Nature of active centers in diene copolymerization initiated by Ziegler-Natta catalysts : The role of a non-transition-metal organic compound

The regularities of butadiene copolymerization with isoprene and the microstructure of the copoIymers produced using a series of catalytic systems NdCl 3 . 3TBP-Al(iso-C 4 H 9 ) 3 , NdCl 3 . 3TBP-Mg(n-C 4 H 9 )(iso-C 8 H 17 ), TiCl 4 -Al(iso-C 4 H 9 ) 3 , and TiCl 4 -Mg(n-C 4 H 9 )(iso-C 8 H 17 ) were studied. It was shown that for the titanium systems, the replacement of an organoaluminum component by an organomagnesium one leads to a complete inversion of the stereospecificity of a lanthanide catalytic system (a cis regulating system turns into a trans regulating one) and causes a significant increase in the content of trans-1,4 units in the produced copolymer (up to 60% in a butadiene part and up to 40% in an isoprene part). Based on experimental evidence, it is inferred that, in the catalytic systems under study, a set of active centers is formed from the R n MtCl 3-n compounds (where Mt = Nd and Ti, n = 1-3) and their combinations with non-transition-metal compounds. These centers are distinguished by reactivity and stereospecificity, and their type and contribution to the stereospecificity of the examined catalysts are controlled by the nature of an alkylating agent, that is, by a non-transition-metal organic compound playing the role of this agent.