Precision Aliphatic Polyesters And Poly(ester Amide)s Via Segmer Assembly Polymerization

Sequence-controlled polymerization is a promising way to establish a precise structure-property relationship and to control the properties of polymers,and is regarded as the Holy Grail of polymer science.Segmer assembly polymerization ideally combines the precision of organic synthesis and the high efficiency of polymerization reaction.Encoded information such as sequence,tacticity and length within monomers can be completely transferred to each repeating unit of periodic copolymers.The macroscopic properties of polymers are fundamentally determined by microstructural information through a bottom-up approach.In this dissertation,a series of periodic polyesters and poly（ester amide）s were synthesized based on segmer assembly polymerization in a facile and efficient pathway.Precise structure-property relationships of the polymers were studied.A novel postpolymerization modification methodology is demonstrated to achieve homo-and hetero-functionalization of hyperbranched polymer by using thiol-Michael chemistries.（1）A family of AB-type monomers（terminal groups areα-olefin and acrylate respectively）were successfully synthesized.Cross-metathesis polymerization of the monomers was demonstrated to be an efficient synthetic method for the generation of periodic aliphatic polyesters,and the relationship between the primary structures（sequence,tacticity）and the thermal and optical properties of these copolymers was investigated.Asymmetric monomers bearing acrylate andα-olefin terminuses with precise structures were synthesized by efficient organic reactions.Cross-metathesis polymerization of the monomers and exhaustive hydrogenation furnished alternating copolymers:poly（glycolic acid-alt-ε-caprolactone）and poly（lactic acid-alt-ε-caprolactone）.Theε-caprolactone-derived monomer unit was generated in situ rather than introduced to tailor made monomers prior to cross-metathesis polymerization.The results showed that methyl side group disrupt the regular packing of the polyester main chain,and the crystalline region is hindered.The former was crystalline state,while the latter was amorphous.Then we improved the synthetic route and successfully obtained two types of copolymers:poly（_D-lactic acid-alt-ε-caprolactone）and poly（_L-lactic acid-alt-ε-caprolactone）,realizing dual control over sequence and tacticity for this system.Stereocomplex is obtained by equimolar mixing of an amorphous,enantiomerically pure _D/L-polyester,which exhibits a sharp melting peak on differential scanning calorimetry curve.（2）A novel postpolymerization modification methodology is demonstrated to achieve homo-and hetero-functionalization of hyperbranched polymer.Terminal and internal acrylates of hyperbranched polymer derived from cross-metathesis polymerization are functionalized in a chemoselective fashion using carefully designed thiol-Michael chemistries.Model reactions between different thiols（benzyl mercaptan and methyl thioglycolate）and acrylates（n-hexyl acrylate and ethyl trans-2-decenoate）by phosphine-initiated and base-catalyzed thiol-Michael reactions are investigated to optimize the modification protocol.Then,high molecular weight hyperbranched polymer P0 is generated through cross-metathesis of AB₂ monomer（bearing oneα-olefin and two acrylates）.Chemo-selective modification of terminal and internal acrylates of hyperbranched polymer was realized in one-step complete（amylamine-catalyzed thiol-Michael addition by using methyl thioglycolate）and sequential modification（DMPP-initiated thiol-Michael addition using benzyl mercaptan,followed by amylamine-catalyzed thiol-Michael reaction using methyl thioglycolate）,which provides a new paradigm of homo-and hetero-functionalization of hyperbranched polymers by using thiol-Michael reaction.（3）A new and efficient strategy for the synthesis of high molecular weight alternatingε-caprolactone/ε-caprolactam copolymers.The thermal property,crystalline structure,surface wettability,upper critical solution temperature behavior and degradation kinetics were studied in detail,which shows a close correlation between structure（composition and sequence）and physicochemical properties of those polymers.Two monomers bearing halogen and carboxylic acid end groups were obtained in a single step and in high yields.Subsequently,repetitive nucleophilic displacement of the monomers furnished poly（ester amide）s with high structural fidelity.It is concluded that physicochemical properties of poly（ε-caprolactone）,poly（ε-caprolactam）andε-caprolactone/ε-caprolactam copolymers,phase transition behaviors in particular,are fundamentally determined by composition and sequence.This step-growth polymerization method shows unique advantages of high efficiency,structural integrity,mild condition and ease of operation.