- Email: [email protected]
Dendritic and hyperbranched polymers: advances in synthesis and applications
683
Dendritic and hyperbranched polymers: advances in synthesis and applications Lois J Hobson* and Robin M Harrison+
Investigations of the synthesis, characterisation and structure-property correlations of dendritic polymers have made significant progress recently. These developments have established better understanding and brought a range of potential applications into consideration.
Figure 1
Divergent synthesis
Addresses IRC in Polymer Science and Technology, Department of Chemistry, Durham University,South Road, Durham DHl 3LE, UK *e-mail: LJ.HobsonQdurham.ac.uk *e-mail: [email protected] Current Opinion in Solid State 8 Materials Science 1997, 2:683-692
Convergent synthesis
Electronic identifier: 1359-0286-002-00683 8 Current Chemistry Ltd ISSN 1359-0286 Cured
Abbreviations CDI 1 ,l ‘-carbonyldiimidazole DB degree of branching DLS dynamic light scattering PAMAM poly(amidoamine) PET poly(ethylene terephthalate) PPE poly(2,6-dimethyl- ,4-diphenylene ether)s PPI poly(propylene imine) REDOR rotational-echo double-resonance SEC size exclusion chromatography
Introduction The concept of cascade molecules was introduced by VGgtle in 1978 [1] and developed during the mid 198Os, by Tomalia [2,3], Newkome [4], Frechet [S] and others. Today dendritic polymers are established as a distinct class of macromolecules. Dendrimers, the best defined structures within this family, can be constructed via two different routes, see Figure 1. The divergent approach was favoured for potential commercial scale development by both DSM (The Netherlands) and Dendritech (USA) in the synthesis of Astramolo poly(propylene imine) (PPI) and Starburst poly(amidoamine) (PAMAM) dendrimers respectively. The convergent approach has been widely adopted for laboratory scale syntheses. In contrast to the iterative sequence of reaction and purification steps associated with dendrimer syntheses, the single step polymerisation of AB, (x > 1) monomers leads to hyperbranched structures. These materials, which are accessible at lower cost than their monodisperse dendrimer analogues, retain some of the structural features and properties of dendrimers and are consequently commercially attractive. As both syntheses and characterisation methods for these types of materials have developed so have attempts to
Opinion in s0ri State 6. Materials Science
Synthetic approaches to dendrimers.
explore structure-property relationships and to establish applications. This review is intended to highlight recent contributions to the field; in particular new syntheses, new concepts and novel architectures. The theoretical questions and implications for potential applications arising from these developments will be discussed.
Synthesis of dendritic materials Dendrimers
One major limitation of dendrimer syntheses is that often they can be prepared only on a relatively small scale due to the inherent difficulty of the chemistry involved. Recent work by Rannard and Davis [6’], using the highly selective reactions of l,l’-carbonyl diimidazole (CDI) with amines, alcohols and carboxylic acids (Figure Za) allows the preparation of a variety of highly functionalised dendritic structures on a relatively large scale (currently hundreds of grams). It has been demonstrated that the intermediate imidazole derivatives show high selectivity in subsequent reactions with amines and alcohols (Figure Zb). Consequently, it is possible to design syntheses in which protection-deprotection chemistry is not required, for example see Figure Zc, where the synthesis of large wedges and dendrimers is achieved via significantly fewer steps than earlier procedures. Esters, amines, ureas, carbonates and urethane linkages can all be selectively introduced at the core, branches and termini of the dendrimer, which should allow tine tuning of polymer properties via structure regulation. Additionally, Rannard and Davis [7’] have described the preparation of statistical co-dendrimers; the analogues of linear statistical copolymers (see Figure Zd). The product
664
Figure
Polymers
2
(a)
(b)
9-
NQl-ea
on
CDI
R-OH,+NH2
R-DH,+R-N”2
CDi
R-2H&,R-NH2
(c) RCH~OH
RrO-pj-Na
Rr-i-R,
<
R++OH. R$OH
3 Ideal Wedges
No Reacbon
Current Opmm
in Sohd State B Materials Scmnce
The highly selective CDI (1 ,l’-carbonyldiimidazole) chemistry developed by Rannard and Davis [6’,7’] to prepare dendritic structures. (a) Versatility of the CDI chemistry. (b) Example of a dendritic wedge prepared using the CDI chemistry. k) Examples of the selectivity of the CDI chemistry. (d) Schematic
representation
of the preparation
of statistical co-dendrimers
consists of a set of dendrimers of the same generation, each with perfect, ideal branching but with a distribution in both structure and molecular weight. These copolymers can be prepared on a large scale in a one-pot reaction.
interest as catalysts as they combine high solubility with easy recovery by dialysis or ultrafiltration techniques, thus combining the advantages of conventional homogeneous catalysts and polymer supported reagents.
Majoral and colleagues [8’] have synthesised one of the largest dendritic materials ever prepared using a two-step procedure from a hexachlorocyclotriphosphazine core. Generations are built by the substitution of chlorine atoms with 4-hydroxybenzaldehyde to give aldehyde terminated materials (Figure 3a), followed by condensation of the peripheral aldehyde functionalities with H,N-N(Me)P(S)C12, regenerating the outer P-Cl groups (Figure 3b). Generation 8 dendrimers have been prepared with up to 768 dichlorophosphorus groups or 1536 aldehyde groups at the periphery.
Van Koten and Jastrzebsli [9] have described silicon-based dendrimers which are functionalised with up to 12 catalytically active nickel or palladium complexes at the periphery, and showed that in cross-coupling reactions, their reactivity is comparable to that of conventional systems. Organoruthenium dendrimers with up to 48 metal centres at the periphery have been prepared by Liao and Moss [lo] using a convergent synthesis based on a ruthenium functionalised wedge (Figure 3c) and the FrCchet methodology.
Several preparations of dendrimers with organometallic groups at the periphery have been reported. These are of
Cuadrado et al. [ 11’1 have functionalised PPI dendrimers with up to 64 ferrocene groups. Reaction of l-(chlorocarbonyl)ferrocene with the dendrimers terminal amine groups introduced the organometallic functionality, and
Dendritic and hyperbranched
polymers Hobson and Harrison
665
Figure 3
(a)
(d)
(b)
fi
0
-mmmplsr
(h)
(e)
/\ , &
OH
cl+ /
o+I
n = 2,3.4.5,6 or 7
Current Opinion in S-Aid State d Materials sdence
Examples of the dendritic structures, building blocks and monomers discussed in the text
complete conversion was supported by NMR and IR spectroscopy. Meijer and coworkers [12’] have used the same PPI dendrimers for the site-specific complexation of copper(H), zinc(I1) and nickel(H) chlorides, and have functionalised the dendrimer periphery with up to 32 metal groups. Given the availability of commercial quantities of the PPI dendrimers, the easy synthesis and varied catalytic ability of metallocene and transition metal complexes, this type of work looks attractive for further research and development. Organometallic dendrimers which contain the metal atom at the branch points rather than at the termini have also received attention. Balzani and coworkers [13’] have developed a building block approach in which each metal centre has a specific function in the material (Figure 3d). To illustrate this concept, luminous and redox active mixed metal third generation dendrimers have been prepared with well defined luminescence and redox properties which can be adjusted by structural modifica-
tions. Further development might see the preparation synthetic light-harvesting arrays using this approach.
of
The assembly of dendritic wedges into larger structures has been investigated by several groups. Tzalis and Tor [ 141 have prepared first generation dendritic wedges with phenanthroline ligands at the focus, and has demonstrated their self-assembly around metal centres. The number and spatial orientations of the dendritic groups in the assembly can be varied by choice of the appropriate metal, making this a versatile technique for controlling architecture. Replacing the focal ligand with groups that form strong hydrogen-bonds is an alternative approach to self-assembled dendrimers. Zimmerman’s [15’,16’] work describes poly(ary1 ether) dendrimers functionalised at the focus with isophthalic acid groups in an orientation that allows the formation of discrete hexameric self-assembled structures (Figure 4a). Experimental evidence suggests that the hexameric structures are formed, with the stability of the
666
Polymers
Figure 4
(a)
(b)
=
afyl ether dendrimer
wedge = atyl ether dendrimer
wedge
aryl ether dendrimer
wedge
PoH
=
Current Opinion in Solid State & Materials Scmnce
Schematic representation for the self-assembly of dendritic wedges using hydrogen-bonding. (b) Approach used by Frkhet et al. [17-J.
aggregates dependent on the solvent and size of the dendritic wedge. Preliminary results from FrCchet’s group [17’] describe a related system applying the self-asseinbly of melamine and cyanuric acid core-functionalised poly(ary1 ether) dendrimer wedges (Figure 4b). This is a more versatile approach as wedges functionalised with different surface groups can be controllably introduced in an alternating fashion around the assembled hexameric structure.
Hyperbranched
polymers
The synthesis and characterisation of hyperbranched polymers has recently been reviewed by Malmstrijm and Hult [18”]. Here, we highlight what we believe to be the most relevant publications during the review period. Polycarbonates are well established as engineering thermoplastics for a variety of applications. It may be expected that the hyperbranched analogues of these materials will prove useful as property modifiers in commercially available systems, or as reactive prepolymers for
(a) Approach used by Zimmerman et a/. [15’,16’]
novel engineering materials. Bolton and Wooley [19] described the first example of an aromatic hyperbranched polycarbonate, prepared from a monomer based on l,l, 1-tris(4’-hydroxyphenyl) ethane (Figure 3e). Two of the ‘hydroxy groups are activated by reaction with CD1 and the third is protected by silylation. Treatment with fluoride ions and potassium hydroxide cleaves the protecting group and initiates polymerisation. Davis and Rannard [ZO’] have used a similar approach to polymerise derivatives of 3,Sdihydroxybenzoic acid (Figure 3f) and 2,2-bis-hydroxymethyl propionic acid (Figure 3g). As no protecting groups are required, polymerisation may be initiated by heating the monomers in the presence of potassium hydroxide to give aromatic and aliphatic hyperbranched polyesters respectively. Using a mixed monomer feed, aliphatic/aromatic hyperbranched copolymers can also be prepared. Wooley and coworkers branched polyfluorinated
[21,22] have prepared hyperpolymers from fluorinated
Dendritic and hyperbranched
Finwe
!i
polymers Hobson and Harrison
667
Figure 6
Hyperbranched polymer perfect branching, irregular growth Perfect dendron
Intrinsic viscosities of polymers with various degrees of branching (DB) as a function of molecular weights. Each point represents an isolated hydrocarbon polymer (CH branch points, C,H, spacers between branch points, and CHs terminal groups) and was obtained by a Monte Carlo simulation. 0, dendrimers of generations l-7; 0, linear chains with the same number of monomer units as respective dendrimers (above M = 10000 they show Gaussian chain behaviour: slope l/2); A, hyperbranched polymers with DB < 0.2; 0, hyperbranched polymers with 0.2 5 DB < 0.5; V, hyperbranched polymers with DB I 0.5. Adapted with permission from [44”1.
