Styrene monomer stereoregular polymer

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

Wullf and Hohn recently described several new stereochemical results (93). They reported the synthesis of a copolymer between a substituted styrene (M ) and methyl methaciylate (M2) having, at least in part, regular. . . M,M M2M MiM2. . . sequences. Polymerization involves the use of a chiral template to which the styrene monomer is loosely bound. After elimination of the template, the polymer shows notable optical activity that must be ascribed to the presence of a chiral stmcture similar to that shown in 53 (here and in other formulas methylene groups are omitted when unnecessaiy for stereochemical information). This constitutes the first stereoregular macromolecular compound having a three monomer unit periodicity. 

It is worth mentioning that a rar.- / ,v -titanocene methylaluminoxane catalyst, such as rac.-Ph2C(Cp)(Ind)TiCl2—[Al(Me)0]x, which yields an isotactic polymer in propylene polymerisation, promotes the syndiospecific polymerisation of styrene [73,100]. This is the first example where two different stereoregular polymers, isotactic and syndiotactic, can be obtained using the same catalyst in the case of two different monomers. 

Polymerizations of various olefin monomers in the presence of polymer matrices which have an interaction with monomers have been carried out. The factors studied include the initiation and the rate enhancement of the polymerizations and the control of stereoregularity or sequence distribution of the resulting polymers. In particular, when monomers and polymer matrices have electric charges, as in the system of poly(styrene sulfonic acid) and vinyl pyridine ), a strong interaction between monomer and the polymer matrix caused a spontaneous polymerization along the polymer matrix, thus forming a polymer complex. But, there are few publications concerned with polycondensation in the presence of a polymer matrix. 

In polymers that exhibit tacticity, the extent of the stereoregularity determines the crystallinity and the physical properties of the polymers. The placement of the monomer units in the polymer is controlled first by the steric and electronic characteristics of the monomer. However, the presence or absence of tacticity, as well as the type of tacticity, is controlled by the catalyst employed in the polymerization reaction. Some common polymers, which can be prepared in specific configuration, include poly(olefins), poly(styrene), poly(methyl methacrylate), and poly(butadiene). 

Apparently very little has been done with simple barium, strontium or calcium alkyl catalysts. One can predict that they should be similar to sodium and potassium catalysts which require special conditions and selected monomers to obtain sufficient cationic attack on the monomer for stereoregular addition. Furukawa (251, 252) has obtained low crystallinity isotactic polymers from styrene and acrylic esters by using complexes of these alkyl metals with ZnR2. However, stereospecificity was attributed to multi-centered coordination involving the two metallic centers (250). 

An optically active polystyrene derivative, 40 ([a]25365 -224° to -283°), was prepared by anionic and radical catalyses.113 The one synthesized through the anionic polymerization of the corresponding styrene derivative using BuLi in toluene seemed to have a high stereoregularity and showed an intense CD spectrum whose pattern was different from those of the monomer and a model compound of monomeric unit 41. In contrast, polymer 42 and a model compound, 43,... 

In spite of the great discoveries by Ziegler and Natta, most synthetic polymers are still made by free-radical reactions. Some of the important homopolymers are poly (vinyl chloride), poly (methyl methacrylate), polystyrene, and low-density polyethylene. Other important polymers made by free-radical reactions contain two or more monomers, for example, the styrene-butadiene rubbers, and the acrylonitrile-butadiene-styrene plastics. Most of these polymers are not stereoregular. A few that are represent the subject of this section. 

The thermal polymerization of -MeOSt has already been mentioned by Staudinger and Dreher [239]. Heating a bulk sample to 90 °C for several days yielded a polymer with a DP = 390 (by viscosity measurements). Later, Russian authors [269] polymerized thermally all three isomers at 100 to 125 °C and found the p and m isomers to polymerize less rapidly than styrene, but the o isomer more rapidly. The stereoregularity of poly(/ -MeOSt) prepared by thermal polymerization in bulk at 60 °C was examined by Yuki et al. [270]. 100-MHz H-NMR spectra showed a rather split signal for the methoxy group and was interpreted in terms of pentad sequences. The analysis of the thermally polymerized sample showed a rather low content of syndiotactic triads. Kawamura et al. [244] studied the C-NMR spectra of o- and -MeOSt polymers prepared with BPO in toluene at 80 °C. They found both polymers to be rich in syndiotactic sequences [o derivative,, = 0.80 p derivative, P = 0.1 (P, = probability of racemic addition of monomer to the growing chain)]. 

Material properties of polymers are determined by their chain miaostmctures. For polymers made from a single monomer type, the above-discussed molecular weight and distribution, chain stereoregularity, head-tail and trans-cis configurations, and so on all play important roles. For copolymers that contain multiple monomer types, chain composition, sequence, as well as their distributions, are added to the important microstmc-ture property list. With these new parameters, almost unlimited number of polymer types can be produced for better balance of properties for commercial applications. Outstanding commercial examples include acrylonittile-butadiene-styrene (ABS), SBS, Acrylan (acrylonittile-vinyl acetate), styrene-butadiene (SBR), butyl mbber (isobutylene-isoprene), Vinylite (vinyl chloride-vinyl acetate), and styrene-maleic anhydride (SMA). 

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