle formation is proposed to proceed through a homogeneous nucleation process where the particles, once nucleated, are stabilized by a continuous adsorption of SNCs from the aqueous phase to compensate for the increase in interfacial area as polymerization continues (Bel Haaj, Thielemans et al., 2014). Films prepared from the SNC-poly(butyl methacrylate) composites showed improved mechanical properties and optical properties when compared to a blend of a poly(butyl methacrylate) latex with SNC, demonstrating the advantage of nanocomposites. More common is the use of CNCs, which can be chemically modified and used as colloidal stabilizers for stabilizing Pickering emulsions (Azzam, Siqueira et al., 2016; Azzam, Heux et al., 2016; Hu, Ballinger, Pelton, & Cranston, 2015; Tang et al., 2014; D. Wang, Jessop, Bouchard, Champagne, & Cunningham, 2015). This concept has been applied to the synthesis of hybrid latex polymers by Ben Mabrouk and co-workers (Ben Mabrouk, Kaddami et al., 2011; Ben Mabrouk, Rei Vilar, Magnin, Belgacem, & Boufi, 2011; Elmabrouk, Wim, Dufresne, & Boufi, 2009; Ben Mabrouk, Salon, Magnin, Belgacem, & Boufi, 2014; Ben Mabrouk, Magnin, Belgacem, & Boufi, 2011). In 2009, they reported a mini-emulsion polymerization approach towards poly(styrene-co-ethyl hexyl acrylate)-CNC nanocomposites (Elmabrouk et al., 2009). Unlike the SNC examples discussed previously, the cellulose whiskers (or CNC, L = 220 ± 20 nm, d = 10 ± 2 nm) (Ben Mabrouk et al., 2014) do not act as Pickering stabilizers and, in fact, have a detrimental effect on the colloidal properties (i.e. the average particle size and colloidal stability) of the hybrid latex. However, the addition of up to 2% of a reactive monomer such as methacryloxypropyl triethyloxysilane, which reacts to form silanol functionalities on the surface of the latex particles, was found to allow for up to 5% of CNC without a significant particle size increase nor the formation of coagulum (Elmabrouk et al., 2009). The same authors (Ben Mabrouk et al., 2014) later reported a thorough liquid and solids state 13 C and 29 Si NMR study of the underlying mechanisms that increased

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Fig. 7. TEM (A-D) and SEM (E-H) images of polystyrene latexes prepared in the presence of 0.1% to 0.7% of OSA-modified SNPs. Reprinted with permission from Pei et al., (2016). Copyright 2016 Royal Society of Chemistry.

latex stability: the hydrophilic silanol functionalities on the hybrid latex particle surface favored adsorption of the CNC through hydrogen bonding. The presence of the hydrophilic silanol groups thus allowed the CNC to act as a Pickering stabilizer. There is, however, an upper limit (i.e. a percolation threshold) to the amount of CNC that could be added, as above a threshold value the CNC can form a 3-dimensional network through hydrogen bonding which resulted in coagulation of the latex (Ben Mabrouk, Rei Vilar et al., 2011). Films cast from nanocomposites containing CNC demonstrated a significant improvement in mechanical properties, due to the high aspect ratio (∼20) and the small particle size of the latex. A different approach towards a poly(methyl methacrylate)-CNC nanocomposite was reported by Sain and Mukhopadhyay (Sain, Ray, & Mukhopadhyay, 2014). CNC (L = 300 nm, d = 10 nm, aspect ratio = 30) was added to a suspension polymerization of methyl methacrylate stabilized by poly(vinyl alcohol). In order to achieve better compatibilization between the poly(methyl methacrylate) matrix and the CNC, the CNC was modified with maleic anhydride prior to polymerization. This modification facilitates covalent bonding of the CNC to the poly(methyl methacrylate) particles through the free radical polymerization process. When films cast of the poly(methyl methacrylate)-CNC nanocomposite were compared to films of the acrylic component and a simple blend of both components, the nanocomposite film showed improved mechanical and thermal properties. Furthermore, the moisture adsorption was considerably lower for the nanocomposite as compared to the blend (Sain et al., 2014). A heterogeneous polymerization approach towards polystyrene grafted on CNC was reported by Morandi and co-workers (Morandi, Heath, & Thielemans, 2009). Hydroxyls available on the surface of the CNC were chemically modified with 2bromoisobutyrylbromide in the presence of a base to introduce initiation sites for the surface-initiated ATRP of styrene. The polystyrene content of the final CNC-g-polystyrene nanoparticles could be controlled up to approx. 20 w/w%, simply by varying the polymerization conditions. The resultant nanoparticles could be dispersed in water and proved effective in scavenging 1,2,4trichlorobenzene, a common organic pollutant found in aqueous (waste) streams.

