Process for producing crystalline vinyl aromatic polymers having a predominantly syndiotactic structure

A process for the production of predominantly syndiotactic vinyl aromatic polymers by polymerizing or copolymerizing vinyl aromatic monomers such as styrene or styrene derivatives in the presence of a catalyst system comprising the product of the reaction between: a) a compound of a transition metal (M) different from tita­nium and containing at least one M-O, M-C, M-N, M-P, M-S or M-halogen bond, andb) an organoaluminium compound containing at least one oxygen atom bound to the aluminium atom or located between two aluminium atoms. Compounds a) and b) can be used as such or supported on a carrier.