Making CO2-based Crystalline&Functional Polymers

CO2emission has aroused great attention in the world, and the focus of this issue has switched from simple CO2capture and landfill to its comprehensive utilization. CO2as starting material for building macromolecular polymers has become an important research field of polymer science. The coupling of epoxide and CO2to produce the biodegradable polycarbonate (CO2-based copolymer) has been regarded as the most promising green polymerization process. Although the commercialization of CO2-based copolymer has already obtained certain progress in recent years, the further development of this green technology is suffering from material performance as well as product type etc. One of the main problems is that all these previously reported CO2copolymers are amorphous and have low glass transition temperatures, which make the strength sharply decrease at enhanced temperatures. In terms of the above bottle-neck problems, in this dissertation, we made an effort to synthesize crystalline CO2-based copolymer. Moreover, we tried to produce functional CO2-based polymers from epoxides with electro-withdrawing groups, such as styrene oxide (SO) and epichlorohydrin (ECH) used in resin industry.1. Binary catalyst systems consisted of unsymmetric enantiopure salenCo(III) complexes and bis(triphenylphosphine)iminium chloride (PPNCl) were developed for the asymmetric alternating copolymerization of CO2and cyclohexene oxide, affording poly(cyclohexene carbonate)s with100%selectivity. The resultant copolymers have more than99%carbonate linkages, indicating the perfect alternating nature. It was found that the competition coordination of (S)-propylene oxide significantly improved the enantioselectivity regarding (S,S)-salenCo(Ⅲ) catalyst systems. With (S)-2-methyltetrahydrofuran as a chiral induction agent, the stereoregularity of the resultant poly(cyclohexene carbonate)(PCHC) is up to98%, the highest record in this asymmetric polymerization catalysis.2. The crystallization behavior of highly stereoregualr PCHC was studied by wide angle X-ray diffraction (WAXD) and atomic force microscopy (AFM). According to the strong diffraction peaks2θ at12.2,17.9,19.0,20.4°. the highly stereoregular PCHC is a typical semi-crystalline thermoplastic, possessing a high Tm of216℃. Crystallization shape observations show that the spherulitic morphology of (R)-PCHC grows in a clockwise spiral from a centre, while that of (S)-PCHC is counterclockwise spiral. Also, it is demonstrated the formation of stereocomplex from the blend of equivalent (R)-PCHC and (S)-PCHC with new strong diffraction peaks2θ=8.6,17.9,21.5°. Unlike its parent polymers, the PCHC stereocomplex presents lath-like dendritic crystal and better thermal deformation ability (Tm= 230℃). This is the first example of crystalline CO2-based polymer since the CO2/epoxide copolymeriztation was first reported in1969.3. Detailed study on the difference in reactivity between styrene oxide (SO) versus PO during the copolymerization with CO2has been conducted, with a focus on the catalytic reactivity, regio-and stereo-selectivity. Furthermore, the alternating copolymerization of CO2and SO to afford the corresponding polycarbonate with more than99%carbonate linkages was achieved with the use of the optimized catalyst systems. A living polymerization process was observed in the CO2/SO copolymrization on the basis of gel permeation chromatography (GPC). In order to assign carbonate signals of poly(styrene carbonate) microstructure, various model compounds to simulate three carbonate linkages of this CO2copolymer, including head-to-head, head-to-tail and tail-to-tail linkages, were synthesized. The highest regioselective ring-opening with96%configuration retention was obtained. Additionally, the reactivity of SO derivatives in the copolymerization with CO2was also tested. The CO2copolymer from4-cholro styrene oxide exhibits excellent thermal stability with a thermolysis temperature up to310℃and a high glass-transition temperature of92℃. The Fineman-Ross plots were performed to determining the monomer reactivity ratios in the CO2/SO/PO and CO2/SO/CHO terpolymerizations. The reactivity ratios indicate that it is favorable to alternating structure of the two different carbonates units, rather than homo-carbonate linkages in the CO2/SO/CHO terpolymers.4. On the basis of the copolymerization of SO/CO2, a systematic comparation of the difference in reactivity between epichlorohydrin (ECH) versus PO during the copolymerization with CO2has been well conducted. The alternating copolymerization of CO2and ECH to afford the corresponding polycarbonate with more than99%carbonate linkages was first achieved with the use of the optimized catalyst systems. Comparative kinetic measurements were performed as a function of reaction temperature to assess the activation barrier for production of cyclic carbonates and polycarbonates for the ECH/CO2and PO/CO2coupling systems. The relative small energy difference (45.4kJ/mol) for the ECH/CO2process accounts for the large quantity of cyclic carbonate produced compared with PO/CO2process. Electrospray ionization mass spectrometry (ESI-MS) in combination with single crystal structures of key intermediates can provide a straightforward approach to insight into the details of the copolymerization of CO2and ECH, regarding sterically hindered organic base7-methyl-1.5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD). A complex with the bulky groups on the3position has been developed for the highly selective copolymerization of CO2/ECH. The highest regioselective ring-opening with97%configuration retention at methine carbon was obtained. Wide angle X-ray diffraction and differencial scanning calorimeter show the stereospecific polymers exhibit good crystallizability with high melting boint at108℃.