4.3. Starch nanoparticles Different processes for the production of starch nanoparticles (SNP) have been receiving increasing attention, both in academia and industry (De´ıborah Le Corre et al., 2010). In academia SNPs predominantly find application in the medical field, due to their biocompatibility, biodegradability and low biofouling (Ip, Tsai, Khimji, Huang, & Liu, 2014; Simi & Emilia Abraham, 2007; X. Wang, Chen, Luo, & Fu, 2016). In industry, the use of SNP nanoparticles is predominately driven by the relatively low price of starch and other key natural and chemical ingredients, combined with the ability to produce them via a very efficient high-volume continuous process. Many other synthetic approaches are available towards SNPs, most notably nanoprecipitation (Tan et al., 2012), sonication (Bel Haaj, Magnin, & Boufi, 2014; Bel Haaj, Magnin, Pétrier, & Boufi, 2013), and reactive extrusion (Giezen, Jongboom, Feil, Gotlieb, & Boersma, 2000; Lee, Bloembergen, & van Leeuwen, 2010; Song, Thio, & Deng, 2011; Wildi, Van Egdom, & Bloembergen, 2007). Contrary to the preparation of SNCs, SNPs are prepared by fully gelatinizing the starch, i.e. removing all crystallinity, followed by a crosslinking step while the starch is in a plasticized amorphous state (Bloembergen, Mclennan, van Leeuwen, & Lee, 2010). Tan and co-workers (Tan et al., 2012) reported a nanoprecipitation process using waxy maize starch chemically modified with acetic anhydride (DS = 2.3) and phthalic anhydride (DS = 0.35). Nanoprecipitation of these hydrophobically modified starch polymers yields SNPs of approx. 270 nm that are able to stabilize toluene-water and undecanol-water emulsions at a loading of 1%. This work was later extended by using the same technique to prepare octenyl succinic anhydride and acetic anhydride modified SNPs that were subsequently used as a Pickering stabilizer in the emulsion polymerization of styrene (see Fig. 7) (Pei et al., 2016). Depending on the amount of modified SNPs added to the polymerization, the final latex particle morphology varied from bare PS particles (e.g. see Fig. 7E) to raspberry-like structures (see e.g. Fig. 7F). The group of Boufi has published a series of papers that reports a high power sonication process towards the production of SNPs ranging from 20 to 100 nm (Bel Haaj, Magnin et al., 2014, 2013; Bel

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Fig. 8. Preparation of composite latex particles (right) from a Pickering emulsion polymerization of butyl methacrylate in the presence of SNP (left). Reprinted and adapted with permission of Bel Haaj (2014). Copyright 2014 Royal Society of Chemistry.

