No abstract

High-density polyethylene (HDPE) is utilized in a multitude of commercial products worldwide due to its broad spectrum of physical properties and low production costs. Molecular weight and dispersity (Đ) are known to affect the tensile and rheological properties of HDPE, but little is known about the influence of the molecular weight distribution (MWD) shape on these properties. In this work, we investigate this matter through the temporal regulation of initiation in a living coordination−insertion polymerization of ethylene. This method provides precise control over the MWD shape which, in turn, offers a systematic study on the influence of MWD shape on the physical properties of HDPE. Through rheological testing, we observe a difference in complex viscosity and shear thinning with opposite MWD skew. However, tensile testing reveals that the MWD skew does not impact the strain at break, signifying the ability to influence HDPE processing without compromising material strength.

Regioselective carbyne-transfer reagents derived from (3,3,3-trifluoroprop-1-yn-1-yl)benzene give access to functionalized ring-opening alkyne metathesis polymerization (ROAMP) initiators [R-CHC≡Mo(OC(CH)(CF))] featuring electron-donating or -withdrawing substituents on the benzylidyne. Kinetic studies and linear free-energy relationships reveal that the initiation step of the ring-opening alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo[a,e][8]annulene exhibits a moderate positive Hammett reaction constant (ρ = +0.36). ROAMP catalysts featuring electron-withdrawing benzylidynes not only selectively increase the rate of initiation (k) over the rate of propagation (k) but also prevent undesired intra- and intermolecular chain-transfer processes, giving access to linear poly-(o-phenylene ethynylene) with narrow molecular weight distribution. The regioselective carbyne transfer methodology and the detailed mechanistic insight enabled the design of a bifunctional ROAMP-reversible addition-fragmentation chain-transfer (RAFT) initiator complex. ROAMP followed by RAFT polymerization yields hybrid poly-(o-phenylene ethynylene)-block-poly-(methyl acrylate) block copolymers.

Recent developments in photocontrolled polymerizations have facilitated the development of previously inaccessible materials. While photocontrolled radical polymerizations have been extensively studied, related processes involving cationic polymerizations are underexplored and limited...

Reversible addition-fragmentation chain-transfer (RAFT) polymerizations are one of the most versatile and powerful polymerization techniques for the synthesis of complex macromolecular architectures. While RAFT polymerizations often give polymers with narrow...