Quantification of branching content in polyolefins is critical to understand, and ultimately tune, the properties of advanced polyolefins, especially for novel commercial thermoplastic elastomers based on polyethylene copolymers. NIST sees a clear industrial need for representative polyolefin measurements and reference materials beyond linear polyethylene, where control and of polyolefin branch length, branch frequency, and dispersity will yield materials with minimal heterogeneity. Herein, a series of highly controlled short-chain branched polyolefins were synthesized by ring-opening metathesis polymerization (ROMP) of alkyl substituted cyclooctene monomers. This synthetic approach utilizes strained cyclic alkenes to generate controlled, regioregular polyalkenamers. Quantitative post-polymerization hydrogenation generates the final branched polyolefin with near ideal head-to-tail addition of monomer, resulting in a fixed branch frequency and branch length across the molar mass distribution for each polymer. This series of low linear density polyethylenes (LLDPE) have comparable molar masses and fixed branching frequency, but systematically varied short-chain branch length. The LLDPEs were analyzed by ambient and high temperature size exclusion chromatography (SEC) with differential refractive index, viscometric, and multi-angle light scattering detectors to quantify molar mass, molar mass distribution (MMD), intrinsic viscosity ([η]), and radius of gyration (Rg) of each polymer. An additional high-temperature flow-through infrared detector also measured the degree of short-chain branching across the elution curve. A systematic decrease of intrinsic viscosity is observed with increasing branch length across the MMD, which follows trends computed from the Monte Carlo numerical path integrator software, ZENO. Rg measurements from high-temperature SEC are also directly compared to those obtained by small-angle neutron scattering (SANS) under comparable solvation conditions. These regularly branched polyolefins can potentially serve as useful standards to calibrate the short-chain branching distribution of polyolefins with higher branching content as well as the branching index correction factor as a function of molar mass. Such calibrations will help improve accuracy in measurements of long-chain and short-chain branching distributions in polyolefins, especially in samples that contain both types of branching.

Synthetic approaches to model reference materials: Design and characterization of architecturally ‘perfect’ branched polyolefins

2019 Fall National Meeting

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