Molecular Weight and Chain Transfer

Unlike heterogeneous Ziegler-Natta catalysts, homogeneous matallocene catalysts often produce only low molecular weight polymers, especially in stereospecific polymerizations. The molecular weight is given by 

T total polymer, Cj -pentane soluble, C n-hexane soluble, C n-heptane soluble, E ether soluble fractions. 

P-Hydrogen Elimination The metal center abstracts a H atom bonded to the jS-C of the growing polymer chain, forming an M-H bond (M = transition metal) and leaving a polymer with an unsaturated end group  

Chain Transfer by Monomer jS-Hydrogen ehmination takes place simultaneously with monomer insertion at the active center without forming the M-H bond  

Chain Transfer to MAO If MAO is used to activate catalyst, the growing polymer chain attached to an active center can exchange with the methyl group of a MAO molecule, forming the Al-terminated polymer chain and the M-CH3 bond in the active center  

P-CH3 Elimination This chain transfer mechanism has been discovered in the polymerization of propylene. The metal center abstracts a CH3 group, instead of a H atom, at the S-carbon of the growing chain, thus forming a M-CH3 bond at the active center and leaving a polymer with an allyhc end group (Resconi et al., 1992)  

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