Synthesis of syndiotactic polystyrene: Reaction mechanisms and catalysis

In this review the scientific and patent literature on the synthesis of syndiotactic poly(styrene) (SPS) is discussed. SPS can be obtained using titanium compounds/methylaluminoxane or titanium compounds/tetra(pentafluorophenyl)borate salts as homogeneous catalytic systems. The titanium ligands affect both catalytic activity and stereoregularity of SPS. In particular, mono(cyclopentadienyl) derivatives give the best results. Zirconium derivatives are also able to produce SPS, but they are less active than titanium homologues. Several studies have elucidated the main aspects of the polymerization mechanism, but a few points still remain obscure. Most authors agree that the syndiotactic configuration arises from phenyl-phenyl repulsive interactions between the last inserted unit of the growing chain and incoming monomer (1,3-asymetric induction). Since SPS forms gel-like or solid precipitates during synthesis, some patents focus on reaction apparatus that is able to control fouling formation. Finally, block, graft and random copolymers of SPS have also been reported in the literature.