Synthesis Of Hyperbranched, Rod-Coil Block Polymers, Polymer Nanogels And Study Of Their Properties

Reversible addition-fragmentation chain transfer(RAFT) living/control free radical polymerization is one of the most versatile methods to obtain well-defined polymers.Many novel structured polymers,such as block,graft,cyclic,star and hyperbranched polymers were synthesized conveniently by RAFT polymerization. Furthermore,the products obtained by RAFT polymerization can easily and high effectively be modified to thiol group capped polymers.Considering the unique reversible coupling and cleaving properties of thiol groups under mild redox circumstance,one can easily realized that the RAFT method should be a potential tool in application of biochemistry and bioconjunction.Aiming to get stimuli response degradable hydrogel,we focus on the well-known smart polymer-PNIPAM(poly(N-isopropylacrylamide)),whose aqueous solution behaved phase transition around its lower critical temperature(LCST).The strategy for synthesis of the novel hydrogel is proposed as follow:firstly,we combine RAFT process and self-condensing vinyl polymerization(SCVCP) to get hyperbranched PNIPAM,then,after aminolysis of the product,the polymer capped with multi-thiols will be obtained,finally,exposing the multi-thiols end-capped PNIPAM to air,the oxidation of the thiol will lead the polymers cross-linked by disulfide bonds.The results of experiment showed that this strategy was feasible when degrees of branching of polymers are high enough.The gels obtained by this strategy were able to decompose when reduced agent,such as Zn powder and DTT were added.Furthermore,at appropriate concentration,the gels with the sizes at nanometer scale were obtained.The aqueous solutions of the nanogels behaved reversible solution to gel trasition when temperature changed,the novel properties disappeared when DTT was added,accompanied by the size of nanogels turning small. The morphologies of the nanogels on fresh mica were different according the methods of preparing:when samples were dried by heating,the fiber like aggregations would be got,while samples were dried by high speed spinning,the aggregations were clusters looked like flowers.The interesting phenomena might contribute to the different states of the nanogel solution at different temperature.Considering the novel properties of PNIPAM and the unique structure of hyperbranched branched polymer,we also investigated the influence of branched structure to the phase transition of aqueous solution of PNIPAM with Micro Calorimetric Investigation.The influences were thought to be expressed by the random distributed hydrophobic units such as benzyl dendritic uints and thiocarbonylthio terminal units.The results showed that the phase transfer temperature(Tp) of aqueous solution of PNIPAM decreased sharply when degree of branching(DB) increased.Furthermore,after thiocarbonylthio end groups were eliminated by amine,the Tp increased greatly,the bigger the DB was,the greater the increasing was.When investigating the relationship of DB and T_p of the aminolized PNIPAM,the almost linear correlation was found,and we proposed this phenomenon might be due to the independent disturbing of hydrophobic benzyl units to water layer around PNIPAM,considering the branching units were comparted by linear units.The results might imply the probability of tuning the phase transfer temperature by changing the degree of branching of hyperbranched PNIPAM.The RAFT polymerizations are also appropriate for heterogeneous systems such as emulsion polymerization and miniemulsion polymerization.Different from traditional heterogeneous systems which need emulsifiers to stabilize the dispersed oil particles,we used ultrasound to encounter the aggregation of the particles caused by the tendency of reducing surface energy.We investigated the kinetic behavior of this system,molecular weights distribution of products obtained by this method.The results showed that the molecular weight distributions and polydispersities of the products were good(PDI＜1.3) and the polymers were end capped by thiocarboylthio groups.But,further analysis showed that the molecular weights obtained by GPC were greatly smaller than the ones calculated by ¹HNMR.We attributed the results mainly to the cleavage of the polymers in ultrasonic field,Aiming to synthesizing cationic nanogel which owns PDMAEMA as core and PEG as shell,we designed two strategies.One strategy is that:firstly,synthesizing hyperbranched PDMAEMA,and then getting multi-thiol end capped PDMAEMA as a core after aminolysis,finally,grafting acrylate group modified PEG to the core to form the shell.But,experiment showed that the inimer we used was not appropriate for RAFT process of the DMAEMA monomer.The other strategy is one-step to get the target polymer by emulsion polymerization with oligomeric PEG modified RAFT agent which behaved as emulsifier.The results showed that with this help of N, N'-methylenebisacrylamide,the obtained particles owned a compacted cross-linked PDMAEMA part and an unfolded PEG part. Self-assembly of rod-coil polymers were investigated as an independent work of this dissertation.In this work,we synthesized the ABA triblock polymers,the A block was polyaniline,which is known as rigid structure for the conjugated structure of the aromatic rings,the B block was poly(ethyl glycol) which acted as a random coil in its good solvents.The block polymers,PANI_x-PEG₁₁₀-PANI_x(PEG) were found self-assembling into sphere structure in THF,which is a good solvent for PEG.When diethyl ether was added to the THF solution of the block polymer,interesting structures such as cross-like rods and helical fibers were found.The results were explained by theory of critical packing parameters if the ABA block could form a hairpin structure byÏ€-Ï€stacking interaction between conjugated chains of PANI.