Manipulating polymer topology is an alternative method to alter the physical properties of polyolefins without the need for extrinsic modifiers. For example, cyclic polymers exhibit different solution and bulk properties as compared to their linear analogs. Thus, investigations into the cyclic analog of commodity polyolefins may provide an insight into changes in the solution and bulk properties. The large-scale synthesis of cyclic polyolefins, however, is synthetically challenging. Herein, we report ring-expansion polymerization (REP) of alkynes by using a tethered tetra-anionic pincer ligand-supported tungsten catalyst. Subsequent post-polymerization modifications of the products provide access to cyclic polymers in large batches. We compared these polymers to their linear analogs using light scattering measurements, solution viscometry, thermal analysis, and rheology. In all cases, differences were observed which could be attributed solely to the cyclic architecture. To obtain visual confirmation of the cyclic topology by atomic force microscopy (AFM), we devised a REP and subsequent atom-transfer radical polymerization technique to generate cyclic bottlebrush polymers, i.e., macrocycles that contain pendant side-chain polymers on nearly every repeat unit of the cyclic chain. Indeed, by grafting side chains of polystyrene and poly(butyl acrylate) from the cyclic polymer backbones, direct visual confirmation via AFM was made possible. To further explore the potential scope of this post-polymerization modification strategy, amphiphilic cyclic bottlebrush polymers were produced by grafting amphiphilic block copolymer side chains onto the cyclic polymer backbone. Interestingly, these polymers were found to self-assemble into a unique morphology of spheres with a single pore in water. In contrast, under identical experimental conditions, linear bottlebrush polymers did not aggregate into this morphology but rather exhibited a porous sphere morphology.

Cyclic Polyolefins: Synthesis, characterization, and post-polymerization modifications

ACS Spring 2022

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