Carbon Dioxide-Derived Poly(Propylene Carbonate) Bottlebrush Polymers: Synthesis, Viscoelastic Properties, and Degradation

Bottlebrush poly(propylene carbonate) (PPC) was synthesized with defined molecular dimensions in order to understand their relationship to viscoelastic properties. Chain-transfer polymerization of propylene oxide with CO2 using a tethered binuclear cobalt(III) salen catalyst afforded norbornene maleimide end-functionalized PPCs that yielded bottlebrush polymers through subsequent ring-opening metathesis polymerization. A series of PPC bottlebrushes were synthesized with varied side-chain (6 < Nsc < 198) and backbone lengths (100 < Nbb < 1,000). Several bottlebrushes were synthesized wherein the maleimide backbone was entangled or unentangled and/or PPC side-chains were entangled as evidenced by the material's plateau modulus measured by small amplitude oscillatory frequency sweeps. The brushes' rubbery moduli ranged from 29 to 485 kPa depending on the dimensions and volume fraction of PPC. As the dimensions of the backbone and side-chain were varied, the material densities and plateau moduli were used to calculate the bottlebrush crowding factor in order to demonstrate that the materials behave as backbone-extended bottlebrushes and not as densely grafted combs. The PPC side-chains could be depolymerized through a chain-end backbiting reaction to yield propylene carbonate or thermally cross-linked through transcarbonation reactions of the side-chains.