Haaj, Thielemans et al., 2014). Their approach starts from a waxy maize starch suspension (1.5%) that is exposed to high frequency ultrasound. The starch granules (5–20 ␮m) are physically broken down due to collisions induced by implosion of cavitation bubbles, until a limiting size of approx. 20–100 nm is reached. Although the starch granules are not gelatinized prior to ultrasound treatment, WAXD analysis showed that the majority of the crystallinity is lost during the process, resulting in relatively amorphous SNPs (Bel Haaj, Magnin et al., 2013). SNPs prepared using this technique (average size of 37 nm, zeta potential = −3 mV) have subsequently been used as Pickering stabilizers in the emulsion polymerization of butyl methacrylate. The ability of the SNPs to act as a Pickering stabilizer was calculated from the energy required to remove the particle from the interface (Levine, Bowen, & Partridge, 1989) and estimated to be 740 kB T, which is substantially larger than the thermal motion energy of a colloidal particle. Consequently, it is expected that once an SNP adsorbs onto the interface, there will be little incentive for it to desorb. The emulsion polymerization results showed that SNP-stabilized poly(butyl methacrylate) latexes could be successfully prepared (see Fig. 8). Pickering stabilization was observed to be the sole stabilization mechanism for latex formulations with an SNP content over 6% (compared to monomer), as was concluded from dynamic light scattering and zeta potential measurements. The hybrid latex particles had a raspberry morphology which illustrates that the latex particle surface is densely packed with SNPs. Films prepared from these nanocomposite latexes showed higher optical transparency and increased mechanical properties when compared to blends of SNP and a poly(butyl methacrylate) latex (Bel Haaj, Magnin et al., 2014). Of the different fabrication routes towards SNPs, reactive extrusion (REX) is likely the most industrially applicable method, which has been applied on commercial scale by EcoSynthetix (Giezen et al., 2000; Wildi et al., 2007). Their original patent describes a process that is based on plasticization of starch using shear forces in the presence of a cross-linker, resulting in SNPs that are on average smaller than 400 nm. These SNPs are commercially available since 2008 and are used as biobased binders in the paper and paperboard and other industries (Bloembergen et al., 2011; Lee et al., 2010). A similar procedure was later also reported in the open literature by the group of Yulin Deng (Song et al., 2011), who reported a detailed study of the effect of various experimental parameters on the size of the SNPs. They found that the presence of cross-linker in particular was very efficient in reducing the average particle size to approx. 160 nm (Song et al., 2011). Copolymerization of these SNPs in an emulsion polymerization process has also been reported

(Bloembergen, van Leeuwen, & Smeets, 2015). The patent application describes an emulsion polymerization of acrylic monomers in the presence of the SNPs. Films of the resulting nanocomposite latex show improved optical transparency and improved solvent resistance. Another interesting use of SNPs was reported by Nikfarjam and co-workers (Nikfarjam, Taheri Qazvini, & Deng, 2013; Nikfarjam, Taheri Qazvini, & Deng, 2014), who described the crosslinked SNPs stabilized Pickering emulsion polymerization of styrene for the preparation of expandable polystyrene beads. The SNPs are produced through precipitation of gelatinized starch using ethanol, followed by a crosslinking step using citric acid (Ma, Jian, Chang, & Yu, 2008). A w/o/w double emulsion was prepared in three steps: 1) a bulk polymerization of styrene and maleic anhydride was performed up to approx. 25% conversion, 2) an SNP dispersion was added and the mixture emulsified under high shear to form an w/o inverse emulsion, and 3) the w/o inverse emulsion was then emulsified in water containing hydroxyethyl cellulose to form an w/o/w double emulsion. Upon polymerization of the resulting suspension polymerization of styrene, polystyrene beads of 3–4 ␮m are obtained that contain 104 -105 small water inclusions depending on the SNP loading used in the emulsification process. The SNPs are postulated to be grafted onto the interface of the inclusions within the polystyrene beads due to hydrogen bonding or esterification between the hydroxyl groups of the SNPs and the maleic anhydride. These beads, due to their high internal water volume fraction, are expected to be a green alternative for expandable polystyrene. A similar approach, except using CNC, has also been reported by this group (Nikfarjam, Taheri Qazvini, & Deng, 2015) and showed far superior water encapsulation efficiency. Furthermore, the authors also showed that the polystyrene beads have excellent expandability to a uniform foam structure.

5. Future perspectives Although the use of carbohydrates for the synthesis of hybrid latex particles is now commonplace both in academia and industry, many opportunities exist for further improvement. Many applications, for example, would benefit from approaches that can increase the biocontent and/or that can impart novel desired properties to the latex product. In particular, the following areas will be discussed (1) alternative latex particle morphologies, (2) the use of nanostructured colloidal carbohydrate-based materials, and (3) saccharide-containing monomers.

