The Synthesis,Structure And Properties Of CO₂-based Linear And Brush Copolymers

In this work, the synthetic methods for chemical fixation of carbon dioxide (CO2) into polymers were studied. Two synthetic routes were used to convert CO2 to various copolymers:(1) CO2 was introduced into the main chain via the copolymerization of CO2 with epoxide, leading to polycarbonate with low Tg, then polyurethane and tri-block copolymer were obtained by chain extension reaction of the resultant copolymer; (2) CO2 was coupled with epoxy side group of a polymer to get cyclic carbonate-functionalized polymers, and well-defined brush copolymers were derivated.The copolymerization of CO2 and epoxides with long alkyl side chain could endow the resulting polycarbonates with low Tg. These CO2-based polycarbonates would be used for the synthesis of biodegradable elastomer. Catalysts that can catalyze the copolymerization of CO2 and these epoxides were rarely reported due to the low reactivity of these monomers. Moreover, CO2-based polycarbonate with low Tg was equally rare. Herein,10-undecenoic acid, which was derived from the plant oil, was successfully used as raw material to synthesize epoxy methyl 10-undecenoate (EMU). The alternating polycarbonates (Fco2> 99%) were obtained by the coplymerization of CO2 and EMU at 40-80℃ using a zinc-cobalt (III) double metal cyanide complex [Zn-Co(III) DMCC] catalyst. The EMU-CO2 copolymers had two hydroxyl (-OH) end groups, which was evidenced by the electrospray ionization-tandem mass spectrometry (ESI-MS). One came from Zn-OH initiation, and the other was produced by the chain transfer reaction of the propagating chain to H2O (or other proton compounds). EMU-CO2 copolymers exhibited pretty low Tgs at-38～44℃, and this could be attributed to the internal plasticization effect of the long alkyl side chain with ester groups. With the low Tg and two hydroxyl end groups, the EMU-CO2 copolymers were further reacted with 4,4’-methylenediphenyl diisocyanate (MDI) to get some new polyurethanes. Furthermore, the EMU-CO2 copolymer could be used as a macroinitiator to initiate ring-opening polymerization (ROP) of a commercial bio-based monomer (L-lactide) via DBU (1,8-diazabicyclo [5.4.0] undec-7-ene) catalysis at mild condition.Up to date, most studies were focused on introducing CO2 into the polymer backbone, while only limited synthetic methods were reported to introduce CO2 into the side chain of a polymer. In the present work, glycidyl methacrylate (GMA), CO2 and methoxypolyethylene glycol amine (MPEG-NH2) were used as the initial materials. PGMA with pending epoxy group was synthesized through atom transfer radical polymerization (ATRP) method, which were then completely converted to the cyclic carbonate groups by coupling reaction with CO2 using [Zn-Co(III) DMCC] as catalyst and cetyltrimethyl ammonium bromide (CTAB) as co-catalyst. The resulting polymer poly(1,3-dioxolan-2-one-4-ylmethyl methacrylate) (PDOMA) had a weight percentage of CO2 of 24 wt% as well as a high Tg of 141.4 ℃ which could be comparable with some commercial engineering plastics. Based on the cyclic carbonate-diamine addition reation, new amphiphilic brush copolymers PGMA-g-PEG with high grafting densities of 97-98% through "grafting onto" method were achieved. The obtained PGMA-g-PEG contained hydroxyl and hydroxyurethane structures that could be used for further chemical modification. Such junction sites between the PGMA backbone and PEG side chain were gernerated by the cyclic carbonate-primary amine addition reaction. The melting temperature (Tm) and crystallization temperature (Tc) of PGMA-g-PEGn increased with the length of the PEG side chain. Furthermore, we observed that these brush copolymers could self-assemble into stable micelles in aqueous solution. The core of the formed micelles was the PGMA main chain accompanied by the shell of the PEG side chain. The micelles were spherical in shape with radius of ca.195 ±31 nm as measured by transmission electron microscopy (TEM). Moreover, the dynamic light scattering (DLS) measurement showed the PGMA-g-PEG44 micelles in aqueous solution had an effective particle size of 420 nm and a polydispersity of 0.292.In summary, two synthetic methods for fixing CO2 into polymers were developed. The resulting CO2-based polymers were linear and brushlike, respectively. The obtained linear polymer had a pretty low Tg, and could be potentially applied in the synthesis of biodegradable elastomer. The amphiphilic brush copolymer of PGMA-g-PEG with high grafting densities was proposed to serve as biomaterial for pharmaceutical application.