Growth syndiotactic

Fig. 7.13, this shifts the vacancy—represented by the square-in the coordination sphere of the titanium to a different site. Syndiotactic regulation occurs if the next addition takes place via this newly created vacancy. In this case the monomer and the growing chain occupy alternating coordination sites in successive steps. For the more common isotactic growth the polymer chain must migrate back to its original position. 

Syndiotactic (Section 31.2) A chain-growth polymer in which the substituents regularly alternate on opposite sides of the backbone. 

Although not a telomerization, it is mentioned here, that syndiotactic 1,2-polybutadienes were prepared in aqueous emulsions with a 7t-allyl-cobalt catalyst [33]. Similarly, chloroprenes were polymerized using aqueous solutions of [PdCl2(TPPMS)2] and [RhCl(TPPMS)3] as catalysts at 40 °C in the presence of an emulsifier and a chain growth regulator (R-SH, R=Cio-Cis) [35]. Despite the usual low reactivity of chlorinated dienes, these reactions proceeded surprisingly fast, leading to quantitative conversion of 10 g chloroprene in 2 hours with only 50 mg of catalyst (approximate TOP = 3500 h- ). 

The stereochemistry of addition to a free centre is mostly determined by interactions between the monomer and active centre during approach to the transition state. In simple cases, represented by equations (34) and (35) only the two primary components will interact, and Bernoulli statistics with a single probability parameter Pm will predominate. For Pm = 0.5, the propagation rate constants of isotactic and syndiotactic growth, kpj and k, will differ... 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

Differently, in the case of secondary chain growth with bis(phenoxy-imine)titanium catalysts, Coates and coworkers reported that chain release occurs exclusively by /3-H hydride transfer from the terminal methyl. This generates an allylic end group as shown in Scheme 15,160 and it has been utilized to produce functionalized syndiotactic propylene oligomers.258... 

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