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The use of carbohydrates for the production of commercial latexes is becoming increasingly mainstream, driven by economic (carbohydrates such as starch are available at lower cost compared to most petroleum-based vinyl monomers and have relatively stable pricing due to being decoupled from fluctuating oil prices) and environmental (introduction of biocontent) advantages. However, the current industrial approach has three significant drawbacks: First, the carbohydrate needs to be reduced in molecular weight before it can be successfully copolymerized in an emulsion polymerization process. Many of the advantageous properties of carbohydrates are related to the high molecular weight of the polymer and are thus lost once the molecular weight is reduced. Second, the final biocontent is limited due to increasing latex viscosity and increasing probability of latex flocculation and coagulation as the biocontent is increased. Typically, the biocontent reported in the patent literature ranges from 10 to 20 wt% of the total polymer mass. Third, as a consequence of the emulsion polymerization process, the carbohydrate is located on the surface of the latex particles, which affects important properties such as the color, latex viscosity and water sensitivity of the dried latex polymer film. Some, if not all, of these disadvantages can be circumvented if the carbohydrates were to be fully encapsulated within the latex particle, an idea that we have been pursuing at EcoSynthetix (Bloembergen et al., 2015). This latex particle morphology would allow for the use of high molecular weight carbohydrate, encapsulated within the latex particle, and thus not contributing to the latex viscosity, even at high biocontent. Furthermore, as the surface of the latex particle consists of conventional synthetic polymers, it may be expected that many of the latex properties are unaltered when compared to a 100% synthetic equivalent latex. This latex particle morphology is unexpected as it is well accepted that in a thermodynamically-driven latex particle morphology the more hydrophilic polymer (i.e. the carbohydrate) would favor the water-particle interface (Sundberg & Durant, 2003). Some examples of similar latex particle morphologies (i.e. hydrophilic core and hydrophobic shell) have been reported in the literature (Hank de Bruyn et al., 2006; McDonald & Devon, 2002; Peng et al., 2008) and it is therefore conceivable that such latex particles can be produced using carbohydrates. However, it should be noted that these examples are purely proof-of-concept and cannot be readily applied to current industrial processes. Nanostructured colloidal carbohydrate-based materials are receiving increasing attention in academia, as well as in industry. Indeed, many of these materials, e.g. cellulose nanocrystals (CNC), cellulose nanofibers (CNF), cellulose fibrils (CF), starch nanocrystals (SNC) and starch nanoparticles (SNP), have demonstrated unique performance attributes and the number of companies producing such materials at industrial scale is increasing. These materials are only recently finding a way into traditional polymerization processes and a few examples have been included in the previous section. Given the colloidal dimensions of most of these nanostructures, these materials can be incorporated into latex particles through adsorption (e.g. Pickering stabilization) and/or grafting chemistry and early results show the promise of these nanocomposite materials. Furthermore, grafting from CNC, SNC, or SNP yields interesting colloidal materials that may find applications in advanced applications such as drug release or sensing. Despite the promise that the nanocrystalline materials hold for many applications, the fact that they are (currently) not available at sufficient scale and at a competitive price-point will hamper their widespread implementation. Significant investments in a number of (pre-commercial) production facilities for CNC/CNF/CF (FPInnovations, Domtar, Kruger in Canada and USDA-Forest Products Labsin Madison, WI, to name a few) (Brinchi, Cotana, Fortunati, & Kenny, 2013) will, to some extent, alleviate this, but there are also some

