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Multi-methacrylated star-shaped, photocurable poly(methyl methacrylate) macromonomers via quasiliving ATRP with suppressed curing shrinkage
Polymer 54 (2013) 6073e6077
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Multi-methacrylated star-shaped, photocurable poly(methyl methacrylate) macromonomers via quasiliving ATRP with suppressed curing shrinkage Amália Szanka, Györgyi Szarka, Béla Iván* Department of Polymer Chemistry, Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, H-1525 Budapest, Pusztaszeri u. 59-67, P. O. Box 17, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 September 2013 Received in revised form 12 September 2013 Accepted 13 September 2013 Available online 22 September 2013
Novel, star-shaped multifunctional poly(methyl methacrylate) (PMMA) macromonomers with welldefined average number of pendant methacrylate groups were synthesized by copolymerizing MMA with 2-hydroxyethyl methacrylate (HEMA) via quasiliving ATRP with a tetrafunctional initiator in methanol at 10 C, followed by methacrylation of the hydroxyl groups of the HEMA units. The resulting tailor-made poly(methyl methacrylate-co-2-methacryloylethyl methacrylate), P(MMA-co-MEMA), multifunctional macromonomers were used as cross-linking agents in photocuring of MMA, a solvent for its own polymer, and thus chemically homogeneous PMMA networks were formed in which the tetrafunctional initiator moiety provides inherent, additional branching points in the resulting cross-linked materials. This approach, even in the presence of relatively low amounts of macromonomers of w35 e45%, provides sol-free products and up to w40% less polymerization shrinkage than that by curing of MMA with a conventional low molecular weight bifunctional methacrylate. These new, unique starshaped PMMA macromonomers are potential cross-linkers in a variety of solvent-free applications where low curing shrinkage and high conversions are critical requirements, such as in several engineering materials, coatings, dental fillings and restorations, bone cements etc. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Poly(methyl methacrylate) Star-shaped PMMA macromonomer by ATRP Reduced photocuring shrinkage with MMA
1. Introduction Well-defined branched multifunctional macromolecules, such as star-shaped and hyperbranched polymers and networks, are of great scientific and technological importance with application possibilities ranging from speciality coatings to engineering materials, nanomedicine, gene delivery etc. (see e.g. Refs. [1e11] and references therein). However, when the final polymer-based objects are made by curing of low molecular weight monomers, polymerization shrinkage is still one of the major problems in a variety of applications of macromolecules. For example, the volume decrease during the curing process is especially important not only in many engineering but in several healthcare processes as well, such as in the case of dental fillings and bone cements in orthopaedic surgeries (see e.g. Refs. [10e22] and references therein). In these instances, poly(methyl methacrylate) (PMMA) is prepared by thermal polymerization or photocuring of MMA in the presence of other
* Corresponding author. E-mail address: [email protected] (B. Iván). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.09.025
components for bone cement or dental restoration, respectively. Dental fillings are made by photocuring of MMA with bifunctional methacrylates to form stable networks. Although several attempts have been reported for decreasing the extent of the harmful polymerization shrinkage in dental applications, especially by monomer design, component variations and process control, this phenomenon is still a critical issue and intensively investigated because it can lead to severe damages in the teeth (see e.g. Refs. [14e22] and references therein). The volume decrease is the result of the formation of the covalent bonds between the low molecular weight monomers, on the one hand. On the other hand, when networks are formed by copolymerization of mono and bi or multifunctional monomers, there is an interval when monomers and branched polymer chains with unreacted polymerizable groups coexist in the polymerization system. Considering this state of the curing process, reduced shrinkage can be expected in the course of the preparation of the final products by copolymerizing suitable branched multifunctional prepolymers as macromonomers with vinyl groups with low molecular weight monomers. In such cases, at least part of the shrinkage would occur during the synthesis of the reactive prepolymers (macromonomers). However, the low solubility of high
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molecular weight polymers for desired applications may prevent the usefulness of such approaches in many cases. For instance, curing of branched methacrylate-telechelic macromonomers of dicarboxylic acids required diluent and resulted in products with low mechanical stiffness [21]. Previous reports on cocuring dimethacrylates with methacrylated hyperbranched polyesters led to monomer leaching and chemically inhomogeneous products [22]. It has to be mentioned that although copolymerization of MMA with other methacrylate-telechelic polymers [23aec], led to conetworks with interesting properties, structural inhomogeneity exists in these materials as well, similar to other polymer conetworks utilizing methacrylate-telechelic macro-cross-linkers, such as polyisobutylene and poly(tetrahydrofuran) [23dej]. In order to overcome these problems, instead of such chemically different functional polymeric materials, cocuring of MMA with branched multifunctional PMMA macromonomers would be the ideal process to obtain compositionally homogeneous PMMA networks with reduced shrinkage possibility. However, to the best of our knowledge, such PMMA macromonomers and their cocuring with MMA have not been reported yet. The projected PMMA macromonomers should possess relatively low molecular weights (MW) and narrow molecular weight distribution (MWD) in order to provide sufficient solubility in MMA, its own monomer, and low viscosity as well. For obtaining well-defined functional polymers with such characteristics, quasiliving polymerizations [24] proceeding via dynamic equilibrium between propagating (living) and nonpropagating (nonliving) polymer chains have gained increased interest in both academia and industry. Among these processes, atom transfer radical polymerization (ATRP) is a suitable tool for synthesizing macromolecules from vinyl monomers with unique structures and functionality (see e.g. Refs. [25aed] for recent reviews). In this context, it has to be mentioned that endfunctional linear PMMAs have been recently prepared by living radical polymerizations for subsequent reactions [26]. In branching polymerizations, however, because copolymerization of (meth)acrylates with divinyl cross-linkers under ATRP conditions can lead to gelation already at low cross-linker/monomer ratios [27], and products with low concentration of pendant unreacted vinyl groups are formed, the application of such copolymerizations for obtaining well-defined, vinyl-containing branched (hyperbranched) polymer structures is very limited. On the basis of this line of thoughts, other branched structures, specifically well-defined multifunctional star macromonomers can thus be taken into account as prepolymers for network syntheses with reduced shrinkage possibility. Herein, we report on the synthesis of four-arm star PMMAs with pendant methacrylate groups via ATRP and subsequent derivatization followed by photocuring these multifunctional macromonomers with its own monomer, methyl methacrylate, as a potential route towards low shrinkage PMMA-based macromolecular compositions. 2. Experimental section Materials. Methyl methacrylate (MMA, 99%, Aldrich) was stirred with CaH2 for 3 h, then passed through neutral alumina column and distilled under reduced pressure. 2-Hydroxyethyl methacrylate (HEMA, 97%, Aldrich) was distilled before use under reduced pressure. 1,1,1,1-tetrakis(20 -bromo-20 -methylpropionyloyxymethyl) methane (TBMPMM) was synthesized according to a literature procedure [28]. Methanol (lab. use, Molar Chemicals) was stirred with 3 A molecular sieves for 3 h and distilled. CuCl (99þ%, Aldrich) was purified by stirring overnight in acetic acid, then filtered and washed with abs. ethanol and diethyl ether, and finally dried. 2,20 Bipyridyl (bpy, 99%, Aldrich), methacryloyl chloride (97%, Fluka), 1,4-butanediol dimethacrylate (BDDMA, Aldrich), 2,2-dimethyoxy1,2-diphenylethan-1-one (97%, Aldrich), triethylamine (98%, VWR),
