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 micros tructure of the copolymers produced using a series of catalytic systems NdC l3 3TBP-Al(iso-C4H9)(3), NdCl3 . 3TBP-Mg(n-C4H9)(iso-C8H17), TiCl4-Al(iso-C 4H9)(3) and TiCl4-Mg(n-C4H9)(iso-C8H17) 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 tra ns regulating one) and causes a significant increase in the content of tran s-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 inferre d that, in the catalytic systems under study, a set of active centers is fo rmed from the R(n)MtCl(3-n) compounds (where Mt = Nd and Ti, II = 1-3) and their combinations with non-transition-metal compounds. These centers are d istinguished by reactivity and stereospecificity, and their type and contri bution to the stereospecificity of the examined catalysts are controlled by the nature of an alkylating agent, that is, by a non-transition-metal orga nic compound playing the role of this agent.