benzyl ether AB, and AB, monomers (Figure 3h, 3i). Polymerisation is said to occur with the displacement of the para-fluorine by the nucleophillic benzylic hydroxy group under basic conditions. The polymers show marked differences in thermal and physical properties, reflecting the different repeat motifs, branching patterns and fluorine contents. Polymer films are extremely hydrophobic, as demonstrated by the measurement of contact angles with water. Feast and coworkers [23”] have recently described the synthesis of PAMAM hyperbranched polymers, prepared by the melt polymerisation of N-acryloyl-a,*diaminoalkane hydrochlorides (Figure 3j). The polymers prepared where n = 2 using a B, core terminator, which are the hyperbranched polymer analogues of PAMAM dendrimers, have been shown by rsN NMR spectroscopy to have very high degrees of branching and exhibit solution viscosity behaviour analogous to true dendrimers (wide inrr~). Polymerisation of AB, monomers via novel mechanisms have been reported. Weber’s [24] group has synthesised and polymerised 4-acetylstyrene (Figure 3k) with a ruthenium catalyst. The.polymerisation step relies on the activation of the hydrogens o&o to the acetyl group towards alkylation by vinyl groups. The polymer has a low DB (degree of branching), as measured by 13C NMR spectroscopy, with 4-7% branched units. Given the simplicity of the monomer and the potential to increase the activity of the catalyst to
perfect branching Current Opinion in Solid State 6 Materials Science
Highly branched macromolecules.
obtain higher DBs and molecular weights, this concept could see development into a viable system for the preparation of functionalised hyperbranched polymers. Matyjaszewski and colleagues [ZS”] have polymerised 4-(chloromethyl)styrene (Figure 31) with a copper (I) bipyridyl catalyst by atom transfer radical polymerisation to give hyperbranched derivatives of polystyrene. Co-polymerisation with styrene gives polystyrene derivatives in which the degree of branching may be controlled by altering the feed ratios. Given the availability and low cost of the monomers and catalyst and the potential application of the materials as property modifying agents for commercial polystyrene, the further development of these ideas may be fruitful.
Variation
in dendritic architecture
A recent review by Tomalia and Esfand [26’] highlights the progressive development of different macromolecular structures over the last twenty years. Dendrigrafts, composite structures containing dendritic elements, are identified as a sub-class of dendritic polymers. Compared to monodisperse dendrimers, dendrigrafts have well defined structures and higher molecular weight and exhibit solution viscosity behaviour reminiscent of pure dendrimers. Results presented by Yin and coworkers (271, suggest that these composite structures are more amenable to certain applications than their dendrimer equivalents, for example as gene transfection agents. This theme of attaching dendritic grafts to a linear backbone has been approached using well established free radical polymerisation chemistry. In an elegant synthesis
688
Polymers
Grubbs
eta/. [28”]
combined
living
free radical
and living
atom transfer radical polymerisation techniques to yield novel graft and dendrigraft structures from a range of commercially available monomers. In all cases, control over molecular weight is demonstrated whilst retaining low polydispersity values. The use of dendrons as initiators for living radical polymerisations, leading to the synthesis of dendritic-linear block copolymers, by the use of polyether dendrons
has been exemplified as macromolecular ini-
tiators for the controlled free radical polymerisation of vinyl monomers [29’]. The approach is shown to be versatile, with variation of both monomer and initiator being possible. Meijer’s group addresses the synthesis of polystyrene/PPI diblock copolymers via a complementary approach. PPI dendrimers, up to generation five, have been synthesised divergently from amine terminated polystyrene [30]. Conductivity measurements area isotherms indicate generation dependent behaviour for these systems.
and pressure amphiphilic
The synthesis of structures combining AB, hyperbranched systems with linear polymers has also been considered. Brenner and Voit [31] report the modification of both acid and alcohol terminated hyperbranched polyesters to introduce azo functionalities at the surface, capable of initiating free radical polymerisation. Using this approach grafts of controlled size and number were introduced from the hyperbranched core. The resultant materials have been shown to exhibit low solution viscosity, an attractive property for coating applications. Hyperbranched polymer films grafted on self-assembled monolayers have also been prepared. In a series of papers Crooks and coworkers [32] describe the synthesis and subsequent modification of surface grafted hyperbranched poly(acrylic acid) films grafted on self-assembled organomercaptan monolayers. Surface carboxylic acid groups, shown to serve as specific metal ion binding sites, can be modified with metallocene derivatives to give electroactive films [33]; while the synthesis of a range of composite and fluorinated derivatives has produced possible materials for passive coatings applications [34’]. Combination of preformed, well defined dendrons and linear polymers via the Williamson ether synthesis to form amphiphilic star copolymers was described by Gitsov and FrCchet [35”]. This work represented the first example of a polymeric system responsive to changes in the surrounding medium by forming mono-molecular micelles with different core shell structures. Modification of the commercially available PPI dendrimers by the introduction of hydrophobic alkyl chains from the corresponding acid chlorides has also been described [36”]. Remarkable selectivity was demonstrated with the use of excess dendrimer; with two products resulting, the modified and unmodified dendrimers, rather than the statistical structure expected. Their capacity to act as dynamic hosts
for guest micellar
molecules structure
Fundamental
provides
of these
evidence
modified
for
an
inverted
dendrimers.
questions
Investigations of dendritic macromolecules have uncovered a number of fundamental questions relating to the structure, reactivity and physical properties of materials within the class. Eventual exploitation of these novel macromolecules may be dependent upon answering these questions. The ongoing problem of molecular weight determination for materials with nonlinear topologies was highlighted by Meijer and coworkers in relation to star-shaped poly(2,6dimethyl-1,4-diphenylene ether)s (PPE). The PPE modified PPI dendrimers were studied in solution and as 50/50wt.% blends with linear poly(ethylene terephthalate), PET [37’]. Significantly it was observed that for stars with grafts of constant arm length but increasing numbers of arms, hydrodynamic volume, measured by SEC (size exclusion chromatography), remained constant while both hydrodynamic radius, determined by DLS (dynamic light scattering), and intrinsic viscosity were shown to decrease at high numbers of arms. Evaluation of solid state shape, size and intramolecular packing of a fifth generation dendrimer by combined REDOR (rotational-echo double-resonance) NMR spectroscopy, distance constrained molecular dynamic simulations and site-specific stable-isotope labelling studies was described by Wooley et al. [38]. This work addresses the concepts of dendritic shape, conformation and packing, critical to many proposed applications. It is clear that much work remains to be done in this area. A recent paper contrasting the physical properties of hyperbranched and dendritic polyesters with both simple linear polymers and linear analogues of dendritic polymers (Pdi > 2 ) [39], concludes that thermal properties are independent of architecture but extremely sensitive to the number and nature of the end-groups. In contrast solubility, viscosity and reactivity were found to be architecture dependent. The validity of this analysis was subsequently questioned by Hawker et a/. [40”] who extended this analysis by contrasting polyether dendrimers with their exact monodisperse linear analogues on the basis of comparing like with like. Previous work by Hawker and Chu [41’] described the effect of manipulating the structure of monomers on the physical properties of hyperABX branched poly(ether ketones). It was demonstrated that interchange of the A and B groups in an ABE polymerisation can dramatically modify the degree of branching of the resultant materials. Frey and coworkers [42”] have redefined the extent of branching in AB, systems (x > l), deriving a general expression based on the numbers of dendritic and linear units present within the structure for AB, systems. In a subsequent series of papers a theoretical consideration of possible methods to enhance the extent of branching in AB, poly-
Dendritic and hyperbranched
mers was presented [43’]. The use of prefabricated dendrons as macromonomers for AB, systems was proposed; an approach already established experimentally [41’]. The second possibility considered to enhance the degree of branching was the design of a system where reactivity of the linear and terminal units is different. The extent of activation of the reactivity of the linear over terminal groups necessary to achieve a high degree of branching was also evaluated theoretically; this has also been described independently by Widmann and Davies [44”]. Related to this concept, addressing the synthesis of hyperbranched polymers with no linear sections, Voit and coworkers [45’] have proposed a synthetic strategy based on the use of ABz monomers where the polymer structure is unstable after reaction of one B group. The slow addition of monomer to a B, core unit has been widely proposed as a method of narrowing polydispersity and increasing the extent of branching [44”,46]. Widmann and Davies went on to consider the intrinsic viscosity/molecular weight relationship for hyperbranched polymers [44”]. Correlation of intrinsic viscosity with a connectivity index, enable Monte Carlo simulations to be bypassed in subsequent work, allowing experimental aspects to be taken into closer account.The predictions of Widmann and Davies [44”], outlined in Figure 5, are supported experimentally by the work of Hobson and Feast [47”], who after showing the branching factor to be a poor indication of topology (Figure 6) approached the question of dendritic character through direct comparison of the physical properties of a hyperbranched AB$B, copolymer, based on the PAMAM structure, with its monodisperse dendritic analogue. This work resulted in the first demonstration of dendritic solution viscosity behaviour for core terminated hyperbranched polymers. Another question arising from the synthesis of hyperbranched polymers is the possibility of intramolecular cyclisation reaction between the A group held at the focal point and the terminal groups. This is of concern not only because in the presence of cyclics molecular weights determined by end-group counting would be overestimates but also the availability of a single focal point could be critical to many proposed applications. To date NMR spectroscopy [48”] and MALDI-TOF (matrix-assisted laser desorption/ionisation-time-of-flight) mass spectrometry [23”,49”] have been reported in the literature as effective techniques to probe the possibility of cyclisation in hyperbranched systems. From results presented in the literature we can generalise that cyclic formation is system specific. For example, within the class of hyperbranched polyesters no significant cyclisation is reported for systems based on 4,4-(4-hydroxyphenol) pentanoic acid, while MALDI-TOF MS indicates that polymers derived from dimethyl-(5hydroxyalkoxy) isophthalates show a high proportion of cyclic structures.
polymers Hobson and Harrison
689
Bharathi and Moore [SO’] describe the synthesis of hyperbranched polymers where cyclic formation is controlled without the addition of a B, core terminating unit. Their approach, via the palladium catalysed polymerisation of di-iodophenylacetylene on a solid support, ensures the availability of a single focal point in the final product, with the additional advantages of control over molecular weight and narrower polydispersity values than commonly shown for standard ABz polymerisations.
