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

A group of polyethylenes synthesized using palladium a-diimine catalysts we re studied using C-13 NMR spectroscopy, intensity light scattering, dynamic light scattering, and viscometry. These catalysts are known to produce bra nched polyethylenes without a-olefin comonomers. The series of polymers stu died were synthesized under conditions of varying ethylene pressure. The po lymers are highly branched and completely amorphous and are thus soluble in common organic solvents at ambient temperatures. Light scattering determin ations of the root-mean-square radius of gyration (R-g) and the molecular w eight M of fractions eluting from a size exclusion chromatograph demonstrat ed that, at a given M, R-g decreased as ethylene pressure decreased. The hy drodynamic parameters-the Stokes radius (RH) from dynamic light scattering and the intrinsic viscosity ([eta]-also decreased. The change in R-g at a c onstant M results from the change in branching topology for the polymers sy nthesized at different ethylene pressures. The parameter R-g(2)/M varies by an order of magnitude for the polymers synthesized under ethylene pressure s varying from 0.1 atm to 500 psi. However, the total branching (methyls pe r 1000 CH2) and the distribution of short branches (methyl, ethyl, propyl, etc.) determined by C-13 NMR remained essentially unchanged. These observat ions 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 st ructure at very low ethylene pressures. Polymers synthesized at the lowest ethylene pressure studied, 0.1 atm, exhibited dilute solution parameters si milar to those observed for dendrimers or many-armed stars, with R-g/R-H be low unity, and a segment density approaching that of a hard sphere.