tetrahydrofuran (THF, lab. use, Molar Chemicals) and hexane (lab. use, Molar Chemicals) were used as received. Copolymerization of MMA and HEMA. The copolymerization of MMA with HEMA for obtaining tetra-arm star copolymers was carried out under ATRP conditions by adapting the process reported by Yagci et al. [29] for the preparation of linear copolymer, i.e. methanol was used as diluent at 10 C. In a typical experiment, a mixture of TBMPMM (0.91 g, 1.24 mmol) as a tetrafunctional initiator, bpy (1.56 g, 9.98 mmol), MMA (10.6 mL, 100.1 mmol), HEMA (1.4 mL, 10.02 mmol) and 8 mL distilled methanol were charged into a 50 mL round bottom flask equipped with a magnetic stirrer. The solution was deoxygenated with three freezeethaw cycles. Subsequently, the reaction mixture was warmed to ambient temperature and CuCl (0.49 g, 4.95 mmol) was added under argon. The solution was immediately freezed, and the deoxygenation was repeated. Then the flask was warmed to 10 C, and after 2 h the reaction was quenched by bubbling oxygen through the mixture for 2 min. Sample was taken to determine the conversion of monomers. Methanol was removed on a rotary evaporator, and the remaining mixture was dissolved in THF and passed through a chromatography column filled with silica and neutral alumina to remove the complex salts. The resulting copolymer was precipitated in hexane, collected by filtration and dried in a vacuum oven at room temperature until constant weight. Synthesis of multifunctional poly(methyl methacrylate-comethacryloylethyl methacrylate) macromonomer (P(MMA-coMEMA)). In a typical experiment, 7 g of P(MMA-co-HEMA) copolymer was dissolved in 100 mL of THF. The solution and 5.5 M excess of triethylamine with regard to the hydroxyl groups were charged into a 250 mL round bottom flask equipped with a magnetic stirrer. It was cooled with ice-water mixture, and 5 M excess of methacryloyl chloride relative to the hydroxyl groups dissolved in 25 mL of THF was added dropwise. After 1 h, the cooling bath was removed, and the solution was stirred another 24 h at room temperature. Then THF was removed on a rotary evaporator, and the remaining mixture was dissolved in toluene. It was extracted twice with 50 mL of 2 wt% sodium hydrogen carbonate solution and distilled water. Finally the polymer solution was passed through a chromatography column filled with neutral alumina, precipitated into hexane, and after collection by filtration, the resulting polymer was dried in a vacuum oven at room temperature until constant weight. Photopolymerization of MMA and BDDMA. 0.0514 g of 2,2dimethyoxy-1,2-diphenylethan-1-one was dissolved in a mixture of 4.5 mL of MMA and 4.5 mL of BDDMA. The solution was charged into a glass tube having predetermined volume and irradiated with a 5000-PC Modular Dymax flood lamp for 3.3 h. Photopolymerization of P(MMA-co-MEMA) macromonomers with MMA. In a typical experiment, 3.5 g of macromonomer was dissolved in 5.5 mL of MMA and 0.046 g of 2,2-dimethyoxy-1,2diphenylethan-1-one was added to the mixture. Then the solution was cured the same way as described for MMA and BDDMA. Characterization. The molecular weight distributions and average molecular weights were determined by gel permeation chromatography (GPC). The GPC equipment was composed of a 515 HPLC Pump and 717 Autosampler, all supplied by Waters, three GPC columns (type MIXED C, Varian), an Agilent 390 RI detector and WinGPC Unichrom software (PSS GmbH). THF was used as eluent with 1 mL/min elution rate at 35 C. Calibration was made with a series of polystyrene standards of narrow MWD (PSS GmbH). Nuclear magnetic resonance (NMR) spectra were recorded in chloroform-d on a 200 MHz Varian spectrometer. The shrinkage (S) was calculated on the basis of density differences:
S ¼ 1
rs rn
A. Szanka et al. / Polymer 54 (2013) 6073e6077
where S is the shrinkage of polymer solution occurred during the photopolymerization, rn is the density of polymer network, rs is the density of the starting monomer mixtures and polymer solutions. A pycnometer was used to determine the corresponding densities. 3. Results and discussion Ethylene glycol dimethacrylate (EGDMA) is a widely used bifunctional monomer for the synthesis of polymer networks (see e.g. Refs. [30aed] and references therein). Copolymerization of EGDMA with MMA produces branched polymers in the early stage of the reaction, and then insoluble polymer network is formed. This dimethacrylate can be considered as the methacryloyl derivative of 2-hydroxyethyl methacrylate (HEMA). Thus, copolymers with welldefined number of HEMA monomer units can serve as a starting material for synthesizing vinyl group containing macromolecules. As shown in Scheme 1, in order to synthesize PMMA with pendant methacrylate groups, first tetra-arm star poly(methacrylate-co-2hydoxyethyl methacrylate) (P(MMA-co-HEMA)) copolymers were synthesized by copolymerization of MMA and HEMA under ATRP conditions with TBMPMM tetrafunctional initiator and the CuCl/bpy catalyst system in methanol at 10 C for 2 h. The initial ratio of these reactants and HEMA was the same in all experiments, [TBMPMM]:[CuCl]:[bpy]:[HEMA] ¼ 1:4:8:8, while MMA was added at a 40, 60 or 80 mol equivalent compared to 1 mol equivalent of TBMPMM. The monomer conversions after 2 h were measured by 1 H NMR spectroscopy. As the data indicate in Table 1, the monomer conversions were almost complete, 99% and 95%, for samples 1 and 2, respectively, and somewhat less in the case of sample 3, which is due to the higher MMA concentration. This polymerization time is much shorter than that reported in the literature for a linear copolymer (66% yield in 15 h) [28] due to the higher concentration of the tetrafunctional initiator in our case. GPC traces of the resulting copolymers are shown in Fig. 1. The shape of the GPC curves as well as the relatively low polydispersity data (Mw/Mn z 1.3e1.6) indicate that only minor chainechain coupling occurs during the applied ATRP process, i.e. carrying out the copolymerization in methanol at 10 C. This is in accordance with literature reports [31] on recombination of growing polymethacrylates under monomer starved conditions, that is at high monomer conversions. The monomer ratios in the purified copolymers agree well with the feed ratios of MMA/HEMA, which clearly show that the composition of the P(MMA-co-HEMA) star polymers can be well designed by selecting the proper feed ratios of the comonomers. The synthesis of methacrylate-functionalized multifunctional PMMA macromonomer was carried out by esterification of the hydroxyl groups of HEMA units in the P(MMA-co-HEMA) random star copolymers with methacryloyl chloride (Scheme 1). As the 1H NMR spectra in Fig. 2 clearly show, successful conversion of the hydroxyl groups was achieved by this process. The NMR signals at 5.64 and 6.16 ppm in the spectrum of the methacrylated P(MMA-
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Table 1 Feed molar ratios of TBMPMM initiator, MMA and HEMA, conversion (Xc), Mn, Mw/ Mn and copolymer compositions of the P(MMA-co-HEMA) copolymers. (Polymerization conditions: TBMPMM/CuCl/bpy ¼ 1/4/8, Ar atmosphere, 10 C, 2 h reaction time in methanol as a solvent). Sample no.
Feed ratio TBMPMM/ MMA/HEMA
Xc (%)a
Mn (g/mol)
Mw/Mn
Copolymer composition (MMA/HEMA)a
1 2 3
0.9/40/8 1/60.1/8 1/80.7/7.6
99 95 79
9500 11,400 11,700
1.59 1.44 1.32
40/7.6 60.1/8.0 80.7/6.8
a
Conversion and copolymer composition were calculated from 1H NMR spectra.