Applications Dendrimers
At the time of writing, as far as can be ascertained, no specific applications for dendrimers have found their way to the market place. Other than the potential applications discussed above, one of the main focuses of recent research has been in the biomedical area. As large quantities of materials are not required and cost is less important, this is an attractive area for research, with many of the test materials having been derived from commercially available dendrimers. Several groups have investigated the ability of dendrimers as ‘carriers’ for small organic molecules [51] or oligonucleotides [SZ] to introduce these complexes into the body or cells. The recent synthesis of an enzymatically degradable dendrimer [53] may advance the area. They have also been proposed as magnetic resonance imaging contrast agents [54], although this research is still in its early stages. Hyperbranched
polymers
The immediate outlook for hyperbranched polymer systems is more favourable, which relates to their relative ease of synthesis and lower cost. This is exemplified by the recent advances which use commercially available starting materials. Investigation of the property modifying attributes of hyperbranched polymers blended with conventional systems has received considerable attention. Modified hyperbranched polyesters can act as tougheners for epoxy-based composites at a fairly low loading (ca 5%) [SS”]. PPI dendrimers have also been used as curing components, and published data implies that enhanced toughness may result [56]. Feast and Davies and coworkers [.57’] have demonstrated that arylester dendrimers blended with PET act as plasticizers or antiplasticizers depending on the dendrimer generation. Aliphatic hyperbranched polyesters with acrylate modified end-groups have successfully been used as a base for thermoset resins [58”]. The relative ease with which the base polymer can be modified to suit particular polymer properties of the materials was highlighted as an important advantage of these materials, and significant advances in this area seem likely.
Summary and outlook Due to extensive investigation over the last decade, the science and technology of dendritic polymers is developing
690
Polymers
into
a mature
area of research.
The
preparation
dendrimers is often based on well established synthetic strategies, or modification of commercially available materials. However, the last year has seen one major advance in this area, and it can be expected that the chemistry developed by Rannard and Davis [6’,7’] will soon join this list of standard synthetic routes to dendrimers, and in some cases supersede them. Investigation of potential applications of dendrimers is now at the forefront of research, although considerable development is still required. Continuing fundamental and applied research in hyperbranched polymers, in both academic and industrial research laboratories, has led to the development of new systems and the development of several applications. The introduction of these materials into the market place in specific, high added value products can be expected within a few years. Materials which combine dendriric and linear architectures are also likely to see continued development, and should lead to a range of interesting new materials.
References and recommended
Polymeric Materiak: Science and Engineering: Vegas, Nevada. ACS; 1997, 77:75-76.
of new
reading
10.
l
of special interest * of outstanding interest
1.
Buhleier E, Wehner W, Vogtle F: Cascade - and nonskid-chain-like synthesis of molecular cavity topologies. Synthesis 1978,55:155158.
2.
Tomalia DA, Baker H, Dewald J, Hall M, Kallos G, Martin S, Roeck J, Ryder J, Smith P: A new class of polymers: starburst dendritic macromolecules. Polymer J 1985, 17:117.
3.
Tomalia DA, Naylor AN, Goddard A: Starburst dendrimers: control of size, shape, surface chemistry, topology and flexibility in the conversion of atoms to macroscopic materials. Angew Chem fnt Ed 1990:138.
4.
Newkome GR, Yao Z-Q, Baker GR, Gupta VK: Cascade molecules. A new approach to micelles. J Org Chem 1985, 50:2003.
5.
Hawker CJ, Frechet JMJ: Preparation of polymers with controlled molecular architecture - a new convergent approach to dendritic macromolecules. J Am Chem Sot 1990, 112:7638-7647.
6. .
Rannard S, Davis N: Synthesis of dendrimers using highly selective chemical reactions. In Proceedings of the American Chemical Society, Division of Polymeric Materials; Science and Engineering: 1997 Sept 8-11; Las Vegas, Nevada. ACS; 1997, 77:63-64. This paper describes the basic chemistry of 1 ,l ‘carbonyldiimidazole and its derivatives. Examples of the type of highly functional dendrimers that may be prepared using this technique are also illustrated. 7. .
Rannard S, Davis N: Synthesis of statistical co-dendrimers and terdendrimers using mixed monomer feeds and high selectivity. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8-l 1; Las Vegas, Nevada. ACS; 1997, 77:160-l 61. Extension of the previous paper in which the1 ,l ‘carbonyldiimidazolel chemistry has been used to prepare statistical co-dendrimers, dendritic analogues of statistical copolymers. 8. .
Launay N, Caminade A-M, Majoral JP: Synthesis of bowl-shaped dendrimers from generation 1 to generation 8. J Organomet Chem 1997, 529:51-58. The paper describes the synthesis of extremely large dendrimers based on main group elements. Given the structure and relative ease of synthesis, the possibility of substituting organometallic groups at the periphery to prepare catalytically active dendrimers looks extremely attractive. 9.
van Koten G, Jastrzebski JTBH: Periphery-functionalized organometallic dendrimers for homogeneous catalysis. In Proceedings of the American Chemical Society, Division of
dendrimers.
Organometa//ics
Cuadrado I, Moran M, Casado CM, Alonso B, Lobete F, Garcia B, lbisate M, Losada J: Ferrocenyl-functionalized poly(propylenimine) dendrimers. Organometallics 1996, 15:5278-5280. An example of a large dendrimer substituted with metallocene groups at the periphery. Modification of the ferrocene groups could lead to catalytically active material, although this is not discussed. 11. .
12. .
Bosman AW, Schenning APHJ, Janssen RAJ, Meijer EW: Welldefined metallodendrimers by site-specific complexation. Chem Ber 1997, 1301725-728. The complexation of M(II) transition metal ions with the terminal groups of poly(propylene imine) dendrimers has been followed by Visible, NMR and ESR spectroscopy. Transmition electron microscopy of the metallodendrimers shows them to be spherical structures with a radius of 3 fl nm. 13. .
Serroni S, Juris A, Venturi M, Campagna S, Resin0 IR, Denti G, Credi A, Balzani V: Polynuclear metal complexes of nanometre size. A versatile synthetic strategy leading to luminescent and redox-active dendrimers made of osmium(ll)-based core and ruthenium(ll)-based units in the branches. J Mater Chem 1997, 711227-l 236. Description of a versatile synthetic approach, intermediate between convergent and divergent syntheses, which allows preparation of mixed metal complexes with up to 22 metal centres. 14.
Papers of particular interest, published within the annual period of review, have been highlighted as:
l
Liao Y-H, Moss JR: Organoruthenium 1996, 15:4307-4316.
1997 Sept 8- 11; Las
Tzalis D, Tor Y: Towards self-assembling dendrimers: metal complexation induces the assembly of hyperbranched structure. Tett Lett 1996, 37:8293- 8296.
15. Zimmerman SC, Zeng F, Reichert DEC, Kolotuchin SV: Self. assembling dendrimers. Science 1996, 271 :1095-l 098. The first example of large dendritic wedges self-assembled by hydrogenbonding interactions. Evidence for discrete hexameric structures is primarily obtained from molecular weight determination of the complexes by standard polymer techniques. Convincing data is provided for several of the smaller complexes but we would argue that the data presented for the highest generation wedge does not necessarily imply aggregation into a discrete hexagonal array, and other interpretations are possible. 16. .
Thiyagarajan P, Zeng F, Ku CY, Zimmerman SC: SANS investigation of self-assembling dendrimers in organic solvents. J Mater Chem 1997, 7:1221-1226. Evidence for the hydrogen-bonded self-assembly of dendritic wedges into discrete hexameric aggregates using small angle neutron scattering. No experimental data was presented for the largest of the structures claimed in [15’1.
17. .
Freeman AW, Vreekamp RH, Frechet JMJ: The self-assembly of convergent dendrimers based on the melamine-cyanuric acid lattice. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8- 11; Las Vegas, Nevada. ACS; 1997,77:138-l 39. Initial experimental data indicates that the materials self-assemble, although it appears that the expected hexamers are in equilibrium with smaller aggregates.
18. Malmstrijm E, Hult A: Hyperbranched polymers a review. JMS.. Rev Macromol Chem Phys C 1997, 37:555-579. An excellent review covering the synthesis, characterisation and properties of hyperbranched polymers up to the end of 1996. 19.
Bolton DH, Wooley KL: Synthesis and characterisation of hyperbranched polycarbonates; Macromolecules 1997, 30:18901896.
20. .
Davis N, Rannard S: Synthesis of hyperbranched polymers using highly selective chemical reactions. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 7997 Sept 8-11; Las Vegas, Nevada. ACS; 1997, 77:158-l 59. The synthesis of both aromatic, aliphatic and mixed hyperbranched polyesters which can be achieved without resorting to protection deprotection chemistry is described. 21.
Mueller A, Kowalewski T, Wooley KL: Hyperbranched polyfluorinated polymers: surface properties and micromechanical behaviour. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8-l 7; Las Vegas, Nevada. ACS; 1997, 77:89-90.
Dendrftic
Stefanescu AD, Wooley KL: Hyperbranched polyfluorinated polymers from AB, monomers. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering. 1997, 771216219.
23.
Hobson LJ, Kenwright AM, Feast WJ: A simple ‘one pot’ route to the hyperbranched analagues of Tomalia’s poly(amidoamine) dendrimers. J Chem Sot Chem Commun 1997:1877-l 878.
The hyperbranched polymer analogues of polyfamidoamine) dendrimers described in this paper have been shown to have near perfect degrees of branching. 24.
Lu P, Paulasaari JK, Weber WP: Hyperbranchad poly(4-acetylstyrene) by ruthenium-catalysad step-growth pdymerisation acetylstyrane. Macromolecules 1996, 29:8583-8586.
of 4-
25. n
Gaynor SG, Edelman, Matyjasxewski K: Synthesis of branched and hyperbranchad polystyranes. Macromolecules 1996,29:10791081. The simple syntheses of hyperbranched derivatives of polystyrene are described. 26.
