Design and Characterization of Model Linear Low-Density Polyethylenes (LLDPEs) by Multidetector Size Exclusion Chromatography

Abstract

Separations of commercial polyethylenes, which often involve mixtures and copolymers of linear, short-chain branched, and long-chain branched chains, can be very challenging to optimize as species with similar hydrodynamic sizes or solubility often coelute in various chromatographic methods. To better understand the effects of polymer structure on the dilute solution properties of polyolefins, a family of model linear low-density polyethylenes (LLDPEs) were synthesized by ring-opening metathesis polymerization (ROMP) of sterically hindered, alkyl-substituted cyclooctenes, followed by hydrogenation. Within this series, the alkyl branch frequency was fixed while systematically varying the short-chain branch length. These model materials were analyzed by ambient- and high-temperature size exclusion chromatography (HT-SEC) to determine their molar mass, intrinsic viscosity ([η]), and degree of short-chain branching across their respective molar mass distributions. Short-chain branching is fixed across the molar mass distribution, based on the synthetic strategy used, and measured values agree with theoretical values for longer alkyl branches, as evident by HT-SEC. Deviation from theoretical values is observed for ethyl branched LLDPEs when calibrated using either α-olefin copolymers (poly­(ethylene-stat-1-octene)) or blends of polyethylene and polypropylene standards. A systematic decrease of intrinsic viscosity is observed with increasing branch length across the entire molar mass distribution. This work demonstrates the applicability of these model materials to deconvolute structure–property relationships using chromatographic separation techniques and is a step toward determining if sequence control can minimize compositional heterogeneity and generate improved standards for determining branching content in commercial polyolefins.

Pushing the Bounds of Olefin Metathesis with Vanadium

“…Monomers based on CPE, CHE, and COE bearing functional groups allow for the synthesis of model, defect-free linear low-density polyethylenes (LLDPEs). [20,[82][83][84] In the case of the low strain monomer cis-cyclooctadiene (COD), the resulting polymer serves as a model for defect-free poly(butadiene). [20,85,86] Highly electron deficient initiator 9 was capable of polymerizing both CPE and COE, however activities were notably lower than with NBE, and Ð values were higher, [80] possibly due to CM of the less sterically protected backbone olefins in the growing polymer chain (Scheme 4).…”

“…Higher strain monomers such as cyclopropene or cyclobutene may also be used to achieve similar polymers, however these monomers tend to be unstable at room temperature or require difficult routes for their synthesis. Monomers based on CPE, CHE, and COE bearing functional groups allow for the synthesis of model, defect‐free linear low‐density polyethylenes (LLDPEs) [20,82–84] . In the case of the low strain monomer cis ‐cyclooctadiene (COD), the resulting polymer serves as a model for defect‐free poly(butadiene) [20,85,86] …”

Recent Advances in Separation-Based Techniques for Synthetic Polymer Characterization

“…HT-GPC was recently used to assist in the understanding of the polymerization mechanism, such as chain walking polymerization, to support synthesis and characterization of model linear low density polyethylene (LLDPE) polymers, , for understanding changes in MWD as a result of hydrogen–deuterium exchange reactions, comprehensive analysis of novel grafted polyethylenes using multidimensional fractionation methods, deformulation of commercial LLDPE materials, and structure performance correlations involving adhesion of waxes . With the great enhanced awareness of environmental sustainability, HT-GPC was recently used to characterize plastic waste in the ocean and plastic residues digested by marine animals. , …”

“…Finding a proper linear reference with the desired composition, molecular weight and distribution can be challenging. The effort of synthesizing model linear polyethylene with exact comonomer content and comonomer type was explored recently . The model LLDPE polymers were synthesized with ring-opening metathesis polymerization, and had short chain branching lengths ranging from 1 to 10 carbons.…”

Topological Characteristics of Chain Molecules with Branching and Molar‐Mass Distributions

“…Typical longchain branched polymers, such as low-density polyethylene (LDPE), are randomly branched polydisperse polymers that are considered to be random mixtures of structural isomers. [6,7] Gel-permeation chromatography (GPC), also known as sizeexclusion chromatography (SEC), [8][9][10][11][12][13] is a popular technique used to fractionate polymers according to their volume dimension [η]M, where M is the polymer molecular weight. Thus, the chain dimensions of various branched polymers have been continuously investigated using combinations of GPC with either LS or [η] measurements.…”