Synthesis And Depolymerization Of Star-Shaped Poly(γ-Butyrolactone)and γ-Butyrolactone-Based Polyvinyl Ester

γ-Butyrolactone(γ-BL)is a cyclic lactone with a stable five-membered ring,which is inexpensive,has a natural and renewable green source,and is a downstream product of succinic acid,which the U.S.Department of Energy recently listed as one of the most suitable biomass-derived compounds to replace petrochemicals.As a five-membered cyclic lactone,γ-BL has an extremely low ring strain and was considered non-polymerizable in the past.However,in 2016,Miao Hong and Eugene Chen reported the first efficient and successful ring-opening polymerization of γ-butyrolactone.and they found that the synthesized poly(γ-butyrolactone)(PγBL)could be 100%thermally depolymerized toγ-BL monomer,which could be utilized for repolymerization,enabling a monomer-polymer-monomer closed-looped process.This landmark work in the field of recyclable polymer research has completely opened up the research field of recyclable polymers,such as PγBL.and has attracted widespread attention.Currently,the design of catalyst systems and the exploration of polymerization methodology are mainly focused on this field.More synthesis and properties ofγ-butyrolactone-based polymer derivatives based on the equilibrium of polymerization and depolymerization have important scientific value and practical value.This thesis successfully achieved controllable star-shaped PγBL by the ring-opening polymerization ofγ-BL initiated by polyhydroxy compounds.In this system,phosphazene base/thiourea as a binary catalytic system plays a vital role in the controllable synthesis of star-shaped PγBL.Secondly,initiators with different hydroxyl numbers determine the number of arms of the final synthesized star-shaped PγBL.The ring-opening polymerization of γ-BL initiated by compounds with varying numbers of hydroxyl groups exhibits different polymerization kinetics,and the synthesized star polymers and linear polymers also exhibit different thermal properties and depolymerization behavior.Based on the unique depolymerization properties of γ-BL,in this thesis,γ-BL was also applied as a raw material to synthesize a new type of vinyl ester monomer with γ-BL ring-opening structure as a side group.Following the classical free radical polymerization,a new type of polyvinyl ester(PVBT)with very different properties from the commonly used polyvinyl acetate(PVAc)was finally obtained.The side groups of PVBT can also be easily removed to obtain polyvinyl alcohol(PVA)by simple operations,and the γ-BL raw material is 100%recovered,which can be reused for synthesizing monomers.The thesis is discussed in detail as follows:(1)First,various catalytic systems,including 4,4-Dimethylaminopyridine/1,3-diisopropyl-2-thiourea(DMAP/TU),1,5,7-triazabicyclo[4.4.0]-5-decene/1,3-diisopropyl-2-thiourea(TBD/TU),sodium hydride/1,3-diisopropyl-2-thiourea(NaH/TU),n-butyllithium/1,3-di Isopropyl-2-thiourea(nBuLi/TU)and superbase phosphazene/1,3-diisopropyl-2-thiourea(tBu-P4/TU),were explored in the ring-opening polymerization of γ-BL.It was found that only the tBu-P4/TU catalytic system achieved the controllable synthesis of PγBL,and the polymerization rate was also the fastest.Then,star-shaped PγBLs with different numbers of arms were synthesized by using compounds with varying numbers of hydroxyl groups as initiators by a nucleus-first-arm strategy,and the influence of solvents and initiators on the kinetics of ring-opening polymerization were explored.When benzyl alcohol was used as the initiator,the polymerization of γ-BL was faster in toluene than in tetrahydrofuran.In addition,in tetrahydrofuran,the order of γ-BL polymerization rate is amylglycerol>benzyl alcohol>pentaerythritol>propylene glycol.In this study,the structures of star-shaped PγBL with different arm numbers were confinned by various characterization methods,such as NMR,MALDI-TOF MS and SEC.Compared with the linear PγBL,star-shaped PγBLs has higher Tg and lower Tm,Tc and Td as the number of polymer arms increases.In addition,the depolymerization rate of star-shaped PyBL is faster than that of linear PyBL,and its depolymerization rate becomes faster and faster as the number of polymer arms increases.(2)Using γ-BL as raw material,a new type of vinyl ester monomer with γ-BL ring-opening structure as the pendant group was synthesized.Subsequently,the corresponding PVBT was synthesized by classical radical polymerization.When the γ-BL ring-opening structure on the pendant group is protected by a tert-butyldiphenylsilyl group,PVBT is stable and can be used for further processing.Compared with PVAc with similar molecular weight,the elongation at break(εB)of PVBT can reach 8100%,which is 30 times that of PVAc.And compared with PVAc,PVBT exhibits stronger thermal stability and lower glass transition temperature(Tg)due to its longer silicon-containing side groups.When the tert-butyldiphenylsilyl protecting group is controllably removed,PVBT will controllably release PVA fragments,generate PVA-PVBT copolymers with different PVA contents,and finally synthesize PVA homopolymers.Notably,the raw material γ-BL can be 100%recovered and reused for monomer synthesis during deprotection.In addition,PVA prepared from PVBT has higher thermal stability and lower Tg than PVA prepared from PVAc.In this paper,the synthesis of star-shaped PγBL and a new type of polyvinyl ester PVBT not only opens up a new way for the performance adjustment of PγBL,but also provides a new idea for expanding the application of γ-BL,which has important scientific value and practical application value.