Tomalia DA, Esfand R: Dendrons, dendrfmers and dendrigrafts. Chem Ind 1997:416-420. b brief overview of macromolecular architectures, highlighting possible areas of application. 27.
polymers Hobson and Harrison
691
37. .
22.
..
and hyperbranched
Yin R, Qin D, Tomalia DA, Kukowska-Latallo, Baker JR: From dendrfmers to dendrfgratts: Synthesis, characterisation and applications. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8-l 1; Las Vegas, Nevada. ACS; 1997,77:206-207.
26.
van Aert HAM, van Genderen MHP, Meijer EW: Star-shaped poly(2,6- dimethyl- ,4-diphenylene ether). Polym Bull 1997, 37~273-280. A paper of topical interest highlighting the problems encountered for molecular weight determination of nonlinear polymers. 36.
Wooley KL, Klug CA, Tasaki K, Schaefer J: Shapes of dendrimers from rotational-echo double-resonance NMR. J Am Chem Sot 1997, 119:53-58.
39.
Hawker CJ, Frechet JMJ: Comparison of linear, hyperbranched and dendritic macromolecules ACS Symp 1996,624:132-l 44.
40. ”
Hawker CJ, Malmstrom E, Frank CW, Kampf JP, Mio C, Prausnitz J: Dendrftic macromolecules: hype or unique materials. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8- 11; Las Vegas, Nevada. ACS; 1997, 77:61-62. The linear and dendritic materials compared are analogous in terms of molecular formula, degree of polymerisation, number of terminal units and having a single focal point functionality. 41.
.
Hawker CJ, Chu F: Hyperbranched polytether ketones): Manipulation of structure and physical properties. Macromolecules
1996,29:4370-4300. Water soluble derivatives shown to act as unimolecular micelles. While through the incorporation of linear or branched units into the monomer structure, modification of the degree of branching between 15% and 71% was illustrated. Holter D, Frey H: Degree of branching in hyperbranched polymers. Acta Polymerica 1997.40130-35. Calculation of DPn by this method relies upon the accuracy by which linear, terminal and branched contributions can be calculated spectroscopically and assumes the absence of cyclic products. 42.
”
Grubbs RB, Hawker CJ, Dao J, Frechet JMJ: A tandem approach to graft and dendritic graft copolymers based on living free radical polymerisatfons. Angew Chem lnt Ed 1997,36:270-272. Synthesis of dendritic graft copolymers by combination of radical polymerisation techniques with independent activation processes. The authors acknowledge the inherent problems of using size exclusion chromtaography to determine molecular weight and polydispersity of dendritic graft structures.
Frey H, Holter D: Degrae of branching in hyparbranchad polymers. 2. Enhancement of the DB: Scope and limitations. Acta Polymerica 1997,48:295-309. The relationship between x and extent of branching for AB, polymers (where x < 1) was also proposed in this paper.
29. .
44. ”
Leduc MR, Hawker CJ, Dao J, Frechet JMJ: Dendritic Initiators for living free radical polymerisations: a versatile approach to the synthesis of dendrftic-linear block copolymers. J Am Chem Sot 1996,118:11111-11116. First reported use of dendrons as macroinitiators in controlled free radical polymerisations leading to hybrid dendritic-linear block copolymers.
30.
van Hest JMC, Delnoye DAP, Baars MWPL, Elissen-Roman, van Genderen MHP, Meijer EW: Polystyrene-poly(propylene imine)
dendrimers: synthesis, characterisatlon and association behaviour of a new class of amphiphiles. Chem Eur J 1996, 12:1616-l
626.
31.
Brenner AR, Voit BI: Free radical grafting from hyparbranched polyesters based on polymerfc azo initiators. Macromol Chem Phys 1996,197:2673-2689.
32.
Zhou Y, Bruening ML, Bergbreiter DE, Crooks RM, Wells M: Preparation of hyperbranchad polymer films grafted on self assembled monolayers. J Am Chem Sot 1996,110:3773-3774.
33.
Zhou Y, Bruening ML, Lui Y, Crooks RM, Bergbreiter DE: Synthesis of hyperbranched, hydrophilic fluorinated surface grafts. Langmuir 1996,12:5519-5521.
34. .
Bruening ML, Zhou Y, Zhoa M, Crooks RM, Bergbreiter DE: Surfacagrafted, hyperbranched poly(acrylic acid) films: new interfaces relevant to sensing and anti corrosion applications. In Proceedings of the American Chemical Societv. Division of Polvmeric Materials: Science and Engineermg: 1997 Sept 8- 11; Las Vegas, Nevada. ACS; 1997, 77:77-70. . This paper describes the electrochemrcal propertres and potentral applications of modified surface grafted poly(acrylic acid) films. 35. ”
Gitsov I, Frechet JMJ: Stimuli-responsive hybrid macromolecules: novel amphiphilic star copolymers with dendrltic groups at the periphery. J Am Chem Sot 1996,118:3765-3766. The significance of this paper is magnified by both the accessibility of the components and the simplicity of the synthetic methodologies used. 36. ”
Stevelmans S, van Hest JMC, Jansen JFGA, van Boxtel DAFJ, de Brabander- van den berg EMM, Meijer EW: Synthesis, characterisation and guest host properties of inverted unimolecular dendritic micelles. J Am Chem Sot 1996, 118:73987399. Surface modification of poly(propylene imine) dendrimers is shown to be extremely selective.
43. .
Widmann AH, Davies GR: Simulation of the intrinsic viscosity of hyperbranchad polymers with varying topology: 1. Step-wise built polymers. Comput Theor Polym Sci 1997, in press. Includes a theoretical evaluation of the intrinsic viscosity-molecular weight relationship for materials with varying degrees of branching. The use of a connectivity index to give a better indication of topology is also discussed. 45.
.
Herzner C, Maier G, Voit B: A synthetic strategy towards hyperbranched polymers wlthout linear units. In Proceedings of
the American Chemical Societv, Division of Polvmeric Materials: Science and Engineering: 7997 Sept 8-l I; Las Vegas, Nevada. ACS: 1997. 77:113. This work presents an interesting concept and the results of polymerisation studies are awaited with interest. 46.
Hasselmann R, Holter D, Frey H: Hyparbranchad polymers preparad via the core-dilution/slow addition technique: computer simulation of molecular weight distribution and degree of branching. Macromolecules 1997, in press.
47. ..
Hobson LJ, Feast WJ: Dendrltic solution viscosity behaviour in cora terminated hyperbranched poly(amidoamlnes). J Chem Sot Chem Commun 1997:2067-2066. A polyfamidoamine) hyperbranched AB?IBe copolymer is shown to display a viscosity/molecular weight profile simrlar to that shown by perfectly regular dendrimers. 48. Chu F, Hawker CJ, Pomery PJ, Hill DJT: Intramolecular cyclisation in ” hyperbranchad polyesters. J PO/~ Sci A 1997,35:1627-l 633. Analysis of hyperbranched polyesters, based on 4,4-(4-hydroxyphenyl) pentanoic acid, by tsC NMR spectroscopy shows intramolecular cyclisation to be insignificant, that is, less than 5%. This conclusion is supported by comparison of Mn values determined spectroscopically by end-group counting and that determined by osmometry. 49. u
Feast WJ, Keeney AJ, Kenwright AM, Parker D: Synthesis structure and cyclics content of hyparbranched polyesters. J Chem Sot Chem Commun 1997,1&l 749-l 750. Matrix-assisted laser desorptionlionisation-time-of-flight mass spectrometry has been demonstrated to be an invaluable diagnostic technique for the evaluation of intramolecular cyclisation products in polycondensation reactions.
50. .
Bharathi P, Moore JS: Solid-supported hyperbranchad polymerisatfon: Evidence for self-limited growth. J Am Chem Sot 1997,119:3391-3392. Control over molecular weight, through the molar ratio of monomer to focal
692
Polymers
point, has been demonstrated 51.
52.
in the range of 5 to 25 kDa.
Watkins DM, Sayed-Sweet Y, Kilmash JW, Turro, NJ, Tomalia DA: Dendrimers with hydrophobic cores and the formation of supramolecular dendrimer-surfactant assembles. Langmuir 1997, 13:3136-3141. Delong R, Stephenson K, Loftus T, Fisher M, Alahari S, Nolting A, Juliano RL: Characterisation of complexes of oligonucleotides with polyamidoamine starburst dendrimers and effects on intracellular delivery. J Pharm Sci 1997, 86:762-764.
53.
Seebach D, Herrmann GF, Lengweiler UD, Abchmann BM, Amrein W: Synthesis and enzymatic degradation of dendrimers from (R)-3-hydroxybutanoic acid and trimesic acid. Angew Chem Int Ed 1996,35:2795-2797.
54.
Tbth E, Pubanz D, Vauthey S, Helm L, Merbach AE: The role of water exchange in attaining maximum relaxivities for dendrimer MRI contrast agents. Chem Eur J 1996, 2:1607-l 615.
Boogh L, Petterson, Kaiser P, Manson J-A: Novel tougheners epoxy-based composites. SAMPE J 1997,1:45-49. The inclusion of low loadings of modlfled hyperbranched polyesters commercial expoxy resin resulted in doubling the toughness of the 55. ..
for to a
composites with no detrimental characteristics. 56.
effect to thermal properties
or processing
Zukas WX, Wilson PM, Gassner JJ: Curing behaviour of a series of amine terminated dendrimers with an epoxy resin. In Proceedmgs of the American Chemical Societv. Division of Polvmeric Materials: Science and Engioeering: 1997 sept 8- 7 1; Las Qegas, Nevada. ACS; 1997, 77:232-233.
57. .
Carr PL, Davies, GR, Feast WJ, Stainton NM, Ward IM: Dielectric and mechanical characterisation of aryl ester dendrimer/PET blends. Polymer 37:2395-2401. A detailed study of the modification of commercial poly(ethylene terephthalate) with aryl ester dendrimers which are fully miscible with the poly(ethylene terephthalate). 58. ..
Johansson M, Rospo G, Hult A: Hyperbranched aliphatic polyesters as a base for thermoset resins. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8-l 1; Las Vegas, Nevada. ACS; 1997, 77:124-l 25. The preparation of thermoset resins based on modified polyesters. The nature of the hyperbranched polymer, which can be controlled, effects the thermal and mechanical properties of the materials.
