Kinetics of Long Chain Branching in Continuous Solution Polymerization of Ethylene Using Constrained Geometry Metallocene

Our high-temperature and high-pressure continuous stirred-tank reactor (CSTR) has been used for the polymerization of ethylene with the constrained geometry metallocene system, [C5Me4(SiMe2NtBu)]TiMe2 (CGC−Ti)/ tris(pentafluorophenyl)boron (TPFPB)/ modified methylaluminoxane (MMAO), in Isopar E solution at 500 psig. Polyethylenes (PE) with long chain branching (LCB) densities up to 0.44 carbons/10 000 carbons, melt flow index ratios of 7.4−25.7, and narrow polydispersity indexes about 2 were synthesized. The polymerization variables included temperature, mean residence time, and ethylene feed concentration. Polyethylene chains terminated mainly by transfer to monomer yielded macromonomers for the LCB formation. The CGC−Ti catalyst appeared to be single site type, and active centers showed an exponential decay with mean residence time. The rate constants and their activation energies of the ethylene propagation, LCB, chain transfer to monomer, and chain transfer to hydrogen were estimated. The presence of methyl side chains in PEs produced at elevated temperatures indicated that the open structure of CGC−Ti catalyst allowed macromonomers to have 2,1-insertion.