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significant engineering challenges to resolve (e.g. the large amounts of concentrated acid used as well as the downstream processing to purify, concentrate, and ultimately dry the CNC) (Brinchi et al., 2013). SNP-like products, on the other hand, are currently already produced at (semi)-commercial scale (e.g. EcoSphereTM by EcoSynthetix, Mater-BiTM by Novamont, and PhytoSpherixTM by Mirexus) and at a competitive price-point. It is therefore conceivable that these SNP materials will find a faster route to market for novel nanocomposite latex materials, especially for high volume industrial applications where the widespread use of a renewable alternative to petro-based chemicals is critical in combatting climate change on a global scale. Another route for the incorporation of biocontent into synthetic latex is via polymerizable saccharide monomers, which can be copolymerized with conventional acrylic and styrenic monomers. This strategy can lead to two different latex particle morphologies, depending on the hydrophilic/hydrophobic balance of the saccharide monomer. When relatively hydrophilic monomers such as acrylated monosaccharides (e.g. based on glucose, mannose, galactose, ribose, or glucosamine) (Abeylath & Turos, 2007; Carlson et al., 2015) are used, the saccharide functionality will be located as a corona on the surface of the latex particle. This strategy was exploited by Abeylath and Turos who prepared poly(ethyl acrylate) latex particles functionalized with a corona of different carbohydrates (Abeylath & Turos, 2007) through an emulsion polymerization process. The final latex particles had a hydrodynamic radius of approx. 40 nm and contained 10% biocontent originating from the carbohydrate acrylate monomers used. The intended application for these glycosylated latex particles was as a drug delivery vehicle for various antibiotics, which the authors demonstrated in a follow-up paper (Abeylath, Turos, Dickey, & Lim, 2008). Mann and co-workers (Mann et al., 2014), prepared glucose decorated polystyrene particles from the emulsion copolymerization of styrene and glucose acrylate, containing up to 8% biocontent. Silver nanoparticles were selectively deposited onto these latex particles (i.e. no free, non-adsorbed silver nanoparticles were present), which could provide an interesting colloid for application ranging from sensing to catalysis. Copolymerization of carbohydrate acrylates is a highly beneficial strategy for high-end applications where the functionality of the saccharide is important. However, as these monomers are specialty chemicals and used in relatively low amounts, this approach offers low biocontent at high cost and is therefore not a suitable solution for more commoditized markets. An alternate approach is to use carbohydrate-based monomers that can effectively (or at least predominantly) be incorporated within the latex particles. This requires relatively hydrophobic carbohydrate-based monomers, which have been reported by EcoSynthetix (EcoMerTM ) (Bloembergen, McLennan, Cassar, & Narayan, 1998; Bloembergen, Nemeth, & McLennan, 2002; Bloembergen et al., 1999; Bloembergen, McLennan, & Lee, 2013) and by Pokerˇznik and Krajnc (Pokerˇznik & Krajnc, 2015) (butyl poly glucoside maleic acid ester). Both polymerizable carbohydrate monomers consist of an alkyl poly glucoside (APG) (i.e. a hydrophobically modified glucose, produced at large scale industrially and commonly used as biosurfactant) (Hill, von Rybinski, & Stoll, 1996) which is functionalized with a polymerizable double bond through esterification using maleic anhydride. The hydrophilic/hydrophobic balance of this monomer can be controlled by changing the alkyl chain length of the alcohol used in the APG synthesis, ranging from polymerizable surfactants (butyl, hexyl) (Pokerˇznik & Krajnc, 2015) to monomer (octyl and higher) (Bloembergen et al., 1999). These sugar macromers have been used to produce a range of latex polymers containing up to 50% of the bio-renewable monomer (Pokerˇznik & Krajnc, 2015; Bloembergen et al., 2013). Furthermore, these monomers can provide additional functionality to the latex, such as repulpability (Bloembergen et al.,