Dendritic and hyperbranched polymers: advances in synthesis and applications Lois J Hobson* and Robin M Harrison+
Investigations of the synthesis, characterisation and structure-property correlations of dendritic polymers have made significant progress recently. These developments have established better understanding and brought a range of potential applications into consideration.
Figure 1
Divergent synthesis
Addresses IRC in Polymer Science and Technology, Department of Chemistry, Durham University,South Road, Durham DHl 3LE, UK *e-mail: LJ.HobsonQdurham.ac.uk *e-mail: [email protected] Current Opinion in Solid State 8 Materials Science 1997, 2:683-692
Convergent synthesis
Electronic identifier: 1359-0286-002-00683 8 Current Chemistry Ltd ISSN 1359-0286 Cured
Abbreviations CDI 1 ,l ‘-carbonyldiimidazole DB degree of branching DLS dynamic light scattering PAMAM poly(amidoamine) PET poly(ethylene terephthalate) PPE poly(2,6-dimethyl- ,4-diphenylene ether)s PPI poly(propylene imine) REDOR rotational-echo double-resonance SEC size exclusion chromatography
Introduction The concept of cascade molecules was introduced by VGgtle in 1978 [1] and developed during the mid 198Os, by Tomalia [2,3], Newkome [4], Frechet [S] and others. Today dendritic polymers are established as a distinct class of macromolecules. Dendrimers, the best defined structures within this family, can be constructed via two different routes, see Figure 1. The divergent approach was favoured for potential commercial scale development by both DSM (The Netherlands) and Dendritech (USA) in the synthesis of Astramolo poly(propylene imine) (PPI) and Starburst poly(amidoamine) (PAMAM) dendrimers respectively. The convergent approach has been widely adopted for laboratory scale syntheses. In contrast to the iterative sequence of reaction and purification steps associated with dendrimer syntheses, the single step polymerisation of AB, (x > 1) monomers leads to hyperbranched structures. These materials, which are accessible at lower cost than their monodisperse dendrimer analogues, retain some of the structural features and properties of dendrimers and are consequently commercially attractive. As both syntheses and characterisation methods for these types of materials have developed so have attempts to
Opinion in s0ri State 6. Materials Science
Synthetic approaches to dendrimers.
explore structure-property relationships and to establish applications. This review is intended to highlight recent contributions to the field; in particular new syntheses, new concepts and novel architectures. The theoretical questions and implications for potential applications arising from these developments will be discussed.
Synthesis of dendritic materials Dendrimers
One major limitation of dendrimer syntheses is that often they can be prepared only on a relatively small scale due to the inherent difficulty of the chemistry involved. Recent work by Rannard and Davis [6’], using the highly selective reactions of l,l’-carbonyl diimidazole (CDI) with amines, alcohols and carboxylic acids (Figure Za) allows the preparation of a variety of highly functionalised dendritic structures on a relatively large scale (currently hundreds of grams). It has been demonstrated that the intermediate imidazole derivatives show high selectivity in subsequent reactions with amines and alcohols (Figure Zb). Consequently, it is possible to design syntheses in which protection-deprotection chemistry is not required, for example see Figure Zc, where the synthesis of large wedges and dendrimers is achieved via significantly fewer steps than earlier procedures. Esters, amines, ureas, carbonates and urethane linkages can all be selectively introduced at the core, branches and termini of the dendrimer, which should allow tine tuning of polymer properties via structure regulation. Additionally, Rannard and Davis [7’] have described the preparation of statistical co-dendrimers; the analogues of linear statistical copolymers (see Figure Zd). The product
664
Figure
Polymers
2
(a)
(b)
9-
NQl-ea
on
CDI
R-OH,+NH2
R-DH,+R-N”2
CDi
R-2H&,R-NH2
(c) RCH~OH
RrO-pj-Na
Rr-i-R,
<
R++OH. R$OH
3 Ideal Wedges
No Reacbon
Current Opmm
in Sohd State B Materials Scmnce
The highly selective CDI (1 ,l’-carbonyldiimidazole) chemistry developed by Rannard and Davis [6’,7’] to prepare dendritic structures. (a) Versatility of the CDI chemistry. (b) Example of a dendritic wedge prepared using the CDI chemistry. k) Examples of the selectivity of the CDI chemistry. (d) Schematic
representation
of the preparation
of statistical co-dendrimers
consists of a set of dendrimers of the same generation, each with perfect, ideal branching but with a distribution in both structure and molecular weight. These copolymers can be prepared on a large scale in a one-pot reaction.
interest as catalysts as they combine high solubility with easy recovery by dialysis or ultrafiltration techniques, thus combining the advantages of conventional homogeneous catalysts and polymer supported reagents.
Majoral and colleagues [8’] have synthesised one of the largest dendritic materials ever prepared using a two-step procedure from a hexachlorocyclotriphosphazine core. Generations are built by the substitution of chlorine atoms with 4-hydroxybenzaldehyde to give aldehyde terminated materials (Figure 3a), followed by condensation of the peripheral aldehyde functionalities with H,N-N(Me)P(S)C12, regenerating the outer P-Cl groups (Figure 3b). Generation 8 dendrimers have been prepared with up to 768 dichlorophosphorus groups or 1536 aldehyde groups at the periphery.
Van Koten and Jastrzebsli [9] have described silicon-based dendrimers which are functionalised with up to 12 catalytically active nickel or palladium complexes at the periphery, and showed that in cross-coupling reactions, their reactivity is comparable to that of conventional systems. Organoruthenium dendrimers with up to 48 metal centres at the periphery have been prepared by Liao and Moss [lo] using a convergent synthesis based on a ruthenium functionalised wedge (Figure 3c) and the FrCchet methodology.
Several preparations of dendrimers with organometallic groups at the periphery have been reported. These are of
Cuadrado et al. [ 11’1 have functionalised PPI dendrimers with up to 64 ferrocene groups. Reaction of l-(chlorocarbonyl)ferrocene with the dendrimers terminal amine groups introduced the organometallic functionality, and
Dendritic and hyperbranched
polymers Hobson and Harrison
665
Figure 3
(a)
(d)
(b)
fi
0
-mmmplsr
(h)
(e)
/\ , &
OH
cl+ /
o+I
n = 2,3.4.5,6 or 7
Current Opinion in S-Aid State d Materials sdence
Examples of the dendritic structures, building blocks and monomers discussed in the text
complete conversion was supported by NMR and IR spectroscopy. Meijer and coworkers [12’] have used the same PPI dendrimers for the site-specific complexation of copper(H), zinc(I1) and nickel(H) chlorides, and have functionalised the dendrimer periphery with up to 32 metal groups. Given the availability of commercial quantities of the PPI dendrimers, the easy synthesis and varied catalytic ability of metallocene and transition metal complexes, this type of work looks attractive for further research and development. Organometallic dendrimers which contain the metal atom at the branch points rather than at the termini have also received attention. Balzani and coworkers [13’] have developed a building block approach in which each metal centre has a specific function in the material (Figure 3d). To illustrate this concept, luminous and redox active mixed metal third generation dendrimers have been prepared with well defined luminescence and redox properties which can be adjusted by structural modifica-
tions. Further development might see the preparation synthetic light-harvesting arrays using this approach.
of
The assembly of dendritic wedges into larger structures has been investigated by several groups. Tzalis and Tor [ 141 have prepared first generation dendritic wedges with phenanthroline ligands at the focus, and has demonstrated their self-assembly around metal centres. The number and spatial orientations of the dendritic groups in the assembly can be varied by choice of the appropriate metal, making this a versatile technique for controlling architecture. Replacing the focal ligand with groups that form strong hydrogen-bonds is an alternative approach to self-assembled dendrimers. Zimmerman’s [15’,16’] work describes poly(ary1 ether) dendrimers functionalised at the focus with isophthalic acid groups in an orientation that allows the formation of discrete hexameric self-assembled structures (Figure 4a). Experimental evidence suggests that the hexameric structures are formed, with the stability of the
666
Polymers
Figure 4
(a)
(b)
=
afyl ether dendrimer
wedge = atyl ether dendrimer
wedge
aryl ether dendrimer
wedge
PoH
=
Current Opinion in Solid State & Materials Scmnce
Schematic representation for the self-assembly of dendritic wedges using hydrogen-bonding. (b) Approach used by Frkhet et al. [17-J.
aggregates dependent on the solvent and size of the dendritic wedge. Preliminary results from FrCchet’s group [17’] describe a related system applying the self-asseinbly of melamine and cyanuric acid core-functionalised poly(ary1 ether) dendrimer wedges (Figure 4b). This is a more versatile approach as wedges functionalised with different surface groups can be controllably introduced in an alternating fashion around the assembled hexameric structure.
Hyperbranched
polymers
The synthesis and characterisation of hyperbranched polymers has recently been reviewed by Malmstrijm and Hult [18”]. Here, we highlight what we believe to be the most relevant publications during the review period. Polycarbonates are well established as engineering thermoplastics for a variety of applications. It may be expected that the hyperbranched analogues of these materials will prove useful as property modifiers in commercially available systems, or as reactive prepolymers for
(a) Approach used by Zimmerman et a/. [15’,16’]
novel engineering materials. Bolton and Wooley [19] described the first example of an aromatic hyperbranched polycarbonate, prepared from a monomer based on l,l, 1-tris(4’-hydroxyphenyl) ethane (Figure 3e). Two of the ‘hydroxy groups are activated by reaction with CD1 and the third is protected by silylation. Treatment with fluoride ions and potassium hydroxide cleaves the protecting group and initiates polymerisation. Davis and Rannard [ZO’] have used a similar approach to polymerise derivatives of 3,Sdihydroxybenzoic acid (Figure 3f) and 2,2-bis-hydroxymethyl propionic acid (Figure 3g). As no protecting groups are required, polymerisation may be initiated by heating the monomers in the presence of potassium hydroxide to give aromatic and aliphatic hyperbranched polyesters respectively. Using a mixed monomer feed, aliphatic/aromatic hyperbranched copolymers can also be prepared. Wooley and coworkers branched polyfluorinated
[21,22] have prepared hyperpolymers from fluorinated
Dendritic and hyperbranched
Finwe
!i
polymers Hobson and Harrison
667
Figure 6
Hyperbranched polymer perfect branching, irregular growth Perfect dendron
Intrinsic viscosities of polymers with various degrees of branching (DB) as a function of molecular weights. Each point represents an isolated hydrocarbon polymer (CH branch points, C,H, spacers between branch points, and CHs terminal groups) and was obtained by a Monte Carlo simulation. 0, dendrimers of generations l-7; 0, linear chains with the same number of monomer units as respective dendrimers (above M = 10000 they show Gaussian chain behaviour: slope l/2); A, hyperbranched polymers with DB < 0.2; 0, hyperbranched polymers with 0.2 5 DB < 0.5; V, hyperbranched polymers with DB I 0.5. Adapted with permission from [44”1.