co-HEMA) confirm that a copolymer with pendant methacrylate groups is formed. The change of chemical shift of the methylene group next to the hydroxyl group in HEMA from 3.82 ppm to 4.35 ppm, as well as the appearance of the NMR signal of the methyl protons adjacent to the vinyl group in the methacrylate at 1.97 ppm corroborate the successful synthesis of multimethacrylated PMMA, that is poly(methyl methacrylate-co-2methacryloylethyl methacrylate) copolymers, P(MMA-co-MEMA), were obtained. The molar ratios of MMA and MEMA in the macromonomers were the same as that of MMA and HEMA in the starting P(MMA-co-HEMA) copolymers (Table 2). Fig. 3 shows the GPC traces of a P(MMA-co-HEMA) random star copolymer (sample 1) and the resulting P(MMA-co-MEMA) macromonomer synthesized from it. The shape of the GPC curve belonging to the P(MMAco-MEMA) macromonomer indicates that detectable side reactions leading to either chain coupling or scission did not occur during the esterification process. In all cases, the Mn and Mw/Mn values decreased by some extent. This can be caused by the purification process. Macromonomers with higher molecular weights might remain on the column filled with neutral alumina even if it was washed with toluene after chromatography. In order to determine whether the presence of a multifunctional star-shaped random copolymer with pendant polymerizable methacrylate functionalities, i.e. P(MMA-co-MEMA) results in efficient network formation and reduced volume shrinkage in photopolymerization of MMA, a series of experiments were carried out. In these photocuring reactions, the P(MMA-co-MEMA) acts as a multifunctional macromolecular cross-linking agent for MMA, on the one hand. It has to be also mentioned that the tetrafunctional initiator moiety in the middle of these chains contributes to the network structure as a cross-linking (branching) point in the forming networks, on the other hand. It has to be also mentioned that the branched star-shaped macromonomer has also better solubility and lower viscosity than that of its linear counterpart with the same molecular weight, and thus more optimal curing compositions can be obtained. Therefore, utilizing the methacrylated four-arm star P(MMA-co-MEMA) as a macromolecular crosslinker for MMA is expected to lead to high cross-linking efficiency
Scheme 1. Synthesis of multi-methacrylated PMMA via copolymerization of MMA with HEMA under quasiliving ATRP conditions (in methanol at 10 C) and subsequent derivatization with methacryloyl chloride (I stands for the TBMPMM tetrafunctional initiator).
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A. Szanka et al. / Polymer 54 (2013) 6073e6077 Table 2 Composition of P(MMA-co-HEMA) random star copolymers, P(MMA-co-MEMA) macromonomers (mol/mol ratios), and Mn and Mw/Mn of the macromonomers.
1 2 3
Sample no.a
Copolymer composition (MMA/HEMA)
Macromonomer composition (MMA/MEMA)b
Mn
Mw/Mn
Mm-1 Mm-2 Mm-3
40/7.6 60.1/8.0 80.7/6.8
40/7.4 60.1/8.0 80.7/6.5
7000 10,200 11,200
1.30 1.31 1.24
a b
20
22
24
26
Elution volume (mL) Fig. 1. GPC traces of P(MMA-co-HEMA) random star copolymers at different feed ratios of comonomers: MMA/HEMA ¼ 40/8 (black line, sample 1), 60/8 (blue line, sample 2) and 80/8 (red line, sample 3). (Polymerization conditions: TBMPMM/CuCl/bpy ¼ 1/4/8, Ar atmosphere, 10 C, 2 h reaction time in methanol.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and densely cross-linked PMMA networks. The polymerization mixtures consisted of MMA, 2,2-dimethoxy-1,4-diphenylethan-1one as photoinitiator and 1,4-butanediol dimethacrylate (BDDMA), used widely in dental fillings, or P(MMA-co-MEMA) multifunctional macromonomer (Table 3). Cross-linking of MMA with BDDMA served as a reference in these polymerizations. MMA was used as not only the curable monomer but as solvent for the methacrylate-functionalized PMMA macromonomers as well, i.e. the photocuring of MMA with P(MMA-co-MEMA) was carried out in the absence of any other solvent or diluting agent. The ratio of MMA and the macromonomer was varied to get mixtures which can be poured into the glass tubes without air bubbles. Thus, solutions of P(MMA-co-MEMA) macromonomers in MMA in 35e45% (m/m) concentrations were prepared. It has to be noted that due to
Sample numbers correspond to sample numbers in Table 1. MEMA ¼ 2-methacryloylethyl methacrylate.
the relatively low MW and narrow MWD of the investigated P(MMA-co-MEMA) samples, solutions with higher concentrations can also be obtained which might be important for certain potential applications. As shown in Table 3, curing MMA with the low molecular weight BDDMA led to high shrinkage of 15.8% and 19.3% with 2% (V/V) and 50% (V/V) BDDMA, respectively. The presence of less than 50% (m/m) P(MMA-co-MEMA) macromolecular crosslinker resulted in significantly less polymerization shrinkage in the range of 12.3e13.7%. When the relative amounts of methacrylate groups in the BDDMA (sample Ph-M-2) and in the macromolecular cross-linkers are in the same range, the difference in the shrinkage with the P(MMA-co-MEMA) is nearly 40% less than that with the low molecular weight BDDMA cross-linking agent. It was also found that the sol fraction determined by extracting the networks with THF was zero in all these networks. These results clearly indicate that P(MMA-co-MEMA) is an effective multifunctional polymeric cross-linker in the photocuring of MMA leading to 100% gel content and significantly less polymerization shrinkage than a conventional low molecular weight cross-linking compound. As shown in Table 3, increasing the relative amount of MMA in the mixture increases somewhat the shrinkage arising from the MMA polymerization in the case of sample Ph-2. 4. Conclusions Polymerization shrinkage is a critical phenomenon in many applications. It is especially important in processes used not only in industrial procedures but also in healthcare, such as dental fillings and restorative materials, bone cements etc. These materials primarily consist of polymethacrylates, mainly PMMA and corresponding low MW bi or multifunctional methacrylate cross-linkers. In order to suppress shrinkage during PMMA formation, we attempted to prepare well-defined, branched macromonomers of PMMA with
Star copolymer Macromonomer
20
22
24
26
Elution volume / mL Fig. 2. 1H NMR spectra of P(MMA-co-HEMA) random star copolymer (lower) and the methacrylated P(MMA-co-MEMA) vinyl group containing multifunctional macromonomer (sample 1).
Fig. 3. GPC traces of P(MMA-co-HEMA) random star copolymer and its corresponding P(MMA-co-MEMA) macromonomer derivative (sample-1 and Mm-1 in Tables 1 and 2, respectively).
A. Szanka et al. / Polymer 54 (2013) 6073e6077 Table 3 Composition of the photocuring reaction mixtures: the weight of the P(MMA-coMEMA) macromonomer (mMm), volume of 1,4-butanediol dimethacrylate (VBDDMA), volume of methyl methacrylate (VMMA), weight of 2,2-dimethyoxy-1,2diphenylethan-1-one photoinitiator (mPhI), time of irradiation (tR) and polymerization shrinkage (S). (The numbers for the photocured samples Ph-1, Ph-2 and Ph-3 correspond to the P(MMA-co-MEMA) sample numbers in Table 2.) Sample no.
mMm (g)
VBDDMA (mL)
VMMA (mL)
mPhI (g)
tR (min)
S (%)
Ph-1 Ph-2 Ph-3 Ph-M-1 Ph-M-2
4.5 3.5 3.5 e e
e e e 4.5 0.2
5.5 7 5.5 4.5 10
0.0500 0.0545 0.0460 0.0514 0.0246
260 200 290 200 150
12.3 13.7 12.3 15.8 19.3
pendant methacrylate groups capable to serve as macromolecular cross-linkers in MMA curing. In such cases, significant part of the polymerization shrinkage can be avoided because it occurs during the synthesis of the prepolymer cross-linker, on the one hand. On the other hand, chemically homogeneous PMMA networks can be prepared by using PMMA macromonomer as cross-linker in the course of curing of MMA. MMA itself is a suitable solvent for PMMA, thus solvent-free curing is possible. When branched, e.g. starshaped PMMA polymacromonomer is used for the curing process, the core of the star polymer acts as an inherent branching (junction) point in the networks providing the possibility for obtaining highly cross-linked materials. In line with these considerations, star-shaped poly(methyl methacrylate-co-2-methacryloylethyl methacrylate)s (P(MMA-co-MEMA)) with relatively low MWs, narrow MWDs and well-defined average number of methacrylate functional groups were prepared via quasiliving ATRP and subsequent chain modification. First, successful synthesis of P(MMA-co-HEMA)s with high yields and predetermined compositions was achieved under quasiliving ATRP conditions with a tetrafunctional initiator in conjunction with CuCl/bpy catalyst system in methanol at 10 C in short reaction times (2 h). It should be mentioned that quasiliving SET-LRP [32] conditions can also be useful as alternative to ATRP for obtaining similar PMMAs. Quantitative transformation of the hydroxyl containing P(MMA-co-HEMA) copolymers was obtained by methacrylation with methacryloyl chloride yielding PMMA macromonomers with well-defined average numbers of pendant methacrylate groups. It was found that the resulting P(MMA-co-MEMA) copolymers are well soluble in MMA, and photocuring of such mixtures led to sol-free networks. The polymerization shrinkage was w40% less than in the case of cocuring MMA with the widely used butanediol dimethacrylate (BDDMA). These results indicate that P(MMA-co-MEMA) is an efficient crosslinker, and it is able to reduce the curing shrinkage which is undesirable in many applications. Thus, the star-shaped multifunctional P(MMA-co-MEMA) macromonomers are potentially useful components in curing compositions where suppressed shrinkage is required, e.g. in coatings, dental fillings, bone cements and other applications as well. Acknowledgements The authors thank Mr. Tamás Ignáth for technical assistance in the GPC measurements. References [1] [2] [3] [4]
Blencowe A, Tan JF, Goh TK, Qiao GG. Polymer 2009;50:5e32. Voit BI, Lederer A. Chem Rev 2009;109:5924e73. Kanibolotsky AL, Perepichka IF, Skabara PJ. Chem Soc Rev 2010;39:2695e728. Tasdelen MA, Kahveci MU, Yagci Y. Prog Polym Sci 2011;36:455e567.