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1999) and degradability (Bloembergen et al., 1998). The significantly higher biocontent that can be achieved, combined with their commercial availability and novel properties, suggests that these sugar macromers could find increasing application in the latex industry. 6. Conclusion In this review article we have provided an overview of the use of carbohydrates to prepare hybrid latex particles. The earlier work in this field focussed on the use of soluble (modified) carbohydrates that could be used as a surfactant/stabilizer, co-monomer, or as a grafting site in various dispersed phase free-radical polymerization processes. This technology has led to the use of low molecular weight carbohydrates as co-monomers in large-scale latex polymer production. More recently, focus has shifted from soluble carbohydrates to colloidal carbohydrate nanostructures, such as granules, nanocrystals, and nanoparticles. These carbohydratebased materials are used to produce surfactant-free or structured latex particles. Although the use of carbohydrates for the synthesis of hybrid latex particles is now commonplace both in academia and industry, many future opportunities exist and we have highlighted (1) alternative latex particle morphologies, (2) the use of nanostructured colloidal carbohydrate-based materials, and (3) saccharide-containing monomers. References Abdel-Halim, E. S. (2013). Preparation of starch/poly(N,N-Diethylaminoethyl methacrylate) hydrogel and its use in dye removal from aqueous solutions. Reactive and Functional Polymers, 73(11), 1531–1536. http://dx.doi.org/10. 1016/j.reactfunctpolym.2013.08.003 Abeylath, S. C., & Turos, E. (2007). Glycosylated polyacrylate nanoparticles by emulsion polymerization. Carbohydrate Polymers, 70(1), 32–37. http://dx.doi. org/10.1016/j.carbpol.2007.02.027 Abeylath, S. C., Turos, E., Dickey, S., & Lim, D. V. (2008). Glyconanobiotics: novel carbohydrated nanoparticle antibiotics for MRSA and Bacillus anthracis. Bioorganic and Medicinal Chemistry, 16(5), 2412–2418. http://dx.doi.org/10. 1016/j.bmc.2007.11.052 Azzam, F., Heux, L., & Jean, B. (2016). Adjustment of the chiral nematic phase properties of cellulose nanocrystals by polymer grafting. Langmuir, 32(17), 4305–4312. http://dx.doi.org/10.1021/acs.langmuir.6b00690 Azzam, F., Siqueira, E., Fort, S., Hassaini, R., Pignon, F., Travelet, C., & Jean, B. (2016). Tunable aggregation and gelation of thermoresponsive suspensions of polymer-grafted cellulose nanocrystals. Biomacromolecules, 17(6), 2112–2119. http://dx.doi.org/10.1021/acs.biomac.6b00344 Bel Haaj, S., Ben Mabrouk, A., Thielemans, W., & Boufi, S. (2013). A one-step miniemulsion polymerization route towards the synthesis of nanocrystal reinforced acrylic nanocomposites. Soft Matter, 9(6), 1975–1984. http://dx.doi. org/10.1039/C2SM27190G Bel Haaj, S., Magnin, A., Pétrier, C., & Boufi, S. (2013). Starch nanoparticles formation via high power ultrasonication. Carbohydrate Polymers, 92(2), 1625–1632. http://dx.doi.org/10.1016/j.carbpol.2012.11.022 Bel Haaj, S., Magnin, A., & Boufi, S. (2014). Starch nanoparticles produced via ultrasonication as a sustainable stabilizer in Pickering emulsion polymerization. RSC Advance, 4(80), 42638–42646. http://dx.doi.org/10.1039/ C4RA06194B Bel Haaj, S., Thielemans, W., Magnin, A., & Boufi, S. (2014). Starch nanocrystal stabilized pickering emulsion polymerization for nanocomposites with improved performance. ACS Applied Materials & Interfaces, 6(11), 8263–8273. http://dx.doi.org/10.1021/am501077e Ben Mabrouk, A., Magnin, A., Belgacem, M. N., & Boufi, S. (2011). Melt rheology of nanocomposites based on acrylic copolymer and cellulose whiskers. Composites Science and Technology, 71(6), 818–827. http://dx.doi.org/10.1016/j. compscitech.2011.01.012 Ben Mabrouk, A., Salon, M. C. B., Magnin, A., Belgacem, M. N., & Boufi, S. (2014). Cellulose-based nanocomposites prepared via mini-emulsion polymerization: understanding the chemistry of the nanocellulose/matrix interface. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 448(1), 1–8. http://dx. doi.org/10.1016/j.colsurfa.2014.01.077 Ben Mabrouk, A., Kaddami, H., Magnin, A., Belgacem, M. N., Dufresne, A., & Boufi, S. (2011). Preparation of nanocomposite dispersions based on cellulose whiskers and acrylic copolymer by miniemulsion polymerization: effect of the silane content. Polymer Engineering & Science, 51(1), 62–70. http://dx.doi.org/10. 1002/pen.21778 Ben Mabrouk, A., Rei Vilar, M., Magnin, A., Belgacem, M. N., & Boufi, S. (2011). Synthesis and characterization of cellulose whiskers/polymer nanocomposite

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