benzyl ether AB, and AB, monomers (Figure 3h, 3i). Polymerisation is said to occur with the displacement of the para-fluorine by the nucleophillic benzylic hydroxy group under basic conditions. The polymers show marked differences in thermal and physical properties, reflecting the different repeat motifs, branching patterns and fluorine contents. Polymer films are extremely hydrophobic, as demonstrated by the measurement of contact angles with water. Feast and coworkers [23”] have recently described the synthesis of PAMAM hyperbranched polymers, prepared by the melt polymerisation of N-acryloyl-a,*diaminoalkane hydrochlorides (Figure 3j). The polymers prepared where n = 2 using a B, core terminator, which are the hyperbranched polymer analogues of PAMAM dendrimers, have been shown by rsN NMR spectroscopy to have very high degrees of branching and exhibit solution viscosity behaviour analogous to true dendrimers (wide inrr~). Polymerisation of AB, monomers via novel mechanisms have been reported. Weber’s [24] group has synthesised and polymerised 4-acetylstyrene (Figure 3k) with a ruthenium catalyst. The.polymerisation step relies on the activation of the hydrogens o&o to the acetyl group towards alkylation by vinyl groups. The polymer has a low DB (degree of branching), as measured by 13C NMR spectroscopy, with 4-7% branched units. Given the simplicity of the monomer and the potential to increase the activity of the catalyst to
perfect branching Current Opinion in Solid State 6 Materials Science
Highly branched macromolecules.
obtain higher DBs and molecular weights, this concept could see development into a viable system for the preparation of functionalised hyperbranched polymers. Matyjaszewski and colleagues [ZS”] have polymerised 4-(chloromethyl)styrene (Figure 31) with a copper (I) bipyridyl catalyst by atom transfer radical polymerisation to give hyperbranched derivatives of polystyrene. Co-polymerisation with styrene gives polystyrene derivatives in which the degree of branching may be controlled by altering the feed ratios. Given the availability and low cost of the monomers and catalyst and the potential application of the materials as property modifying agents for commercial polystyrene, the further development of these ideas may be fruitful.
Variation
in dendritic architecture
A recent review by Tomalia and Esfand [26’] highlights the progressive development of different macromolecular structures over the last twenty years. Dendrigrafts, composite structures containing dendritic elements, are identified as a sub-class of dendritic polymers. Compared to monodisperse dendrimers, dendrigrafts have well defined structures and higher molecular weight and exhibit solution viscosity behaviour reminiscent of pure dendrimers. Results presented by Yin and coworkers (271, suggest that these composite structures are more amenable to certain applications than their dendrimer equivalents, for example as gene transfection agents. This theme of attaching dendritic grafts to a linear backbone has been approached using well established free radical polymerisation chemistry. In an elegant synthesis
688
Polymers
Grubbs
eta/. [28”]
combined
living
free radical
and living
atom transfer radical polymerisation techniques to yield novel graft and dendrigraft structures from a range of commercially available monomers. In all cases, control over molecular weight is demonstrated whilst retaining low polydispersity values. The use of dendrons as initiators for living radical polymerisations, leading to the synthesis of dendritic-linear block copolymers, by the use of polyether dendrons
has been exemplified as macromolecular ini-
tiators for the controlled free radical polymerisation of vinyl monomers [29’]. The approach is shown to be versatile, with variation of both monomer and initiator being possible. Meijer’s group addresses the synthesis of polystyrene/PPI diblock copolymers via a complementary approach. PPI dendrimers, up to generation five, have been synthesised divergently from amine terminated polystyrene [30]. Conductivity measurements area isotherms indicate generation dependent behaviour for these systems.
and pressure amphiphilic
The synthesis of structures combining AB, hyperbranched systems with linear polymers has also been considered. Brenner and Voit [31] report the modification of both acid and alcohol terminated hyperbranched polyesters to introduce azo functionalities at the surface, capable of initiating free radical polymerisation. Using this approach grafts of controlled size and number were introduced from the hyperbranched core. The resultant materials have been shown to exhibit low solution viscosity, an attractive property for coating applications. Hyperbranched polymer films grafted on self-assembled monolayers have also been prepared. In a series of papers Crooks and coworkers [32] describe the synthesis and subsequent modification of surface grafted hyperbranched poly(acrylic acid) films grafted on self-assembled organomercaptan monolayers. Surface carboxylic acid groups, shown to serve as specific metal ion binding sites, can be modified with metallocene derivatives to give electroactive films [33]; while the synthesis of a range of composite and fluorinated derivatives has produced possible materials for passive coatings applications [34’]. Combination of preformed, well defined dendrons and linear polymers via the Williamson ether synthesis to form amphiphilic star copolymers was described by Gitsov and FrCchet [35”]. This work represented the first example of a polymeric system responsive to changes in the surrounding medium by forming mono-molecular micelles with different core shell structures. Modification of the commercially available PPI dendrimers by the introduction of hydrophobic alkyl chains from the corresponding acid chlorides has also been described [36”]. Remarkable selectivity was demonstrated with the use of excess dendrimer; with two products resulting, the modified and unmodified dendrimers, rather than the statistical structure expected. Their capacity to act as dynamic hosts
for guest micellar
molecules structure
Fundamental
provides
of these
evidence
modified
for
an
inverted
dendrimers.
questions
Investigations of dendritic macromolecules have uncovered a number of fundamental questions relating to the structure, reactivity and physical properties of materials within the class. Eventual exploitation of these novel macromolecules may be dependent upon answering these questions. The ongoing problem of molecular weight determination for materials with nonlinear topologies was highlighted by Meijer and coworkers in relation to star-shaped poly(2,6dimethyl-1,4-diphenylene ether)s (PPE). The PPE modified PPI dendrimers were studied in solution and as 50/50wt.% blends with linear poly(ethylene terephthalate), PET [37’]. Significantly it was observed that for stars with grafts of constant arm length but increasing numbers of arms, hydrodynamic volume, measured by SEC (size exclusion chromatography), remained constant while both hydrodynamic radius, determined by DLS (dynamic light scattering), and intrinsic viscosity were shown to decrease at high numbers of arms. Evaluation of solid state shape, size and intramolecular packing of a fifth generation dendrimer by combined REDOR (rotational-echo double-resonance) NMR spectroscopy, distance constrained molecular dynamic simulations and site-specific stable-isotope labelling studies was described by Wooley et al. [38]. This work addresses the concepts of dendritic shape, conformation and packing, critical to many proposed applications. It is clear that much work remains to be done in this area. A recent paper contrasting the physical properties of hyperbranched and dendritic polyesters with both simple linear polymers and linear analogues of dendritic polymers (Pdi > 2 ) [39], concludes that thermal properties are independent of architecture but extremely sensitive to the number and nature of the end-groups. In contrast solubility, viscosity and reactivity were found to be architecture dependent. The validity of this analysis was subsequently questioned by Hawker et a/. [40”] who extended this analysis by contrasting polyether dendrimers with their exact monodisperse linear analogues on the basis of comparing like with like. Previous work by Hawker and Chu [41’] described the effect of manipulating the structure of monomers on the physical properties of hyperABX branched poly(ether ketones). It was demonstrated that interchange of the A and B groups in an ABE polymerisation can dramatically modify the degree of branching of the resultant materials. Frey and coworkers [42”] have redefined the extent of branching in AB, systems (x > l), deriving a general expression based on the numbers of dendritic and linear units present within the structure for AB, systems. In a subsequent series of papers a theoretical consideration of possible methods to enhance the extent of branching in AB, poly-
Dendritic and hyperbranched
mers was presented [43’]. The use of prefabricated dendrons as macromonomers for AB, systems was proposed; an approach already established experimentally [41’]. The second possibility considered to enhance the degree of branching was the design of a system where reactivity of the linear and terminal units is different. The extent of activation of the reactivity of the linear over terminal groups necessary to achieve a high degree of branching was also evaluated theoretically; this has also been described independently by Widmann and Davies [44”]. Related to this concept, addressing the synthesis of hyperbranched polymers with no linear sections, Voit and coworkers [45’] have proposed a synthetic strategy based on the use of ABz monomers where the polymer structure is unstable after reaction of one B group. The slow addition of monomer to a B, core unit has been widely proposed as a method of narrowing polydispersity and increasing the extent of branching [44”,46]. Widmann and Davies went on to consider the intrinsic viscosity/molecular weight relationship for hyperbranched polymers [44”]. Correlation of intrinsic viscosity with a connectivity index, enable Monte Carlo simulations to be bypassed in subsequent work, allowing experimental aspects to be taken into closer account.The predictions of Widmann and Davies [44”], outlined in Figure 5, are supported experimentally by the work of Hobson and Feast [47”], who after showing the branching factor to be a poor indication of topology (Figure 6) approached the question of dendritic character through direct comparison of the physical properties of a hyperbranched AB$B, copolymer, based on the PAMAM structure, with its monodisperse dendritic analogue. This work resulted in the first demonstration of dendritic solution viscosity behaviour for core terminated hyperbranched polymers. Another question arising from the synthesis of hyperbranched polymers is the possibility of intramolecular cyclisation reaction between the A group held at the focal point and the terminal groups. This is of concern not only because in the presence of cyclics molecular weights determined by end-group counting would be overestimates but also the availability of a single focal point could be critical to many proposed applications. To date NMR spectroscopy [48”] and MALDI-TOF (matrix-assisted laser desorption/ionisation-time-of-flight) mass spectrometry [23”,49”] have been reported in the literature as effective techniques to probe the possibility of cyclisation in hyperbranched systems. From results presented in the literature we can generalise that cyclic formation is system specific. For example, within the class of hyperbranched polyesters no significant cyclisation is reported for systems based on 4,4-(4-hydroxyphenol) pentanoic acid, while MALDI-TOF MS indicates that polymers derived from dimethyl-(5hydroxyalkoxy) isophthalates show a high proportion of cyclic structures.
polymers Hobson and Harrison
689
Bharathi and Moore [SO’] describe the synthesis of hyperbranched polymers where cyclic formation is controlled without the addition of a B, core terminating unit. Their approach, via the palladium catalysed polymerisation of di-iodophenylacetylene on a solid support, ensures the availability of a single focal point in the final product, with the additional advantages of control over molecular weight and narrower polydispersity values than commonly shown for standard ABz polymerisations.