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[5] Kim DG, Kang H, Han S, Lee J. ACS Appl Mater Interfaces 2012;4:5898e906. [6] Schallon A, Synatschke CV, Jerome V, Müller AHE, Freitag R. Biomacromolecules 2012;13:3463e74. [7] Zeng XW, Tao W, Mei L, Huang LG, Tan CY, Feng SS. Biomaterials 2013;34: 6058e67. [8] Han QR, Chen XH, Niu YH, Zhao B, Wang BX, Mao C, et al. Langmuir 2013;29: 8402e9. [9] Ikeda T, Oikawa D, Shimasaki T, Teramoto N, Shibata M. Polymer 2013;54: 3206e16. [10] (a) Jiang T, He Y, Jian Y, Nie J. Prog Org Coat 2012;75:398e403; (b) Ji L, Chang W, Cui M, Nie J. J Photochem Photobiol A Chem 2013;252: 216e21. [11] Ifkovits JL, Burdick JA. Tissue Eng 2007;13:2369e85. [12] Griza S, Ueki MM, Souza DHG, Cervieri A, Strohaecker TR. J Mech Behav Biomed Mater 2013;18:29e36. [13] Kinzl M, Boger A, Zysset PK, Pahr DH. J Biomech 2012;45:1478e84. [14] (a) Moszner N, Hirt T. J Polym Sci Part A Polym Chem 2012;50:4369e402; (b) Tabatabai M, Garska B, Fischer U, Moszner N, Utterodt A, Ritter H. Polym Int 2012;61:407e12; (c) Szaloki M, Gall J, Bukovinszki K, Borbely J, Hegedus Cs. React Funct Polym 2013;73:465e73. [15] Schneider LFJ, Cavalcante LM, Silikas N. J Dent Biomech 2010:1e14 [ID 131630]. [16] Alnazzawi A, Watts DC. Dent Mater 2012;28:1240e9. [17] Zhong H, Zhou X, Cai Q, Yang X. Adv Mater Res 2013;647:46e52. [18] Leprince JG, Palin WM, Hadis MA, Devaux J, Leloup G. Dent Mater 2013;29: 139e56. [19] Beigi S, Yeganeh H, Atai M. Dent Mater 2013;29:777e87. [20] Liu D, Liu F, He J, Lassila LVJ, Vallitu PK. J Mater Sci Mater Med 2013;24: 1595e603. [21] Klee JE, Walz U, Hölter D, Frey H, Mülhaupt R. Angew Makromol Chem 1998;260:71e5. [22] Wan Q, Rumpf D, Schricker SR, Mariotti A, Culbertson BM. Biomacromolecules 2001;2:217e22. [23] (a) Sugimoto H, Nishino G, Koyama H, Daimatsu K, Inomata K, Nakashini E. J Appl Polym Sci 2012;124:1316e22; (b) Aranaz I, Carrasco S, Tardajos MG, Elvira C, Reinecke H, Lopez D, et al. Polym Chem 2011;2:709e13; (c) Carrot G, Schmitt B, Lutz P. Polym Bull 1998;40:181e8; }di G, Scherble J, Thomann R, Mülhaupt R. Mac(d) Iván B, Haraszti M, Erdo romol Symp 2005;227:265e73; (e) Haraszti M, Tóth E, Iván B. Chem Mater 2006;16:4952e8; (f) Fodor Cs, Kali G, Iván B. Macromolecules 2011;44:4496e502; (g) Fodor Cs, Iván B. J Polym Sci Part A Polym Chem 2011;49:4729e34; (h) Domján A, Fodor Cs, Kovács Sz, Marek T, Iván B, Süvegh K. Macromolecules 2012;45:7557e65; (i) Fodor Cs, Domján A, Iván B. Polym Chem 2013;4:3714e24; (j) Kali G, Vavra K, László K, Iván B. Macromolecules 2013;46:5337e44. [24] (a) Iván B. Macromol Chem Phys 2000;201:2621e8; (b) Iván B. Macromol Symp 1994;88:201e15; (c) Verebélyi K, Szabó Á, Iván B. Polymer 2012;53:4940e6. [25] (a) Ayres N. Polym Rev 2011;51:138e62; (b) Matyjaszewski K. Macromolecules 2012;45:4015e39; (c) He W, Jiang H, Zhang L, Cheng Z, Zhu X. Polym Chem 2013;4:2919e38; (d) Yamago S, Nakamura Y. Polymer 2013;54:981e94. [26] (a) Greving N, Keul H, Millaruelo M, Weberskirch R, Moeller M. Eur Polym J 2013;49:235e46; (b) Greving N, Keul H, Millaruelo M, Weberskirch R, Moeller M. Macromol Chem Phys 2012;213:1465e74; (c) Gadwal I, Khan A. Polym Chem 2013;4:2440e4. [27] (a) Bannister I, Billingham NC, Armes SP, Rannard SP, Findlay P. Macromolecules 2006;39:7483e92; (b) Rosselgong J, Armes SP. Macromolecules 2010;43:2145e56; (c) Gao H, Matyjaszewski K. Prog Polym Sci 2009;34:317e50; (d) . In: Yan D, Gao C, Frey H, editors. Hyperbranched polymers: synthesis, properties, and applications. Hoboken: John Wiley & Sons Inc; 2011. [28] Ydens I, Degée P, Dubois P, Libiszowski J, Duda A, Penczek S. Macromol Chem Phys 2003;204:171e9. [29] Durmaz YY, Kumbaraci V, Demirel AL, Talinli N, Yagci Y. Macromolecules 2009;42:3743e9. [30] (a) Shirin-Abadi AR, Mahdavian AR, Khoee S. Macromolecules 2011;44: 7405e14; (b) Matsumoto K, Talukdar B, Endo T. Polym Bull 2011;66:771e8; (c) Krasznai DJ, McKenna TFL, Cunningham MF, Champagne P, Smeets NMB. Polym Chem 2012;3:992e1001; (d) Byun H-S, Yang D-S, Cho S-H. Polymer 2013;54:589e95. [31] (a) Liu P, Jin L, Hu J, Wang C. Polym Int 2004;53:136e41; (b) Jin L, Liu P, Hu J, Wang C. Polym Int 2004;53:142e8; (c) Zhou Y-N, Li J-J, Luo Z-H. J Polym Sci Part A Polym Chem 2012;50:3052e66. [32] (a) Wang G, Lu M, Wu H. Polym Bull 2012;69:417e27; (b) Fleischmann S, Percec V. J Polym Sci Part A Polym Chem 2010;48:4889e93; (c) Wang WX, Zhang ZB, Zhu J, Zhou NC, Zhu XL. J Polym Sci Part A Polym Chem 2009;47:6316e27.
