Synthesis And Characterization Of Graft Copolymers And Their Derivatives With Alternating Grafts

Synthesis and properties of nonlinear copolymers have attracted much attentionrecently. The development of “living”/controlled polymerization techniques such as atomtransfer radical polymerization (ATRP), reversible addition-fragmentation chain transferpolymerization (RAFT), and ring-opening polymerization (ROP) has enabled facilesynthesis of well-defined complex macromolecular architectures. As well known,N-substituted maleimide (MI) and styrene (St) belongs to typical comonomer pairs to formalternating copolymers via copolymerization. At present, this system has been used toachieve many types of alternating copolymers, linear copolymers with alternatingcomonomer units and dendritic-linear block copolymers. To the best of our knowledge, theSt-MI copolymerization system has been scarcely utilized to prepare star polymers,branched polymers, graft polymers and their derivatives with alternating sequences ofmonomer units and pendent chains. Therefore, it is extremely important to generalize theSt-MI copolymerization system to construct these complex macromolecular architecturesand study their properties and potential applications. This research was aimed at synthesisand characterization of graft copolymers and their derivatives with alternating grafts bycombination of controlled polymerization, linking reaction and St-MI copolymerizationsystem. Some nonlinear polymers with alternating PEG and PCL side chains weresynthesized, and main examples comprised disulfide-linked symmetric A2mB2nC2starliketerpolymers, couplable A(BC)mD heterografted comblike block quaterpolymers, and A2B2miktobrush copolymers with alternating V-shaped grafts. On this basis, versatilepostpolymerization reactions such as click chemistry and quaternization were used totrigger topological transforms. In addition, the structure-property relationships wereinvestigated, and the potential applications were also studied. The main contents are listedas follows.The research in Part1aimed at facile synthesis and versatile topologicaltransformation of disulfide-linked symmetric A2mB2nC2starlike terpolymers. First, RAFTcopolymerization of vinyl benzyl-terminated PEG (St-PEG) with N-(2-hydroxyethyl)maleimide (HEMI) in an equimolar feed ratio afforded well-defined starlike S-CP(St-PEG-alt-HEMI)(denoted as PSH). Second, PSH-mediated chainextension polymerization of vinyl monomers such as N,N-dimethylaminoethylmethacrylate (DMA), N-isopropylacrylamide (NIPAM) and tert-butyl acrylate (tBA)afforded (PEG)2m(PM)2stars with controlled chain length of PM arms. Last, ROP ofε-caprolactone (CL) initiated with (PEG)2m(PM)2mmacroinitiators gave(PEG)2m(PCL)2n(PM)2starlike terpolymers. Meanwhile, hydrolysis was used to generate(PEG)2m(PCL)2n(PAA)2. As evidenced by1H NMR and GPC-MALLS, the resultantcopolymers had well-controlled molecular weights, low polydispersity indices (PDI=1.08-1.18) and alternating PEG and PCL segments (m≈n≈3). Moreover, the presence ofdisulfide and dithiobenzoate moieties in starlike copolymers allowed versatile topologicaltransformation. By postpolymerization reactions including reduction, aminolysis andoxidative coupling, topological transformations among (PEG)2m(PCL)2n(PM)2,(PEG)m(PCL)nPM and comblike-linear multiblock copolymer could be efficientlyperformed. Upon pH and reduction stimuli, the drug release kinetics of DOX-loadedaggregates could be efficiently adjusted, revealing the potential application in biomedicalfield.The research in Part2was focused on synthesis and versatile postpolymerizationmodification of A(BC)mD comblike block quaterpolymers. RAFT polymerization of DMAmediated by prop-2-ynyl4-(benzodithioyl)-4-cyanopentanoate (PBCP), and followed byRAFT copolymerization of St-PEG and maleimidic poly(ε-caprolactone)(MI-PCL) togenerate A(BC)mtoothbrushlike copolymers. After chain extension polymerization of vinylmonomers and selective hydrolysis of PtBA segment, some A(BC)mD (A=PDMA; B=PEG; C=PCL; D=PNIPAM, PtBA, PMA and PAA) copolymers were successfullyachieved. The general strategy has some advantages such as straightforward synthesis andwell-controlled molecular weight, microstructure and chemical constitution. Thesequaterpolymers possessed an alkyne, a dithiobenzoate moiety, and multiple tertiary aminoand hydroxyl functionalities, allowing versatile postmodification reactions. Thecombination of end group transformation and azide-alkyne “click” reaction affordedcomblike-linear multiblock copolymer and dendritic graft copolymer, and Menschutkinreaction between A(BC)mD copolymer and bromide-functionalized small molecule/macromolecule gave ion-bearing A’(BC)mD comblike block copolymer andA(BC)mD-graft-E toothbrushlike copolymer. The preparation of two types of novelarchitectures revealed the great potential of postpolymerization modification in novel architecture construction and topological transformation. Preliminary results indicated thatthe introduction of various functionalities and polymer segments into brushlike copolymersvia the quaternization process could affect the chain relaxation and melting behaviors,solubility, and surface wettability of polymer films. This research afforded a facile way tosynthesize sequence-regulated comblike block copolymers and their derivatives, whichhave a great potential in materials science and nanotechnology.The research in last part aimed at facile synthesis and properties of well-defined A2B2miktobrush copolymers with alternating V-shaped grafts. Two types of inimers MI(OH)2and St(Br)2were synthesized and subjected to RAFT copolymerization, and followed byelimination reaction of dithiobenzoate to afford poly(St(Br)2-alt-MI(OH)2)(PSM)macroinitiator with multiple bromine and hydroxyl functionalities. Then, ATRP, ROP andhydrolysis were performed to synthesize A2B2(A=PSt, PNIPAM, PMEMA, PtBA, PAA)miktobrush copolymers with alternating V-shaped grafts. The resultant V-shapedcopolymers had well-controlled molecular weight and relatively low polydispersity (PDI=1.07-1.22). The copolymers with PAA and PNIPAM segments were self-assembled intocopolymer micelles, and they could efficiently encapsulate an anticancer drug, doxorubicin(DOX) and perform controlled release sensitive to pH and temperature. As compared withtheir linear analogues and random miktobrushes, miktobrush copolymers with alternatingV-shaped grafts are expected to exhibit unique physicochemical properties.In summary, this study was primarily focused on continuous development of efficientapproaches to the synthesis of graft copolymers with alternating grafts. The St-MIcopolymerization system was successfully used to synthesize novel graft copolymers andtheir derivatives with alternating grafts. Versatile postpolymerization modification allowedirreversible and reversible transformations among different macromolecular architectures,and their physicochemical properties were also preliminarily investigated. Our study notonly enriches the family of sequence controlled polymers but accelerates the developmentof advanced polymer synthesis. The success in facile synthesis of novel sequence-regulatedbrushlike polymers and their derivatives stands for latest advance in topologicalconstruction of nonlinear copolymers.