Applications Dendrimers
At the time of writing, as far as can be ascertained, no specific applications for dendrimers have found their way to the market place. Other than the potential applications discussed above, one of the main focuses of recent research has been in the biomedical area. As large quantities of materials are not required and cost is less important, this is an attractive area for research, with many of the test materials having been derived from commercially available dendrimers. Several groups have investigated the ability of dendrimers as ‘carriers’ for small organic molecules [51] or oligonucleotides [SZ] to introduce these complexes into the body or cells. The recent synthesis of an enzymatically degradable dendrimer [53] may advance the area. They have also been proposed as magnetic resonance imaging contrast agents [54], although this research is still in its early stages. Hyperbranched
polymers
The immediate outlook for hyperbranched polymer systems is more favourable, which relates to their relative ease of synthesis and lower cost. This is exemplified by the recent advances which use commercially available starting materials. Investigation of the property modifying attributes of hyperbranched polymers blended with conventional systems has received considerable attention. Modified hyperbranched polyesters can act as tougheners for epoxy-based composites at a fairly low loading (ca 5%) [SS”]. PPI dendrimers have also been used as curing components, and published data implies that enhanced toughness may result [56]. Feast and Davies and coworkers [.57’] have demonstrated that arylester dendrimers blended with PET act as plasticizers or antiplasticizers depending on the dendrimer generation. Aliphatic hyperbranched polyesters with acrylate modified end-groups have successfully been used as a base for thermoset resins [58”]. The relative ease with which the base polymer can be modified to suit particular polymer properties of the materials was highlighted as an important advantage of these materials, and significant advances in this area seem likely.
Summary and outlook Due to extensive investigation over the last decade, the science and technology of dendritic polymers is developing
690
Polymers
into
a mature
area of research.
The
preparation
dendrimers is often based on well established synthetic strategies, or modification of commercially available materials. However, the last year has seen one major advance in this area, and it can be expected that the chemistry developed by Rannard and Davis [6’,7’] will soon join this list of standard synthetic routes to dendrimers, and in some cases supersede them. Investigation of potential applications of dendrimers is now at the forefront of research, although considerable development is still required. Continuing fundamental and applied research in hyperbranched polymers, in both academic and industrial research laboratories, has led to the development of new systems and the development of several applications. The introduction of these materials into the market place in specific, high added value products can be expected within a few years. Materials which combine dendriric and linear architectures are also likely to see continued development, and should lead to a range of interesting new materials.
References and recommended
Polymeric Materiak: Science and Engineering: Vegas, Nevada. ACS; 1997, 77:75-76.
of new
reading
10.
l
of special interest * of outstanding interest
1.
Buhleier E, Wehner W, Vogtle F: Cascade - and nonskid-chain-like synthesis of molecular cavity topologies. Synthesis 1978,55:155158.
2.
Tomalia DA, Baker H, Dewald J, Hall M, Kallos G, Martin S, Roeck J, Ryder J, Smith P: A new class of polymers: starburst dendritic macromolecules. Polymer J 1985, 17:117.
3.
Tomalia DA, Naylor AN, Goddard A: Starburst dendrimers: control of size, shape, surface chemistry, topology and flexibility in the conversion of atoms to macroscopic materials. Angew Chem fnt Ed 1990:138.
4.
Newkome GR, Yao Z-Q, Baker GR, Gupta VK: Cascade molecules. A new approach to micelles. J Org Chem 1985, 50:2003.
5.
Hawker CJ, Frechet JMJ: Preparation of polymers with controlled molecular architecture - a new convergent approach to dendritic macromolecules. J Am Chem Sot 1990, 112:7638-7647.
6. .
Rannard S, Davis N: Synthesis of dendrimers using highly selective chemical reactions. In Proceedings of the American Chemical Society, Division of Polymeric Materials; Science and Engineering: 1997 Sept 8-11; Las Vegas, Nevada. ACS; 1997, 77:63-64. This paper describes the basic chemistry of 1 ,l ‘carbonyldiimidazole and its derivatives. Examples of the type of highly functional dendrimers that may be prepared using this technique are also illustrated. 7. .
Rannard S, Davis N: Synthesis of statistical co-dendrimers and terdendrimers using mixed monomer feeds and high selectivity. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8-l 1; Las Vegas, Nevada. ACS; 1997, 77:160-l 61. Extension of the previous paper in which the1 ,l ‘carbonyldiimidazolel chemistry has been used to prepare statistical co-dendrimers, dendritic analogues of statistical copolymers. 8. .
Launay N, Caminade A-M, Majoral JP: Synthesis of bowl-shaped dendrimers from generation 1 to generation 8. J Organomet Chem 1997, 529:51-58. The paper describes the synthesis of extremely large dendrimers based on main group elements. Given the structure and relative ease of synthesis, the possibility of substituting organometallic groups at the periphery to prepare catalytically active dendrimers looks extremely attractive. 9.
van Koten G, Jastrzebski JTBH: Periphery-functionalized organometallic dendrimers for homogeneous catalysis. In Proceedings of the American Chemical Society, Division of
dendrimers.
Organometa//ics
Cuadrado I, Moran M, Casado CM, Alonso B, Lobete F, Garcia B, lbisate M, Losada J: Ferrocenyl-functionalized poly(propylenimine) dendrimers. Organometallics 1996, 15:5278-5280. An example of a large dendrimer substituted with metallocene groups at the periphery. Modification of the ferrocene groups could lead to catalytically active material, although this is not discussed. 11. .
12. .
Bosman AW, Schenning APHJ, Janssen RAJ, Meijer EW: Welldefined metallodendrimers by site-specific complexation. Chem Ber 1997, 1301725-728. The complexation of M(II) transition metal ions with the terminal groups of poly(propylene imine) dendrimers has been followed by Visible, NMR and ESR spectroscopy. Transmition electron microscopy of the metallodendrimers shows them to be spherical structures with a radius of 3 fl nm. 13. .
Serroni S, Juris A, Venturi M, Campagna S, Resin0 IR, Denti G, Credi A, Balzani V: Polynuclear metal complexes of nanometre size. A versatile synthetic strategy leading to luminescent and redox-active dendrimers made of osmium(ll)-based core and ruthenium(ll)-based units in the branches. J Mater Chem 1997, 711227-l 236. Description of a versatile synthetic approach, intermediate between convergent and divergent syntheses, which allows preparation of mixed metal complexes with up to 22 metal centres. 14.
Papers of particular interest, published within the annual period of review, have been highlighted as:
l
Liao Y-H, Moss JR: Organoruthenium 1996, 15:4307-4316.
1997 Sept 8- 11; Las
Tzalis D, Tor Y: Towards self-assembling dendrimers: metal complexation induces the assembly of hyperbranched structure. Tett Lett 1996, 37:8293- 8296.
15. Zimmerman SC, Zeng F, Reichert DEC, Kolotuchin SV: Self. assembling dendrimers. Science 1996, 271 :1095-l 098. The first example of large dendritic wedges self-assembled by hydrogenbonding interactions. Evidence for discrete hexameric structures is primarily obtained from molecular weight determination of the complexes by standard polymer techniques. Convincing data is provided for several of the smaller complexes but we would argue that the data presented for the highest generation wedge does not necessarily imply aggregation into a discrete hexagonal array, and other interpretations are possible. 16. .
Thiyagarajan P, Zeng F, Ku CY, Zimmerman SC: SANS investigation of self-assembling dendrimers in organic solvents. J Mater Chem 1997, 7:1221-1226. Evidence for the hydrogen-bonded self-assembly of dendritic wedges into discrete hexameric aggregates using small angle neutron scattering. No experimental data was presented for the largest of the structures claimed in [15’1.
17. .
Freeman AW, Vreekamp RH, Frechet JMJ: The self-assembly of convergent dendrimers based on the melamine-cyanuric acid lattice. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8- 11; Las Vegas, Nevada. ACS; 1997,77:138-l 39. Initial experimental data indicates that the materials self-assemble, although it appears that the expected hexamers are in equilibrium with smaller aggregates.
18. Malmstrijm E, Hult A: Hyperbranched polymers a review. JMS.. Rev Macromol Chem Phys C 1997, 37:555-579. An excellent review covering the synthesis, characterisation and properties of hyperbranched polymers up to the end of 1996. 19.
Bolton DH, Wooley KL: Synthesis and characterisation of hyperbranched polycarbonates; Macromolecules 1997, 30:18901896.
20. .
Davis N, Rannard S: Synthesis of hyperbranched polymers using highly selective chemical reactions. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 7997 Sept 8-11; Las Vegas, Nevada. ACS; 1997, 77:158-l 59. The synthesis of both aromatic, aliphatic and mixed hyperbranched polyesters which can be achieved without resorting to protection deprotection chemistry is described. 21.
Mueller A, Kowalewski T, Wooley KL: Hyperbranched polyfluorinated polymers: surface properties and micromechanical behaviour. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8-l 7; Las Vegas, Nevada. ACS; 1997, 77:89-90.
Dendrftic
Stefanescu AD, Wooley KL: Hyperbranched polyfluorinated polymers from AB, monomers. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering. 1997, 771216219.
23.
Hobson LJ, Kenwright AM, Feast WJ: A simple ‘one pot’ route to the hyperbranched analagues of Tomalia’s poly(amidoamine) dendrimers. J Chem Sot Chem Commun 1997:1877-l 878.
The hyperbranched polymer analogues of polyfamidoamine) dendrimers described in this paper have been shown to have near perfect degrees of branching. 24.
Lu P, Paulasaari JK, Weber WP: Hyperbranchad poly(4-acetylstyrene) by ruthenium-catalysad step-growth pdymerisation acetylstyrane. Macromolecules 1996, 29:8583-8586.
of 4-
25. n
Gaynor SG, Edelman, Matyjasxewski K: Synthesis of branched and hyperbranchad polystyranes. Macromolecules 1996,29:10791081. The simple syntheses of hyperbranched derivatives of polystyrene are described. 26.