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Polymer communication
Multi-methacrylated star-shaped, photocurable poly(methyl methacrylate) macromonomers via quasiliving ATRP with suppressed curing shrinkage Amália Szanka, Györgyi Szarka, Béla Iván* Department of Polymer Chemistry, Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, H-1525 Budapest, Pusztaszeri u. 59-67, P. O. Box 17, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 September 2013 Received in revised form 12 September 2013 Accepted 13 September 2013 Available online 22 September 2013
Novel, star-shaped multifunctional poly(methyl methacrylate) (PMMA) macromonomers with welldefined average number of pendant methacrylate groups were synthesized by copolymerizing MMA with 2-hydroxyethyl methacrylate (HEMA) via quasiliving ATRP with a tetrafunctional initiator in methanol at 10 C, followed by methacrylation of the hydroxyl groups of the HEMA units. The resulting tailor-made poly(methyl methacrylate-co-2-methacryloylethyl methacrylate), P(MMA-co-MEMA), multifunctional macromonomers were used as cross-linking agents in photocuring of MMA, a solvent for its own polymer, and thus chemically homogeneous PMMA networks were formed in which the tetrafunctional initiator moiety provides inherent, additional branching points in the resulting cross-linked materials. This approach, even in the presence of relatively low amounts of macromonomers of w35 e45%, provides sol-free products and up to w40% less polymerization shrinkage than that by curing of MMA with a conventional low molecular weight bifunctional methacrylate. These new, unique starshaped PMMA macromonomers are potential cross-linkers in a variety of solvent-free applications where low curing shrinkage and high conversions are critical requirements, such as in several engineering materials, coatings, dental fillings and restorations, bone cements etc. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Poly(methyl methacrylate) Star-shaped PMMA macromonomer by ATRP Reduced photocuring shrinkage with MMA
1. Introduction Well-defined branched multifunctional macromolecules, such as star-shaped and hyperbranched polymers and networks, are of great scientific and technological importance with application possibilities ranging from speciality coatings to engineering materials, nanomedicine, gene delivery etc. (see e.g. Refs. [1e11] and references therein). However, when the final polymer-based objects are made by curing of low molecular weight monomers, polymerization shrinkage is still one of the major problems in a variety of applications of macromolecules. For example, the volume decrease during the curing process is especially important not only in many engineering but in several healthcare processes as well, such as in the case of dental fillings and bone cements in orthopaedic surgeries (see e.g. Refs. [10e22] and references therein). In these instances, poly(methyl methacrylate) (PMMA) is prepared by thermal polymerization or photocuring of MMA in the presence of other
* Corresponding author. E-mail address: [email protected] (B. Iván). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.09.025
components for bone cement or dental restoration, respectively. Dental fillings are made by photocuring of MMA with bifunctional methacrylates to form stable networks. Although several attempts have been reported for decreasing the extent of the harmful polymerization shrinkage in dental applications, especially by monomer design, component variations and process control, this phenomenon is still a critical issue and intensively investigated because it can lead to severe damages in the teeth (see e.g. Refs. [14e22] and references therein). The volume decrease is the result of the formation of the covalent bonds between the low molecular weight monomers, on the one hand. On the other hand, when networks are formed by copolymerization of mono and bi or multifunctional monomers, there is an interval when monomers and branched polymer chains with unreacted polymerizable groups coexist in the polymerization system. Considering this state of the curing process, reduced shrinkage can be expected in the course of the preparation of the final products by copolymerizing suitable branched multifunctional prepolymers as macromonomers with vinyl groups with low molecular weight monomers. In such cases, at least part of the shrinkage would occur during the synthesis of the reactive prepolymers (macromonomers). However, the low solubility of high
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molecular weight polymers for desired applications may prevent the usefulness of such approaches in many cases. For instance, curing of branched methacrylate-telechelic macromonomers of dicarboxylic acids required diluent and resulted in products with low mechanical stiffness [21]. Previous reports on cocuring dimethacrylates with methacrylated hyperbranched polyesters led to monomer leaching and chemically inhomogeneous products [22]. It has to be mentioned that although copolymerization of MMA with other methacrylate-telechelic polymers [23aec], led to conetworks with interesting properties, structural inhomogeneity exists in these materials as well, similar to other polymer conetworks utilizing methacrylate-telechelic macro-cross-linkers, such as polyisobutylene and poly(tetrahydrofuran) [23dej]. In order to overcome these problems, instead of such chemically different functional polymeric materials, cocuring of MMA with branched multifunctional PMMA macromonomers would be the ideal process to obtain compositionally homogeneous PMMA networks with reduced shrinkage possibility. However, to the best of our knowledge, such PMMA macromonomers and their cocuring with MMA have not been reported yet. The projected PMMA macromonomers should possess relatively low molecular weights (MW) and narrow molecular weight distribution (MWD) in order to provide sufficient solubility in MMA, its own monomer, and low viscosity as well. For obtaining well-defined functional polymers with such characteristics, quasiliving polymerizations [24] proceeding via dynamic equilibrium between propagating (living) and nonpropagating (nonliving) polymer chains have gained increased interest in both academia and industry. Among these processes, atom transfer radical polymerization (ATRP) is a suitable tool for synthesizing macromolecules from vinyl monomers with unique structures and functionality (see e.g. Refs. [25aed] for recent reviews). In this context, it has to be mentioned that endfunctional linear PMMAs have been recently prepared by living radical polymerizations for subsequent reactions [26]. In branching polymerizations, however, because copolymerization of (meth)acrylates with divinyl cross-linkers under ATRP conditions can lead to gelation already at low cross-linker/monomer ratios [27], and products with low concentration of pendant unreacted vinyl groups are formed, the application of such copolymerizations for obtaining well-defined, vinyl-containing branched (hyperbranched) polymer structures is very limited. On the basis of this line of thoughts, other branched structures, specifically well-defined multifunctional star macromonomers can thus be taken into account as prepolymers for network syntheses with reduced shrinkage possibility. Herein, we report on the synthesis of four-arm star PMMAs with pendant methacrylate groups via ATRP and subsequent derivatization followed by photocuring these multifunctional macromonomers with its own monomer, methyl methacrylate, as a potential route towards low shrinkage PMMA-based macromolecular compositions. 2. Experimental section Materials. Methyl methacrylate (MMA, 99%, Aldrich) was stirred with CaH2 for 3 h, then passed through neutral alumina column and distilled under reduced pressure. 2-Hydroxyethyl methacrylate (HEMA, 97%, Aldrich) was distilled before use under reduced pressure. 1,1,1,1-tetrakis(20 -bromo-20 -methylpropionyloyxymethyl) methane (TBMPMM) was synthesized according to a literature procedure [28]. Methanol (lab. use, Molar Chemicals) was stirred with 3 A molecular sieves for 3 h and distilled. CuCl (99þ%, Aldrich) was purified by stirring overnight in acetic acid, then filtered and washed with abs. ethanol and diethyl ether, and finally dried. 2,20 Bipyridyl (bpy, 99%, Aldrich), methacryloyl chloride (97%, Fluka), 1,4-butanediol dimethacrylate (BDDMA, Aldrich), 2,2-dimethyoxy1,2-diphenylethan-1-one (97%, Aldrich), triethylamine (98%, VWR),
