Novel Branching Topology in Polyethylenes As Revealed by Light Scattering and ¹³C NMR

Abstract

A group of polyethylenes synthesized using palladium R-diimine catalysts were studied using 13 C NMR spectroscopy, intensity light scattering, dynamic light scattering, and viscometry. These catalysts are known to produce branched polyethylenes without R-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 (Rg) and the molecular weight M of fractions eluting from a size exclusion chromatograph demonstrated that, at a given M, Rg decreased as ethylene pressure decreased. The hydrodynamic parameterssthe Stokes radius (RH) from dynamic light scattering and the intrinsic viscosity ([η])salso decreased. The change in Rg at a constant M results from the change in branching topology for the polymers synthesized at different ethylene pressures. The parameter Rg 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 13 C 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/RH below unity, and a segment density approaching that of a hard sphere.

Z. Guan, Control of polymer topology through late‐transition‐metal catalysis, J Polym Sci Part A: Polym Chem (2003), 41(22), 3680–3692

“…However, because the branching density and short-chain distribution remained relatively constant, as shown by 13 C NMR, 54,55 this large change in R g could only be attributed to a change in the branching topology. This led to our unambiguous conclusion that the PE topology changed with P E : as P E decreased, the PE topology became more and more dendritic.…”

“…54,55 Because the insertion rate had a first-order dependence on the ethylene concentration but chain walking was independent of ethylene, varying P E was thought to provide the simplest way of controlling the relative rates of insertion and chain walking. It was expected that at high P E values, insertion would be relatively fast because the ethylene concentration was relatively high in the polymerization solution.…”

“…Specifically, we combined static and dynamic light scattering, solution viscosity, neutron scattering, and atomic force microscopy (AFM) to investigate the global topology of PEs made at different pressures. 50,54,55 By correlating the local structural information from NMR with the global structural information from the aforementioned studies coupled with statistical modeling, 64 we finally elucidated the global topology of PEs made at different pressures (Fig. 3).…”

“…54,55 Because the online multiangle light scattering detector measured both the molecular weight (M) and size of the polymer [radius of gyration (R g )] for every fraction of the polymer eluted from the SEC column, we could compare R g 's at the same M value for different samples, and this revealed that at a constant value of M, R g for PE made at 34 atm was about three times R g for PE made at 0.1 atm. For a linear, flexible polymer forming a random coil, R g scaled as M 1/ 2.…”

Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization

“…Assign polymer branching according to the literature. [26][27][28] Note: An Agilent/Varian 600 MHz spectrometer at 130 °C was used for 13 C NMR analysis.…”