Tomalia DA, Esfand R: Dendrons, dendrfmers and dendrigrafts. Chem Ind 1997:416-420. b brief overview of macromolecular architectures, highlighting possible areas of application. 27.
polymers Hobson and Harrison
691
37. .
22.
..
and hyperbranched
Yin R, Qin D, Tomalia DA, Kukowska-Latallo, Baker JR: From dendrfmers to dendrfgratts: Synthesis, characterisation and applications. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8-l 1; Las Vegas, Nevada. ACS; 1997,77:206-207.
26.
van Aert HAM, van Genderen MHP, Meijer EW: Star-shaped poly(2,6- dimethyl- ,4-diphenylene ether). Polym Bull 1997, 37~273-280. A paper of topical interest highlighting the problems encountered for molecular weight determination of nonlinear polymers. 36.
Wooley KL, Klug CA, Tasaki K, Schaefer J: Shapes of dendrimers from rotational-echo double-resonance NMR. J Am Chem Sot 1997, 119:53-58.
39.
Hawker CJ, Frechet JMJ: Comparison of linear, hyperbranched and dendritic macromolecules ACS Symp 1996,624:132-l 44.
40. ”
Hawker CJ, Malmstrom E, Frank CW, Kampf JP, Mio C, Prausnitz J: Dendrftic macromolecules: hype or unique materials. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8- 11; Las Vegas, Nevada. ACS; 1997, 77:61-62. The linear and dendritic materials compared are analogous in terms of molecular formula, degree of polymerisation, number of terminal units and having a single focal point functionality. 41.
.
Hawker CJ, Chu F: Hyperbranched polytether ketones): Manipulation of structure and physical properties. Macromolecules
1996,29:4370-4300. Water soluble derivatives shown to act as unimolecular micelles. While through the incorporation of linear or branched units into the monomer structure, modification of the degree of branching between 15% and 71% was illustrated. Holter D, Frey H: Degree of branching in hyperbranched polymers. Acta Polymerica 1997.40130-35. Calculation of DPn by this method relies upon the accuracy by which linear, terminal and branched contributions can be calculated spectroscopically and assumes the absence of cyclic products. 42.
”
Grubbs RB, Hawker CJ, Dao J, Frechet JMJ: A tandem approach to graft and dendritic graft copolymers based on living free radical polymerisatfons. Angew Chem lnt Ed 1997,36:270-272. Synthesis of dendritic graft copolymers by combination of radical polymerisation techniques with independent activation processes. The authors acknowledge the inherent problems of using size exclusion chromtaography to determine molecular weight and polydispersity of dendritic graft structures.
Frey H, Holter D: Degrae of branching in hyparbranchad polymers. 2. Enhancement of the DB: Scope and limitations. Acta Polymerica 1997,48:295-309. The relationship between x and extent of branching for AB, polymers (where x < 1) was also proposed in this paper.
29. .
44. ”
Leduc MR, Hawker CJ, Dao J, Frechet JMJ: Dendritic Initiators for living free radical polymerisations: a versatile approach to the synthesis of dendrftic-linear block copolymers. J Am Chem Sot 1996,118:11111-11116. First reported use of dendrons as macroinitiators in controlled free radical polymerisations leading to hybrid dendritic-linear block copolymers.
30.
van Hest JMC, Delnoye DAP, Baars MWPL, Elissen-Roman, van Genderen MHP, Meijer EW: Polystyrene-poly(propylene imine)
dendrimers: synthesis, characterisatlon and association behaviour of a new class of amphiphiles. Chem Eur J 1996, 12:1616-l
626.
31.
Brenner AR, Voit BI: Free radical grafting from hyparbranched polyesters based on polymerfc azo initiators. Macromol Chem Phys 1996,197:2673-2689.
32.
Zhou Y, Bruening ML, Bergbreiter DE, Crooks RM, Wells M: Preparation of hyperbranchad polymer films grafted on self assembled monolayers. J Am Chem Sot 1996,110:3773-3774.
33.
Zhou Y, Bruening ML, Lui Y, Crooks RM, Bergbreiter DE: Synthesis of hyperbranched, hydrophilic fluorinated surface grafts. Langmuir 1996,12:5519-5521.
34. .
Bruening ML, Zhou Y, Zhoa M, Crooks RM, Bergbreiter DE: Surfacagrafted, hyperbranched poly(acrylic acid) films: new interfaces relevant to sensing and anti corrosion applications. In Proceedings of the American Chemical Societv. Division of Polvmeric Materials: Science and Engineermg: 1997 Sept 8- 11; Las Vegas, Nevada. ACS; 1997, 77:77-70. . This paper describes the electrochemrcal propertres and potentral applications of modified surface grafted poly(acrylic acid) films. 35. ”
Gitsov I, Frechet JMJ: Stimuli-responsive hybrid macromolecules: novel amphiphilic star copolymers with dendrltic groups at the periphery. J Am Chem Sot 1996,118:3765-3766. The significance of this paper is magnified by both the accessibility of the components and the simplicity of the synthetic methodologies used. 36. ”
Stevelmans S, van Hest JMC, Jansen JFGA, van Boxtel DAFJ, de Brabander- van den berg EMM, Meijer EW: Synthesis, characterisation and guest host properties of inverted unimolecular dendritic micelles. J Am Chem Sot 1996, 118:73987399. Surface modification of poly(propylene imine) dendrimers is shown to be extremely selective.
43. .
Widmann AH, Davies GR: Simulation of the intrinsic viscosity of hyperbranchad polymers with varying topology: 1. Step-wise built polymers. Comput Theor Polym Sci 1997, in press. Includes a theoretical evaluation of the intrinsic viscosity-molecular weight relationship for materials with varying degrees of branching. The use of a connectivity index to give a better indication of topology is also discussed. 45.
.
Herzner C, Maier G, Voit B: A synthetic strategy towards hyperbranched polymers wlthout linear units. In Proceedings of
the American Chemical Societv, Division of Polvmeric Materials: Science and Engineering: 7997 Sept 8-l I; Las Vegas, Nevada. ACS: 1997. 77:113. This work presents an interesting concept and the results of polymerisation studies are awaited with interest. 46.
Hasselmann R, Holter D, Frey H: Hyparbranchad polymers preparad via the core-dilution/slow addition technique: computer simulation of molecular weight distribution and degree of branching. Macromolecules 1997, in press.
47. ..
Hobson LJ, Feast WJ: Dendrltic solution viscosity behaviour in cora terminated hyperbranched poly(amidoamlnes). J Chem Sot Chem Commun 1997:2067-2066. A polyfamidoamine) hyperbranched AB?IBe copolymer is shown to display a viscosity/molecular weight profile simrlar to that shown by perfectly regular dendrimers. 48. Chu F, Hawker CJ, Pomery PJ, Hill DJT: Intramolecular cyclisation in ” hyperbranchad polyesters. J PO/~ Sci A 1997,35:1627-l 633. Analysis of hyperbranched polyesters, based on 4,4-(4-hydroxyphenyl) pentanoic acid, by tsC NMR spectroscopy shows intramolecular cyclisation to be insignificant, that is, less than 5%. This conclusion is supported by comparison of Mn values determined spectroscopically by end-group counting and that determined by osmometry. 49. u
Feast WJ, Keeney AJ, Kenwright AM, Parker D: Synthesis structure and cyclics content of hyparbranched polyesters. J Chem Sot Chem Commun 1997,1&l 749-l 750. Matrix-assisted laser desorptionlionisation-time-of-flight mass spectrometry has been demonstrated to be an invaluable diagnostic technique for the evaluation of intramolecular cyclisation products in polycondensation reactions.
50. .
Bharathi P, Moore JS: Solid-supported hyperbranchad polymerisatfon: Evidence for self-limited growth. J Am Chem Sot 1997,119:3391-3392. Control over molecular weight, through the molar ratio of monomer to focal
692
Polymers
point, has been demonstrated 51.
52.
in the range of 5 to 25 kDa.
Watkins DM, Sayed-Sweet Y, Kilmash JW, Turro, NJ, Tomalia DA: Dendrimers with hydrophobic cores and the formation of supramolecular dendrimer-surfactant assembles. Langmuir 1997, 13:3136-3141. Delong R, Stephenson K, Loftus T, Fisher M, Alahari S, Nolting A, Juliano RL: Characterisation of complexes of oligonucleotides with polyamidoamine starburst dendrimers and effects on intracellular delivery. J Pharm Sci 1997, 86:762-764.
53.
Seebach D, Herrmann GF, Lengweiler UD, Abchmann BM, Amrein W: Synthesis and enzymatic degradation of dendrimers from (R)-3-hydroxybutanoic acid and trimesic acid. Angew Chem Int Ed 1996,35:2795-2797.
54.
Tbth E, Pubanz D, Vauthey S, Helm L, Merbach AE: The role of water exchange in attaining maximum relaxivities for dendrimer MRI contrast agents. Chem Eur J 1996, 2:1607-l 615.
Boogh L, Petterson, Kaiser P, Manson J-A: Novel tougheners epoxy-based composites. SAMPE J 1997,1:45-49. The inclusion of low loadings of modlfled hyperbranched polyesters commercial expoxy resin resulted in doubling the toughness of the 55. ..
for to a
composites with no detrimental characteristics. 56.
effect to thermal properties
or processing
Zukas WX, Wilson PM, Gassner JJ: Curing behaviour of a series of amine terminated dendrimers with an epoxy resin. In Proceedmgs of the American Chemical Societv. Division of Polvmeric Materials: Science and Engioeering: 1997 sept 8- 7 1; Las Qegas, Nevada. ACS; 1997, 77:232-233.
57. .
Carr PL, Davies, GR, Feast WJ, Stainton NM, Ward IM: Dielectric and mechanical characterisation of aryl ester dendrimer/PET blends. Polymer 37:2395-2401. A detailed study of the modification of commercial poly(ethylene terephthalate) with aryl ester dendrimers which are fully miscible with the poly(ethylene terephthalate). 58. ..
Johansson M, Rospo G, Hult A: Hyperbranched aliphatic polyesters as a base for thermoset resins. In Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering: 1997 Sept 8-l 1; Las Vegas, Nevada. ACS; 1997, 77:124-l 25. The preparation of thermoset resins based on modified polyesters. The nature of the hyperbranched polymer, which can be controlled, effects the thermal and mechanical properties of the materials.