Novel branching topology in polyethylenes as revealed by light scattering and 13C NMR

A group of polyethylenes synthesized using palladium a-diimine catalysts were studied using C-13 NMR spectroscopy, intensity light scattering, dynamic light scattering, and viscometry. These catalysts are known to produce branched polyethylenes without a-olefin comonomers. The series of polymers studied were synthesized under conditions of varying ethylene pressure. The polymers are highly branched and completely amorphous and are thus soluble in common organic solvents at ambient temperatures. Light scattering determinations of the root-mean-square radius of gyration (R-g) and the molecular weight M of fractions eluting from a size exclusion chromatograph demonstrated that, at a given M, R-g decreased as ethylene pressure decreased. The hydrodynamic parameters-the Stokes radius (RH) from dynamic light scattering and the intrinsic viscosity ([eta]-also decreased. The change in R-g at a constant M results from the change in branching topology for the polymers synthesized at different ethylene pressures. The parameter R-g(2)/M varies by an order of magnitude for the polymers synthesized under ethylene pressures varying from 0.1 atm to 500 psi. However, the total branching (methyls per 1000 CH2) and the distribution of short branches (methyl, ethyl, propyl, etc.) determined by C-13 NMR remained essentially unchanged. These observations indicate the branching topology changes with polymerization pressure. Polymer topology varies from predominantly linear with many short branches at higher ethylene pressures to a densely branched, arborescent globular structure at very low ethylene pressures. Polymers synthesized at the lowest ethylene pressure studied, 0.1 atm, exhibited dilute solution parameters similar to those observed for dendrimers or many-armed stars, with R-g/R-H below unity, and a segment density approaching that of a hard sphere.