Synergetic Organocatalysis For The Efficient Synthesis Of Renewable Polyesters

Synthetic polymers have become indispensable materials essential for modern human civilization.However,most synthetic polymers are obtained from petroleum-based feedstocks and cannot be naturally degraded,causing severe worldwide environmental consequences.Therefore,the conversion of renewable feedstocks to sustainable polymers,which not only significantly reduces the dependence of petroleum-based feedstocks,but also produces functional polymer materials such as polyamino acids,polyesters,and polycarbonates bearing pendant functional groups,has become one of the most important frontiers in polymeric chemistry.Catalysis presents an enabling tool to facilitate the development of sustainable polymers,and the development of renewable feedstocks to sustainable polymers has been driven by the innovations of new and efficient catalysts.However,typical H-bonding organocatalysts,such as thioureas,are subject to a single catalytic site,limited activation mode,and are still challenging in achieving stereoselective polymerization of emerging monomers to produce stereoregular polymers.In nature,enzyme achieved high efficiency and selectivity through synergistic fashion between multiple and inequivalent catalytic sites for substrate activation.Inspired by this,the design and development of organocatalytic systems with differentiated catalytic sites for the synthesis of sustainable polymers is of great significance.The main results obtained in this dissertation are summarized as follows:1 We proposed a strategy that the tug-of-war between two distinct catalytic sites enables fast and selective ring-opening copolymerizations(ROCOP).We designed and synthesized a series of H-bond-donor/Lewis-acidic-boron organocatalyst featuring two distinct catalytic sites in one molecule,which are simple and efficient to synthesize and can be modularly tuned.The ring-opening copolymerization of epoxides with anhydride mediated by these modular,and tunable catalysts were at the upper end for the organocatalysts field as a whole,and is also 3 × faster than the heterodinuclear Al(Ⅲ)/K(Ⅰ)catalyst reported by Williams,and exhibts high selectivity(>99%polyester selectivity).The results of 1H NMR,11B NMR spectra and crystal structure analysis indicated a tug-of-war mechanism.The quantum chemical studies further support the hypothesis of a chain tug-of-war mechanism.Thiourea can exert a dual H-bonding interaction to perform the "tug-of-war" on the carboxylate propagating chain-end,which facilitates the binding of epoxide to boron center and enhances the nucleophilicity of the propagating intermediates,thus resulting in exceptionally high activity while maintaining perfect polyester selectivity.This mechanism indeed provides a rationale for the maximal activity that observed in the medium-acidic thiourea system:the less-acidic thiourea diminishes its ability to pull off the propagating chain-end from boron center,disfavoring epoxide activation,whereas the more-acidic thiourea typically reduces the nucleophilic attack.2 Based on the previous research work,we designed and synthesized a series of H-bond-donor/Lewis-acidic-boron organocatalysts possessing three distinct catalytic sites and showed that these can catalyze rapid and selective ROCOP of propylene oxide with carbon dioxide at mild conditions(25-50℃)with>99%polymer selectivity(polycarbonate/cyclic carbonate)and quantitative CO2 uptake.Experiments and quantum chemical studies further revealed a tug-of-war mechanism between three differentiated catalytic sites.One boron catalytic site is involved in activating the epoxide monomer,and the other boron is used to stabilize the propagating chain-end.Thiourea can exert a dual H-bonding interaction to perform the "tug-of-war" on the carbonate propagating chain-end,which enhances the nucleophilicity of the propagating intermediates.This mechanism provides a rationale for the maximal activity that observed in the medium-acidic thiourea system:the less-acidic and more-acidic thiourea reduce the nucleophilic attack.3 We developed chiral phosphoric acid catalyzed stereoselective ring-opening polymerization of racemic eight-membered cyclic diolide(rac-DL).The influence of steric and electronic effect of catalyst substituents on stereoselective ring-opening polymerization were investigated,and the gradient isotactic multiblock polyester with controllable molecular weight and molecular weight distribution was obtained.The experimental and calculation analysis provide a plausible insight for the formation of the gradient isotactic multiblock polyester.In kinetically,R-chain or S-chain show a kinetic preference for the selection of same chirality monomer implying the presence of chain-end control mechanism(CEM).Nevertheless,thermodynamically,the overall formation of a new S-chain is always preferred by chiral phosphoric acid catalyst in comparison with a new R-chain,indicating that an enantiomorphic-site control mechanism(SEM)also occurs.Taken together,these results demonstrate that this stereoselective ROP proceeds through a combination of CEM and SEM to give isotactic multiblock polyesters.4 In addition to the above works,we also synthesized sustainable aliphatic polyamides from renewable 4-hydroxyproline.We describe that 4-hydroxyproline(4-HYP),a renewable resource,was readily converted to its corresponding bicyclic bridged lactam monomers bearing pendant Boc-protected group,and oligo-ethylene glycol group.Lithium hexamethyldisilazide(LiHMDS)-mediated polymerization of the resulting monomers exhibited a controlled feature,affording sustainable aliphatic polyamides with number-average molecular weight up to 73 kg/mol and a low molecular weight distribution(D<1.28).Overall,this work can lead to a novel kind of functional and sustainable polyamides with potential applications,including degradable plastics,and drug delivery.