tetrahydrofuran (THF, lab. use, Molar Chemicals) and hexane (lab. use, Molar Chemicals) were used as received. Copolymerization of MMA and HEMA. The copolymerization of MMA with HEMA for obtaining tetra-arm star copolymers was carried out under ATRP conditions by adapting the process reported by Yagci et al. [29] for the preparation of linear copolymer, i.e. methanol was used as diluent at 10 C. In a typical experiment, a mixture of TBMPMM (0.91 g, 1.24 mmol) as a tetrafunctional initiator, bpy (1.56 g, 9.98 mmol), MMA (10.6 mL, 100.1 mmol), HEMA (1.4 mL, 10.02 mmol) and 8 mL distilled methanol were charged into a 50 mL round bottom flask equipped with a magnetic stirrer. The solution was deoxygenated with three freezeethaw cycles. Subsequently, the reaction mixture was warmed to ambient temperature and CuCl (0.49 g, 4.95 mmol) was added under argon. The solution was immediately freezed, and the deoxygenation was repeated. Then the flask was warmed to 10 C, and after 2 h the reaction was quenched by bubbling oxygen through the mixture for 2 min. Sample was taken to determine the conversion of monomers. Methanol was removed on a rotary evaporator, and the remaining mixture was dissolved in THF and passed through a chromatography column filled with silica and neutral alumina to remove the complex salts. The resulting copolymer was precipitated in hexane, collected by filtration and dried in a vacuum oven at room temperature until constant weight. Synthesis of multifunctional poly(methyl methacrylate-comethacryloylethyl methacrylate) macromonomer (P(MMA-coMEMA)). In a typical experiment, 7 g of P(MMA-co-HEMA) copolymer was dissolved in 100 mL of THF. The solution and 5.5 M excess of triethylamine with regard to the hydroxyl groups were charged into a 250 mL round bottom flask equipped with a magnetic stirrer. It was cooled with ice-water mixture, and 5 M excess of methacryloyl chloride relative to the hydroxyl groups dissolved in 25 mL of THF was added dropwise. After 1 h, the cooling bath was removed, and the solution was stirred another 24 h at room temperature. Then THF was removed on a rotary evaporator, and the remaining mixture was dissolved in toluene. It was extracted twice with 50 mL of 2 wt% sodium hydrogen carbonate solution and distilled water. Finally the polymer solution was passed through a chromatography column filled with neutral alumina, precipitated into hexane, and after collection by filtration, the resulting polymer was dried in a vacuum oven at room temperature until constant weight. Photopolymerization of MMA and BDDMA. 0.0514 g of 2,2dimethyoxy-1,2-diphenylethan-1-one was dissolved in a mixture of 4.5 mL of MMA and 4.5 mL of BDDMA. The solution was charged into a glass tube having predetermined volume and irradiated with a 5000-PC Modular Dymax flood lamp for 3.3 h. Photopolymerization of P(MMA-co-MEMA) macromonomers with MMA. In a typical experiment, 3.5 g of macromonomer was dissolved in 5.5 mL of MMA and 0.046 g of 2,2-dimethyoxy-1,2diphenylethan-1-one was added to the mixture. Then the solution was cured the same way as described for MMA and BDDMA. Characterization. The molecular weight distributions and average molecular weights were determined by gel permeation chromatography (GPC). The GPC equipment was composed of a 515 HPLC Pump and 717 Autosampler, all supplied by Waters, three GPC columns (type MIXED C, Varian), an Agilent 390 RI detector and WinGPC Unichrom software (PSS GmbH). THF was used as eluent with 1 mL/min elution rate at 35 C. Calibration was made with a series of polystyrene standards of narrow MWD (PSS GmbH). Nuclear magnetic resonance (NMR) spectra were recorded in chloroform-d on a 200 MHz Varian spectrometer. The shrinkage (S) was calculated on the basis of density differences:
S ¼ 1
rs rn
A. Szanka et al. / Polymer 54 (2013) 6073e6077
where S is the shrinkage of polymer solution occurred during the photopolymerization, rn is the density of polymer network, rs is the density of the starting monomer mixtures and polymer solutions. A pycnometer was used to determine the corresponding densities. 3. Results and discussion Ethylene glycol dimethacrylate (EGDMA) is a widely used bifunctional monomer for the synthesis of polymer networks (see e.g. Refs. [30aed] and references therein). Copolymerization of EGDMA with MMA produces branched polymers in the early stage of the reaction, and then insoluble polymer network is formed. This dimethacrylate can be considered as the methacryloyl derivative of 2-hydroxyethyl methacrylate (HEMA). Thus, copolymers with welldefined number of HEMA monomer units can serve as a starting material for synthesizing vinyl group containing macromolecules. As shown in Scheme 1, in order to synthesize PMMA with pendant methacrylate groups, first tetra-arm star poly(methacrylate-co-2hydoxyethyl methacrylate) (P(MMA-co-HEMA)) copolymers were synthesized by copolymerization of MMA and HEMA under ATRP conditions with TBMPMM tetrafunctional initiator and the CuCl/bpy catalyst system in methanol at 10 C for 2 h. The initial ratio of these reactants and HEMA was the same in all experiments, [TBMPMM]:[CuCl]:[bpy]:[HEMA] ¼ 1:4:8:8, while MMA was added at a 40, 60 or 80 mol equivalent compared to 1 mol equivalent of TBMPMM. The monomer conversions after 2 h were measured by 1 H NMR spectroscopy. As the data indicate in Table 1, the monomer conversions were almost complete, 99% and 95%, for samples 1 and 2, respectively, and somewhat less in the case of sample 3, which is due to the higher MMA concentration. This polymerization time is much shorter than that reported in the literature for a linear copolymer (66% yield in 15 h) [28] due to the higher concentration of the tetrafunctional initiator in our case. GPC traces of the resulting copolymers are shown in Fig. 1. The shape of the GPC curves as well as the relatively low polydispersity data (Mw/Mn z 1.3e1.6) indicate that only minor chainechain coupling occurs during the applied ATRP process, i.e. carrying out the copolymerization in methanol at 10 C. This is in accordance with literature reports [31] on recombination of growing polymethacrylates under monomer starved conditions, that is at high monomer conversions. The monomer ratios in the purified copolymers agree well with the feed ratios of MMA/HEMA, which clearly show that the composition of the P(MMA-co-HEMA) star polymers can be well designed by selecting the proper feed ratios of the comonomers. The synthesis of methacrylate-functionalized multifunctional PMMA macromonomer was carried out by esterification of the hydroxyl groups of HEMA units in the P(MMA-co-HEMA) random star copolymers with methacryloyl chloride (Scheme 1). As the 1H NMR spectra in Fig. 2 clearly show, successful conversion of the hydroxyl groups was achieved by this process. The NMR signals at 5.64 and 6.16 ppm in the spectrum of the methacrylated P(MMA-
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Table 1 Feed molar ratios of TBMPMM initiator, MMA and HEMA, conversion (Xc), Mn, Mw/ Mn and copolymer compositions of the P(MMA-co-HEMA) copolymers. (Polymerization conditions: TBMPMM/CuCl/bpy ¼ 1/4/8, Ar atmosphere, 10 C, 2 h reaction time in methanol as a solvent). Sample no.
Feed ratio TBMPMM/ MMA/HEMA
Xc (%)a
Mn (g/mol)
Mw/Mn
Copolymer composition (MMA/HEMA)a
1 2 3
0.9/40/8 1/60.1/8 1/80.7/7.6
99 95 79
9500 11,400 11,700
1.59 1.44 1.32
40/7.6 60.1/8.0 80.7/6.8
a
Conversion and copolymer composition were calculated from 1H NMR spectra.
