Separations of commercial polyolefins, which often involve mixtures and copolymers of linear, short-chain branched, and long-chain branched polyethylenes, can be very challenging to optimize as species with similar hydrodynamic sizes or solubility often co-elute using various chromatographic methods. To better understand the effects of polyolefin structure on the dilute solution properties of polyolefins, a family of short-chain branched polyolefins with control of alkyl branch frequency and branch length were synthesized by ring-opening metathesis polymerization (ROMP) of 1-alkyl-trans-cyclooctene monomers. This synthetic approach utilizes highly strained cyclic alkenes to generate controlled, regioregular polyalkenamers. Quantitative post-polymerization hydrogenation generates the final branched polyolefins with ideal head-to-tail addition of monomer, resulting in a fixed branch frequency and branch length across the molar mass distribution. A series of linear polyolefins with comparable molar masses, but varied short-chain branch length, were analyzed by high-temperature size exclusion chromatography with infrared, refractive index, viscometric, and multi-angle light scattering detectors to quantify molar mass, molar mass distribution, intrinsic viscosity ([η]), radius of gyration (Rg), and degree of short-chain branching. A systematic decrease of intrinsic viscosity is observed with increasing branch length across the entire molar mass distribution. Rg measurements were also confirmed by offline high-temperature small angle neutron scattering measurements (SANS). These short-chain branched polyolefins can serve as useful standards to calibrate the branching index factor as a function of molar mass so long-chain and short-chain branching distributions in polyolefins can be more accurately measured. Statistical and block copolymers of branched and linear polyethylenes were also synthesized. Comparative analysis of [η] and Rg of the copolymers with fixed branch frequency homopolymers was also conducted to determine differences in the polymer persistence length.

Characterization of polyolefins with precise control of branch frequency and branch length

256th National Meeting (2018)

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