Syndiotactic copolymerisation

The general mechanistic features of the ethene/CO copolymerisation cycle (Scheme 7.2) are substantially valid also for styrene. In particular, the propagation steps are similar for both alkenes and consist of subsequent alternated migratory insertions of alkyl to CO and of acyl to olefin, with P-chelate and y-chelate resting states. The structures of the first intermediates in the syndiotactic copolymerisation of styrene derivatives with CO have been determined by an in situ NMR study using [(Pr DAB)Pd(Me)(NCMe)]BAr4 as precursor (Scheme 7.21) [38]. 

The syndiospecific polymerisation of propylene with soluble vanadium-based Ziegler Natta catalysts is not completely regiospecific [389 392], i.e. the monomer unit enchainment is not entirely head-to-tail. In addition to syndiotactic stereoblocks, the polymer also contains sterically irregular stereoblocks. The whole polymerisation can be thus described as a copolymerisation with four head-to-tail and tail-to-tail stages [2,379]. 

Similarly, the same catalysts that promote the syndiospecific polymerisation of styrene also polymerise ethylene and a-olefins [106,107], ring-substituted styrenes [6] and conjugated dienes [44,74,108-110], These monomers can also be copolymerised with each other [111-114], Substituted styrenes, which yield syndiotactic polymers by polymerisation run with syndiospecific catalysts, form copolymers with styrene the polymerisation rate increases with increasing nucleophilicity of the comonomer. The random copolymers formed are co-syndiotactic [6,111,112]. 

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