co-HEMA) confirm that a copolymer with pendant methacrylate groups is formed. The change of chemical shift of the methylene group next to the hydroxyl group in HEMA from 3.82 ppm to 4.35 ppm, as well as the appearance of the NMR signal of the methyl protons adjacent to the vinyl group in the methacrylate at 1.97 ppm corroborate the successful synthesis of multimethacrylated PMMA, that is poly(methyl methacrylate-co-2methacryloylethyl methacrylate) copolymers, P(MMA-co-MEMA), were obtained. The molar ratios of MMA and MEMA in the macromonomers were the same as that of MMA and HEMA in the starting P(MMA-co-HEMA) copolymers (Table 2). Fig. 3 shows the GPC traces of a P(MMA-co-HEMA) random star copolymer (sample 1) and the resulting P(MMA-co-MEMA) macromonomer synthesized from it. The shape of the GPC curve belonging to the P(MMAco-MEMA) macromonomer indicates that detectable side reactions leading to either chain coupling or scission did not occur during the esterification process. In all cases, the Mn and Mw/Mn values decreased by some extent. This can be caused by the purification process. Macromonomers with higher molecular weights might remain on the column filled with neutral alumina even if it was washed with toluene after chromatography. In order to determine whether the presence of a multifunctional star-shaped random copolymer with pendant polymerizable methacrylate functionalities, i.e. P(MMA-co-MEMA) results in efficient network formation and reduced volume shrinkage in photopolymerization of MMA, a series of experiments were carried out. In these photocuring reactions, the P(MMA-co-MEMA) acts as a multifunctional macromolecular cross-linking agent for MMA, on the one hand. It has to be also mentioned that the tetrafunctional initiator moiety in the middle of these chains contributes to the network structure as a cross-linking (branching) point in the forming networks, on the other hand. It has to be also mentioned that the branched star-shaped macromonomer has also better solubility and lower viscosity than that of its linear counterpart with the same molecular weight, and thus more optimal curing compositions can be obtained. Therefore, utilizing the methacrylated four-arm star P(MMA-co-MEMA) as a macromolecular crosslinker for MMA is expected to lead to high cross-linking efficiency
Scheme 1. Synthesis of multi-methacrylated PMMA via copolymerization of MMA with HEMA under quasiliving ATRP conditions (in methanol at 10 C) and subsequent derivatization with methacryloyl chloride (I stands for the TBMPMM tetrafunctional initiator).
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A. Szanka et al. / Polymer 54 (2013) 6073e6077 Table 2 Composition of P(MMA-co-HEMA) random star copolymers, P(MMA-co-MEMA) macromonomers (mol/mol ratios), and Mn and Mw/Mn of the macromonomers.
1 2 3
Sample no.a
Copolymer composition (MMA/HEMA)
Macromonomer composition (MMA/MEMA)b
Mn
Mw/Mn
Mm-1 Mm-2 Mm-3
40/7.6 60.1/8.0 80.7/6.8
40/7.4 60.1/8.0 80.7/6.5
7000 10,200 11,200
1.30 1.31 1.24
a b
20
22
24
26
Elution volume (mL) Fig. 1. GPC traces of P(MMA-co-HEMA) random star copolymers at different feed ratios of comonomers: MMA/HEMA ¼ 40/8 (black line, sample 1), 60/8 (blue line, sample 2) and 80/8 (red line, sample 3). (Polymerization conditions: TBMPMM/CuCl/bpy ¼ 1/4/8, Ar atmosphere, 10 C, 2 h reaction time in methanol.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and densely cross-linked PMMA networks. The polymerization mixtures consisted of MMA, 2,2-dimethoxy-1,4-diphenylethan-1one as photoinitiator and 1,4-butanediol dimethacrylate (BDDMA), used widely in dental fillings, or P(MMA-co-MEMA) multifunctional macromonomer (Table 3). Cross-linking of MMA with BDDMA served as a reference in these polymerizations. MMA was used as not only the curable monomer but as solvent for the methacrylate-functionalized PMMA macromonomers as well, i.e. the photocuring of MMA with P(MMA-co-MEMA) was carried out in the absence of any other solvent or diluting agent. The ratio of MMA and the macromonomer was varied to get mixtures which can be poured into the glass tubes without air bubbles. Thus, solutions of P(MMA-co-MEMA) macromonomers in MMA in 35e45% (m/m) concentrations were prepared. It has to be noted that due to
Sample numbers correspond to sample numbers in Table 1. MEMA ¼ 2-methacryloylethyl methacrylate.
the relatively low MW and narrow MWD of the investigated P(MMA-co-MEMA) samples, solutions with higher concentrations can also be obtained which might be important for certain potential applications. As shown in Table 3, curing MMA with the low molecular weight BDDMA led to high shrinkage of 15.8% and 19.3% with 2% (V/V) and 50% (V/V) BDDMA, respectively. The presence of less than 50% (m/m) P(MMA-co-MEMA) macromolecular crosslinker resulted in significantly less polymerization shrinkage in the range of 12.3e13.7%. When the relative amounts of methacrylate groups in the BDDMA (sample Ph-M-2) and in the macromolecular cross-linkers are in the same range, the difference in the shrinkage with the P(MMA-co-MEMA) is nearly 40% less than that with the low molecular weight BDDMA cross-linking agent. It was also found that the sol fraction determined by extracting the networks with THF was zero in all these networks. These results clearly indicate that P(MMA-co-MEMA) is an effective multifunctional polymeric cross-linker in the photocuring of MMA leading to 100% gel content and significantly less polymerization shrinkage than a conventional low molecular weight cross-linking compound. As shown in Table 3, increasing the relative amount of MMA in the mixture increases somewhat the shrinkage arising from the MMA polymerization in the case of sample Ph-2. 4. Conclusions Polymerization shrinkage is a critical phenomenon in many applications. It is especially important in processes used not only in industrial procedures but also in healthcare, such as dental fillings and restorative materials, bone cements etc. These materials primarily consist of polymethacrylates, mainly PMMA and corresponding low MW bi or multifunctional methacrylate cross-linkers. In order to suppress shrinkage during PMMA formation, we attempted to prepare well-defined, branched macromonomers of PMMA with
Star copolymer Macromonomer
20
22
24
26
Elution volume / mL Fig. 2. 1H NMR spectra of P(MMA-co-HEMA) random star copolymer (lower) and the methacrylated P(MMA-co-MEMA) vinyl group containing multifunctional macromonomer (sample 1).
Fig. 3. GPC traces of P(MMA-co-HEMA) random star copolymer and its corresponding P(MMA-co-MEMA) macromonomer derivative (sample-1 and Mm-1 in Tables 1 and 2, respectively).
A. Szanka et al. / Polymer 54 (2013) 6073e6077 Table 3 Composition of the photocuring reaction mixtures: the weight of the P(MMA-coMEMA) macromonomer (mMm), volume of 1,4-butanediol dimethacrylate (VBDDMA), volume of methyl methacrylate (VMMA), weight of 2,2-dimethyoxy-1,2diphenylethan-1-one photoinitiator (mPhI), time of irradiation (tR) and polymerization shrinkage (S). (The numbers for the photocured samples Ph-1, Ph-2 and Ph-3 correspond to the P(MMA-co-MEMA) sample numbers in Table 2.) Sample no.
mMm (g)
VBDDMA (mL)
VMMA (mL)
mPhI (g)
tR (min)
S (%)
Ph-1 Ph-2 Ph-3 Ph-M-1 Ph-M-2
4.5 3.5 3.5 e e
e e e 4.5 0.2
5.5 7 5.5 4.5 10
0.0500 0.0545 0.0460 0.0514 0.0246
260 200 290 200 150
12.3 13.7 12.3 15.8 19.3
pendant methacrylate groups capable to serve as macromolecular cross-linkers in MMA curing. In such cases, significant part of the polymerization shrinkage can be avoided because it occurs during the synthesis of the prepolymer cross-linker, on the one hand. On the other hand, chemically homogeneous PMMA networks can be prepared by using PMMA macromonomer as cross-linker in the course of curing of MMA. MMA itself is a suitable solvent for PMMA, thus solvent-free curing is possible. When branched, e.g. starshaped PMMA polymacromonomer is used for the curing process, the core of the star polymer acts as an inherent branching (junction) point in the networks providing the possibility for obtaining highly cross-linked materials. In line with these considerations, star-shaped poly(methyl methacrylate-co-2-methacryloylethyl methacrylate)s (P(MMA-co-MEMA)) with relatively low MWs, narrow MWDs and well-defined average number of methacrylate functional groups were prepared via quasiliving ATRP and subsequent chain modification. First, successful synthesis of P(MMA-co-HEMA)s with high yields and predetermined compositions was achieved under quasiliving ATRP conditions with a tetrafunctional initiator in conjunction with CuCl/bpy catalyst system in methanol at 10 C in short reaction times (2 h). It should be mentioned that quasiliving SET-LRP [32] conditions can also be useful as alternative to ATRP for obtaining similar PMMAs. Quantitative transformation of the hydroxyl containing P(MMA-co-HEMA) copolymers was obtained by methacrylation with methacryloyl chloride yielding PMMA macromonomers with well-defined average numbers of pendant methacrylate groups. It was found that the resulting P(MMA-co-MEMA) copolymers are well soluble in MMA, and photocuring of such mixtures led to sol-free networks. The polymerization shrinkage was w40% less than in the case of cocuring MMA with the widely used butanediol dimethacrylate (BDDMA). These results indicate that P(MMA-co-MEMA) is an efficient crosslinker, and it is able to reduce the curing shrinkage which is undesirable in many applications. Thus, the star-shaped multifunctional P(MMA-co-MEMA) macromonomers are potentially useful components in curing compositions where suppressed shrinkage is required, e.g. in coatings, dental fillings, bone cements and other applications as well. Acknowledgements The authors thank Mr. Tamás Ignáth for technical assistance in the GPC measurements. References [1] [2] [3] [4]
Blencowe A, Tan JF, Goh TK, Qiao GG. Polymer 2009;50:5e32. Voit BI, Lederer A. Chem Rev 2009;109:5924e73. Kanibolotsky AL, Perepichka IF, Skabara PJ. Chem Soc Rev 2010;39:2695e728. Tasdelen MA, Kahveci MU, Yagci Y. Prog Polym Sci 2011;36:455e567.
