Neodimium catalyzed diolefin polymerization

Abstract

After about thirty years since the Ziegler-Natta discoveries, the multifaceted aspects of stereospecific catalysis have been hardly clarified, notwithstanding the large amount of research devoted to this subject. In the same time, extremely active catalysts and simplified processes have been pointed out, particularly in the field of monoolefins. As far as conjugated diolefins are concerned, U [1] and Nd [2] based catalysts have been developed in the last years, besides to more conventional systems based on Ti, V, Co and Ni. A general characteristic of the quoted catalyst systems is the remarkable amount of alkylAl as cocatalyst, required for achieving the best behaviour. Therefore an insight on the reasons of such a large stoichiometric excess is both of theoretical and economical relevance. The possibility of obtaining catalytic complexes with Al/Co = 2 for instance, was early described [3]. Nd catalyst behaves similarly to other systems but shows unusual peculiarities such as: low chain transfer activity with diolefin monomer as well as with monoolefins (ethylene, propene, 1-butene), hydrogen and Et 2Zn; very high sensitivity to cyclopentadiene; low sensitivity to CO; poisoning with alkylMg and NaAlEt 2H 2; partial deactivation with hydrocarbons; capability of homo and copolymerizing both butadiene and isoprene to high cis polymers, surprising thermal stability. A Nd catalyst is composed of: i) Nd compound, halogen containing or not, able to react with; ii) alkylAl unhalogenated compound and iii) metallic halide or organic halogen-derivative, when halogen is absent in i). The halogen function can be partially displayed by water. By carrying out the catalyst preparation from different Nd and Al compounds and from different halogen donors in hexane and/ or in THF solution, variously active mixtures are obtained. In some cases complexes containing Nd, Al, Cl and alkyl groups in simple ratios to each other can be isolated which are able to polymerize butadiene. The activity of the above mentioned complexes changes with composition and in some cases is comparable with that of the corresponding homogeneous system in which Al/Nd ratio is extremely higher.