John C. Chadwick

Advances in propene polymerization using MgCl2-supported catalysts. Fundamental aspects and the role of electron donors

The fundamental factors determining the performance of state-of-the-art MgCl 2 -supported catalysts for polypropylene are becoming increasingly evident. Polymer yield, isotacticity, molecular weight and molecular weight distribution are dependent on the regio- and stereoselectivity of the active species. Chain transfer with hydrogen after the occasional regioirregular (2,1-) insertion has a strong effect on molecular weight and is the main reason for the high hydrogen response shown by high-activity catalysts containing diether donors. Hydrogen response is also dependent on stereoselectivity. The probability of a stereo- or regioirregular insertion can be related to the lability of donor coordination in the vicinity of the active species. Results with different catalyst systems can be interpreted on the basis of a propagation model involving interconverting active species, such that polypropylene produced using MgCl 2 -supported catalysts can be regarded as a stereoblock polymer comprising (highly) isotactic sequences, moderately isotactic (isotactoid) sequences and syndiotactoid sequences. Strongly coordinating donors will give stereoregular polymers in which highly isotactic sequences predominate.

Factors influencing chain transfer with monomer and with hydrogen in propene polymerization using MgCl2-supported Ziegler-Natta catalysts

13C and 1H NMR analysis of polypropylene prepared using various MgCl2-supported catalysts at different polymerization temperatures at different hydrogen concentrations has revealed that chain transfer with monomer, yielding vinylidene-terminated chains, is essentially unaffected by hydrogen but is dependent on catalyst selectivity. Chain transfer with hydrogen and with monomer are therefore processes which operate independently of each other, but in both cases, the incidence of chain transfer increases with decreasing catalyst (site) selectivity. Chain transfer with monomer takes place after primary insertion, while chain transfer with hydrogen takes place after either primary or secondary insertion. Chain initiation via a secondary insertion into a Ti-H species leads either to a 2,3-dimethylbutyl terminal unit, as has recently been observed in metallocene-catalyzed polymerizations, or to a further transfer reaction with hydrogen, generating traces of propane.