6077
[5] Kim DG, Kang H, Han S, Lee J. ACS Appl Mater Interfaces 2012;4:5898e906. [6] Schallon A, Synatschke CV, Jerome V, Müller AHE, Freitag R. Biomacromolecules 2012;13:3463e74. [7] Zeng XW, Tao W, Mei L, Huang LG, Tan CY, Feng SS. Biomaterials 2013;34: 6058e67. [8] Han QR, Chen XH, Niu YH, Zhao B, Wang BX, Mao C, et al. Langmuir 2013;29: 8402e9. [9] Ikeda T, Oikawa D, Shimasaki T, Teramoto N, Shibata M. Polymer 2013;54: 3206e16. [10] (a) Jiang T, He Y, Jian Y, Nie J. Prog Org Coat 2012;75:398e403; (b) Ji L, Chang W, Cui M, Nie J. J Photochem Photobiol A Chem 2013;252: 216e21. [11] Ifkovits JL, Burdick JA. Tissue Eng 2007;13:2369e85. [12] Griza S, Ueki MM, Souza DHG, Cervieri A, Strohaecker TR. J Mech Behav Biomed Mater 2013;18:29e36. [13] Kinzl M, Boger A, Zysset PK, Pahr DH. J Biomech 2012;45:1478e84. [14] (a) Moszner N, Hirt T. J Polym Sci Part A Polym Chem 2012;50:4369e402; (b) Tabatabai M, Garska B, Fischer U, Moszner N, Utterodt A, Ritter H. Polym Int 2012;61:407e12; (c) Szaloki M, Gall J, Bukovinszki K, Borbely J, Hegedus Cs. React Funct Polym 2013;73:465e73. [15] Schneider LFJ, Cavalcante LM, Silikas N. J Dent Biomech 2010:1e14 [ID 131630]. [16] Alnazzawi A, Watts DC. Dent Mater 2012;28:1240e9. [17] Zhong H, Zhou X, Cai Q, Yang X. Adv Mater Res 2013;647:46e52. [18] Leprince JG, Palin WM, Hadis MA, Devaux J, Leloup G. Dent Mater 2013;29: 139e56. [19] Beigi S, Yeganeh H, Atai M. Dent Mater 2013;29:777e87. [20] Liu D, Liu F, He J, Lassila LVJ, Vallitu PK. J Mater Sci Mater Med 2013;24: 1595e603. [21] Klee JE, Walz U, Hölter D, Frey H, Mülhaupt R. Angew Makromol Chem 1998;260:71e5. [22] Wan Q, Rumpf D, Schricker SR, Mariotti A, Culbertson BM. Biomacromolecules 2001;2:217e22. [23] (a) Sugimoto H, Nishino G, Koyama H, Daimatsu K, Inomata K, Nakashini E. J Appl Polym Sci 2012;124:1316e22; (b) Aranaz I, Carrasco S, Tardajos MG, Elvira C, Reinecke H, Lopez D, et al. Polym Chem 2011;2:709e13; (c) Carrot G, Schmitt B, Lutz P. Polym Bull 1998;40:181e8; }di G, Scherble J, Thomann R, Mülhaupt R. Mac(d) Iván B, Haraszti M, Erdo romol Symp 2005;227:265e73; (e) Haraszti M, Tóth E, Iván B. Chem Mater 2006;16:4952e8; (f) Fodor Cs, Kali G, Iván B. Macromolecules 2011;44:4496e502; (g) Fodor Cs, Iván B. J Polym Sci Part A Polym Chem 2011;49:4729e34; (h) Domján A, Fodor Cs, Kovács Sz, Marek T, Iván B, Süvegh K. Macromolecules 2012;45:7557e65; (i) Fodor Cs, Domján A, Iván B. Polym Chem 2013;4:3714e24; (j) Kali G, Vavra K, László K, Iván B. Macromolecules 2013;46:5337e44. [24] (a) Iván B. Macromol Chem Phys 2000;201:2621e8; (b) Iván B. Macromol Symp 1994;88:201e15; (c) Verebélyi K, Szabó Á, Iván B. Polymer 2012;53:4940e6. [25] (a) Ayres N. Polym Rev 2011;51:138e62; (b) Matyjaszewski K. Macromolecules 2012;45:4015e39; (c) He W, Jiang H, Zhang L, Cheng Z, Zhu X. Polym Chem 2013;4:2919e38; (d) Yamago S, Nakamura Y. Polymer 2013;54:981e94. [26] (a) Greving N, Keul H, Millaruelo M, Weberskirch R, Moeller M. Eur Polym J 2013;49:235e46; (b) Greving N, Keul H, Millaruelo M, Weberskirch R, Moeller M. Macromol Chem Phys 2012;213:1465e74; (c) Gadwal I, Khan A. Polym Chem 2013;4:2440e4. [27] (a) Bannister I, Billingham NC, Armes SP, Rannard SP, Findlay P. Macromolecules 2006;39:7483e92; (b) Rosselgong J, Armes SP. Macromolecules 2010;43:2145e56; (c) Gao H, Matyjaszewski K. Prog Polym Sci 2009;34:317e50; (d) . In: Yan D, Gao C, Frey H, editors. Hyperbranched polymers: synthesis, properties, and applications. Hoboken: John Wiley & Sons Inc; 2011. [28] Ydens I, Degée P, Dubois P, Libiszowski J, Duda A, Penczek S. Macromol Chem Phys 2003;204:171e9. [29] Durmaz YY, Kumbaraci V, Demirel AL, Talinli N, Yagci Y. Macromolecules 2009;42:3743e9. [30] (a) Shirin-Abadi AR, Mahdavian AR, Khoee S. Macromolecules 2011;44: 7405e14; (b) Matsumoto K, Talukdar B, Endo T. Polym Bull 2011;66:771e8; (c) Krasznai DJ, McKenna TFL, Cunningham MF, Champagne P, Smeets NMB. Polym Chem 2012;3:992e1001; (d) Byun H-S, Yang D-S, Cho S-H. Polymer 2013;54:589e95. [31] (a) Liu P, Jin L, Hu J, Wang C. Polym Int 2004;53:136e41; (b) Jin L, Liu P, Hu J, Wang C. Polym Int 2004;53:142e8; (c) Zhou Y-N, Li J-J, Luo Z-H. J Polym Sci Part A Polym Chem 2012;50:3052e66. [32] (a) Wang G, Lu M, Wu H. Polym Bull 2012;69:417e27; (b) Fleischmann S, Percec V. J Polym Sci Part A Polym Chem 2010;48:4889e93; (c) Wang WX, Zhang ZB, Zhu J, Zhou NC, Zhu XL. J Polym Sci Part A Polym Chem 2009;47:6316e27.