To develop next-generation branched polyolefin reference materials, NIST is exploring families of branched polyolefins, synthesized by controlled polymerization methods that can be correlated to a linear ‘parent’ polyethylene. This synthetic approach utilizes a highly strained cyclic alkene with an active metathesis catalyst to prepare well-controlled polyalkenamers that can be hydrogenated post-polymerization to polyolefins. Placement of alkyl substitution on the ring to take advantage of steric interactions between monomer and the catalyst generates ideal regioregular control of the polyalkenamers. Perfect head-to-tail addition of monomer places the branch point at a set interval along the backbone, resulting in a fixed branch frequency and branch length across the molar mass distribution.<br/>A series of linear and branched polyolefins with comparable degree of polymerization, but varied branch length, were analyzed by high temperature gel permeation chromatography with infrared detection (GPC-IR) to quantify molar mass, molar mass distribution, intrinsic viscosity, and degree of long and short chain branching. The synergistic combination of additional physical detectors used (refractive index, viscometer, and online multi-angle light scattering) permitted measurement of polymer conformational constants, such as the contraction ratio and the fractal dimension. These values provide valuable information about the solution properties of these materials as a function of branch length. Mixtures of the branched polymers at unknown compositions were also analyzed to determine if all species present could be identified and accurately quantified.

Characterization of polyolefins with precise branch frequency and tunable branch length by GPC-IR

254th National Meeting (2017)

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