Block Index for Characterizing Olefin Block Copolymers

Abstract

Summary: Olefin block copolymers produced by chain shuttling catalysis exhibit crystallinity characteristics that are distinct from what would be expected for typical random olefin copolymers with comparable monomer compositions produced from either 'single-site' or heterogeneous catalysis. Olefin block copolymers produced by chain shuttling catalysis have a statistical multiblock architecture. A unique structural feature of olefin-based block copolymers is that the intra-chain distribution of comonomer is segmented (statistically non-random). Fractionating an olefin block copolymer by preparative temperature rising elution fractionation, TREF, results in fractions that have much higher comonomer content than comparable fractions of a random copolymer collected at an equivalent TREF elution temperature. We have developed a ''block index'' methodology which quantifies the deviation from the expected monomer composition versus the analytical temperature rising elution fractionation, ATREF, elution temperature. When interpreted properly, this index indicates the degree to which the intra-chain comonomer distribution is segmented or blocked. The unique crystallization behavior of block copolymers determine the magnitude of the block index values because the highly crystalline segments along an otherwise non-crystalline chain tend to dominate the ATREF (and DSC) temperature distributions.

Chemical Composition Distribution and Temperature Rising Elution Fractionation of Linear Olefin Block Copolymers

“…To modify the TREF model for OBCs, one important additional factor that needs to be considered is the systematic shift to higher elution temperatures that is observed when TREF profiles of OBCs are compared to those of ethylene/ α‐olefin random copolymers with the same α‐olefin molar fraction. This temperature shift is quantified using the definition of block index ( BI ) introduced by Li Pi Shan and Hazlitt, where T x is the peak TREF elution temperature ( T e ) of an OBC fraction of narrow chemical composition, T x 0 is the T e of the equivalent narrow‐composition random copolymer, T A is the T e of chains composed of a single hard block, and T AB is the T e of a random copolymer with the same average composition of the whole OBC. This parameter can be used to interpret the degree to which the intra‐chain comonomer distribution is blocked; the larger the BI , the more “blocky” the polymer will be.…”

“…For each simulated OBC chain, the average comonomer content was used to estimate the block index ( BI ) using a linear relationship based on results previously reported, where CC is the mole fraction of α‐olefin of the OBC chains generated via Monte Carlo simulation.…”

“…While physical properties of OBCs have been thoroughly investigated, characterizing their detailed chain microstructures is a challenging task. Although temperature rising elution fractionation (TREF) of OBCs has been reported, a relationship between TREF results and chain microstructural distributions is difficult to be established because TREF does not directly probe the block length and composition distributions of these polymers.…”

Understanding the Formation of Linear Olefin Block Copolymers with Dynamic Monte Carlo Simulation

“…[1][2][3][4][5][6][7] OBCs have higher heat and abrasion resistances, and better processability than conventional polyolefin elastomers. [8][9][10][11] Because OBC may have many blocks, mathematical models are needed to quantify how polymerization conditions affect their microstructures. A detailed mathematical model describes how the complex OBC microstructure evolves during polymerization, providing useful insights on how to control these microstructures.…”

Recent Advances in High-Temperature Fractionation of Polyolefins

“…Shan and Hazlitt [29] were successful in developing a "block index methodology" by collecting fractions from P-TREF and then subjecting them to A-TREF. They noticed that olefin block copolymer fractions eluting at the same TREF temperature have much higher comonomer contents than comparable fractions of random copolymers.…”