Styrene based monomers

There have been few synthetic reports employing these monomers beyond the Ballard work, most likely as a result of presumed high cost and monomer availability. However, the performance and stability demonstrated by these materials in fuel cells may spur further developments in this area. The above-reported copolymers are believed to be random systems both in the chemical composition of the copolymer backbone and with regard to sulfonic acid attachment. Novel methods have been developed for the controlled polymerization of styrene-based monomers to form block copolymers. If one could create block systems with trifluorostyrene monomers, new morphologies and PEM properties with adequate stability in fuel cell systems might be possible, but the mechanical behavior would need to be demonstrated. 

The following sections detail the literature reports pertaining to the synthesis of block copolymers using nitroxide-mediated polymerization techniques. The sections are organized according to monomer type and generally follow the historical development of the particular subsection. Most literature on nitroxide mediated preparation of block copolymers is found for the styrene-based monomers, and is summarized first. This is followed by acrylates and dienes, as they were the next monomers to be studied. These sections are followed by more recent work with vinyl pyridine, acrylamides, and maleic anhydride. The final section deals with methacrylates. This is presented last to stress the importance of developing new nitroxides that can successfully be used for the homopolymerization of methacrylate-based monomers. 

Other LC-based copolymers incorporating styrene-based monomers were prepared by Ober et al. [149] who chain extended pAcOSt-TEMPO (Mn=7000, Mw/Mn=1.18) with [(4 -methoxyphenyl)4-oxybenzoate]-6-hexyl (4-vinylbenzoate) (MPVB, Fig. 10). The reactions were controlled, with molecular weights ranging from Mn=12,600-23,000 and Mw/Mn=1.19-1.44. The content of pMPVB in the copolymer determined by XH NMR increased as the molar ratio of the MPVB to pAcOSt-TEMPO increased [149]. For two out of the three copolymers prepared a smectic-isotropic transition was observed however, it was at a value lower than expected based on the composition of the copolymer, even after annealing. X-Ray diffraction patterning indicated that the copolymer was oriented in a lamellar morphology and that the smectic layers were perpendicular to the block copolymer lamellae [149]. 

Nitroxides have been used to prepare numerous block copolymers. Initially, using the TEMPO moiety, only styrene-based monomers could be incorporated into copolymers, but with the use of new nitroxides like DEPN and BPPN, the list has expanded to include acrylate-type monomers, as well as dienes, something that previously could only be accomplished through ionic mechanisms. Unfortunately, chain extension of either St or diene-based macroinitiator with an acry-... 

All the CRP methods have strengths that can be exploited in particular systems. TEMPO is essentially useful only for the polymerization of styrene-based monomers, whether for the preparation of statistical or block copolymers [38]. The radicals generated through the self-initiation of St help to moderate the rate of polymerization by consuming any excess TEMPO generated by termination reactions, which will not occur with other monomers. Acrylate monomers, for example, are very sensitive to the concentration of free TEMPO and therefore its build-up causes the polymerization to stop. The use of different nitroxides and alkoxyamines like DEPN [73] and TMPAH [71], which provide higher equilibrium constants and allow for faster polymerization rates, has also enabled the homo- and copolymerizations of acrylate monomers, as well as for St at lower temperatures. Block order is important, however, and chain end functionality is reduced when TMPAH functional polymers are chain extended with BA. This may... 

The TEMPO method tends to be successful only when styrene-based monomers are part of the system, but are usually imsuccessful when acrylates, and in particular methacrylates, are used. The newer nitroxides can be more successful with acrylates, but methyacrylates still present a problem when preparing block copolymers with nitroxide-mediated reactions. 

Expanded polymers or copolymers, preferably based on vinyHdene chloride, acrylonitrile, vinyl chloride, acronyl or styrene-based monomer, or a mixture of these monomers 0.015-0.200 804300 Optimum 0.3-1.5% range from 0.1% to 10% [129]... 

Scheme 6 Silicone rubber 18 by copolymerization of an oligomeric dimettylsiloxane 15 with a vinyl-containing siloxane 16 and styrene-based monomer 17 using chloroplatinic acid.

Figure 14 Styrene-based monomers for the homopolymerization with a Cp TiMe3/B(C6F5)3 catalyst system without aluminum compounds.

For styrene, the conversion of monomer per hour rises from —0.1% at 60°C to about 14% at 140°C. Thus, the effect has to be encountered, especially for polymerizations at higher temperatures. Furthermore, when a styrene-based monomer is to be purified by distillation, the addition of inhibitors and distillation at reduced pressure is advisable in order to avoid the distillate from becoming viscous. Another difficulty occurring during distillation is the formation of polymer in the column, which can also be prevented by distilling in vacuo. The initiation of a styrene-based monomer is assumed to involve a (4-1-2) cycloaddition of the Diels-Alder type with a subsequent hydrogen transfer from the dimer to another monomer molecule ... 

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