Synthesis and Dilute Solution Properties of Precision Short-Chain Branched Poly(ethylene) Block Copolymers Derived...

Short-chain branching (SCB) is widely applied in poly(ethylene) as a convenient platform for tuning processability and final properties of the material; however, inter- and intramolecular variations in branch distribution make linear low-density poly(ethylene) (LLDPE) difficult to characterize effectively, especially when considering different branch distributions and copolymer structures (e.g., blocky, statistical). The synthesis and characterization of model materials is an important step to increase the understanding of the structure-property relationships of polyolefins and how these behaviors can be used to identify and analytically separate different branching structures. Herein, a model block copolymer with regioregular branch spacing in the "A" block and a linear "B" block is synthesized from the one-pot sequential ring-opening metathesis polymerization of 1-butyl-trans-cyclooctene (C4) and 1,5-cis,cis-cyclooctadiene (COD), respectively. Excess 3-bromopyridine (BrP) is added to the polymerization of COD to reversibly deactivate the Ru catalyst center, affording improved control over polymerization kinetics and limiting secondary metathesis to yield precisely branched block copolymers. Hydrogenation and high-temperature size exclusion chromatography (HT-SEC) of these model materials reveal a systematic decrease in intrinsic viscosity ([η]) with increasing branch content and a similar decrease in the radius of gyration (R _g) associated with the compaction of chains with increasing SCB content. Moreover, the block copolymers have higher values of [η] than their statistical copolymer counterparts, indicating that branch distribution plays a role in the dilute solution properties of short-chain branched polyolefins.