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 MgCl2-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 MgCl2-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.