The Synthesis,Characterization And Mechanism Of CO₂?COS?-Based Block Copolymers

Carbon dioxide?CO₂?and carbonyl sulfide?COS?are two important Cl monomers which have been explored to couple with various epoxides producing aliphatic polycarbonates?APCs?or polythiocarbonates.The structure and properties of linear APCs greatly limited its vast application.In this work we mainly focused on the building of block copolymers based on CO₂?or COS?/epoxides copolymerization via a one-pot/one-step strategy.We also combined living/controlled radical polymerization and immortal copolymerization to synthesize well-defined poly?thiocarbonate?-block-polyacrylate.In addition,we further investigated the oxygen/sulfur exchange reaction?O/S ER?during the COS/propylene oxide?PO?copolymerization.First,heterogeneous catalysis of zinc-cobalt double-metal cyanide complex[Zn-Co???DMCC]for the synthesis of poly??-caprolactone??PCL?is described for the first time.Zn-Co???DMCC in companied with epoxides could initiate the ring-opening polymerization?ROP?of s-caprolactone??-CL?effectively.A key feature of this system is the ability to afford PCL with high molecular weights.Zn-Co?III?DMCC with cyclohexene oxide?CHO?with molar ratios of 1-1.5:100 afforded PCL with high number-average molecular weight?Mn?of up to 177.7 kg/mol determined by the gel permeation chromatography?GPC?.In addition,results of 1H NMR and MALDI-TOF mass spectrometry suggested that CHO acted as a bi-functional initiator for the ROP of s_CL through the formation of zinc alkoxide via CHO ring-opening,which was the active site for the polymerization.Second,we investigated the first example of multiblock copolymer?MBC?with biodegradable polycarbonate and polyester blocks.It is a long-standing challenge to combine mixed monomers into MBC in a one-pot/one-step polymerization manner.MBC were synthesized from highly efficient one-pot/one-step polymerization of cyclohexene oxide?CHO?;,CO₂ and ?-caprolactone??-CL?in the presence of zinc-cobalt double metal cyanide complex and stannous octoate[Sn?Oct?₂].In this protocol,two cross-chain exchange reactions?CCER?occurred at dual catalysts respectively and connected two independent chain propagation procedures?i.e.,polycarbonate formation and polyester formation?simultaneously in a block-by-block manner,affording MBC without tapering structure.The multiblock structure of MBC was determined by the rate ratio of CCER to the two chain propagations and could be simply tuned by various kinetic factors.This protocol is also of significance due to partial utilization of renewable CO₂ and improved mechanical properties of the resultant MBC.Third,the origin of the regio-defects in the process of synthesizing poly?monothiocarbonate?through PO/COS copolymerization is disclosed.Small quantities of regio-defects in a regio-/stereoregular polymer weaken its tacticity and properties.We clarified the origin of the regio-defects in the process of PO/COS copolymerization,using?salen?CrCI complex accompanied by bis?triphenylphosphoranylidene?ammonium chloride?PPNC1?.Quantitative characterization results from the MALDI-TOFMS and ¹H??¹3?C?NMR spectroscopy suggested that the chain transfer reaction resulted in the regio-defect in the final copolymer,i.e.,tail-to-tail?T-T?diad and dithiocarbonate unit?DTC?.The chain transferring to water in the reaction system led to the production of a?salen?Cr-OH intermediate,which initiated the copolymerization via either attacking PO first to result in formation of a T-T diad or first activating COS to produce mercapto?-SH?end-capped dormant chains via decarboxylation,thus generating a DTC unit in the final product through another chain transfer reaction and regrowth of the chain.The content of regio-defect in the final copolymer was directly related to the water content in the system.It is essential to reduce the regio-defect for an immortal COS/PO copolymerization reaction by eliminating trace amounts of water.We also demonstrated the application of a-OH,co-OH poly?propylene monothiocarbonate?for synthesizing a well-defined ABA triblock copolymer,polystyrene-block-poly?propylene monothiocarbonate?-block-polystyrene?PS-b-PPMTC-b-PS?,with a Mn of 10.8 kg/mol and a D of 1.08 via an atom transfer radical polymerization?ATRP?method.Finally,in order to synthesize block copolymer via a one-pot/one-step strategy,we explored a new system of trimethylene oxide?OX?/COS/?-CL terpolymerization catalyzed by?salen?CrCl complex accompanied with 1,5,7-triazabicyclo[4.4.0]dec-5-ene?TBD?.DSC tests and ¹H??¹3?C?NMR spectroscopy suggested that PCL was introduced into the PMTC chain in a block or segment manner.The crystallization of PCL was dramatically restricted by the PMTC segments while the solubility of PMTC was greatly improved by the incorporation of PCL segments.Model diblock copolymer PCL-b-PMTC was synthesized via sequential polymerization of ?-CL and then COS/OX in one-pot and the junction units were determined.Further investigation on the mechanism and the copolymer sequence is under work.In summary,immortal polymerization was explored in depth to synthesize block copolymers by controlling either the structure of active center during polymerization or the structure of end groups after polymerization.On the base of immortal polymerization process,we developed a "catalysts complex" for converting mixed monomers into block copolymers via one-pot/one-step strategy and also successfully synthesized COS-based block copolymer via a combination of two polymerization mechanisms or synthetic methods.Enhanced properties were obtained due to the block structure.