Study On Synthetic Methodology Of Tricomponent Miktoarm Star Polymers

This study was aimed at systematic synthesis of ABC, AB2C2 and AB4C4-type tricomponent miktoarm star polymers by combination of ring-opening polymerization (ROP), atom transfer radical polymerization (ATRP) and click reaction. Meanwhile, functional chain transfer agents (CTA) bearing styrene unit was synthesized and branched polymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) copolymerization of these CTAs with vinyl monomers. The main contents are shown as follows.In Part 1, ABC, AB2C2 and AB4C4-tpye miktoarm star polymers were synthesized and characterized. A trifunctional initiator 3-hydroxymethyl-5-prop-2-ynyloxybenzyl 2-bromo-2-methylpropionate (HPB) bearing alkynyl, hydroxyl and alkyl bromide moieties was synthesized and characerized by EA, NMR and FT-IR. Azide-functionalized methoxy polyethylene glycol (MPEG) was also synthesized. ABC and AB2C2 star polymers in which A was MPEG, B was poly(methyl methacrylate) (PMMA), polystyrene (PSt) or poly(butyl acrylate) (PBA), and C was polycaprolactone (PCL) were then obtained by combination of ATRP (normal ATRP and AGET ATRP), ROP and click reaction. The results indicated that the reaction conditions for ATRP, ROP and click reaction were compatible and could be efficiently used to synthesize target ABC and AB2C2 miktoarm star polymers via stepwise, two-step or one-pot method. These star polymers usually had controlled molecular weight and relatively low polydispersity (typically less than 1.2). However, fractional precipitation was usually necessary to obtain highly pure star polymers without homopolymer and diblock copolymer impurities.Meanwhile, a pentafunctional initiator containing one alkynyl moiety, two hydroxyl groups and two alkyl bromides was synthesized and used to prepare 9-arm AB4C4 star polymer. Unfortunately, its polydispersity was relatively high, so the reaction conditions need further optimization. The resultant star polymers were characterized by NMR, FT-IR, GPC and DSC. Part of amphiphilic star polymers was subjected to self-assembly in aqueous phase, and the results determined by dynamic laser light scattering showed they could form nanoobjects with average diameter of 100-200 nm and particle size distribution between 0.09 and 0.30.In Part 2, functional RAFT agents comprising styrene unit were synthesized, and branched polymers were obtained by RAFT copolymerization of these CTAs with vinyl monomers. Three kinds of styrene-based CTAs benzyl 4-vinylbenzodithioate (BVBD), S-4-vinylbenzyl S'-propyltrithiocarbonate (VBPT) and S-4-vinylbenzyl S'-3-(trimethoxysilyl)propyltrithiocarbonate (VTPT) were synthesized, and their chemical structures were confirmed by EA, NMR and FT-IR. Copolymerization of these CTAs with MA, MMA, St, DMA, DVB and EGDMA via RAFT process was performed. As compared with polymers obtained by RAFT copolymerization using feed ratio of [MA]0:[VBPT]0 = 5:1/10:1/20:1, the polymer obtained by polymerization using feed ratio of [MA]0:[VBPT]0 = 50:1 had much lower molecular weight. When RAFT copolymerization ([MA]0:[VBPT]0:[AIBN]0 = 100:1:0.1, [MA]0 = 3.0 mol/L) was conducted in toluene at 60 oC, the polymerization kinetics was investigated in detail. The polymerization could be classified into three stages. At early stage with conversion less than 30%, RAFT polymerization was performed smoothly, and most of the resultant polymer was PMA homopolymer with styrene unit in the chain end. At second stage in which monomer conversion was ranged between 30% and 80%, the polymerization rate was significantly increased, the kinetics was deviated from first-order linear relationship, and branched polymers were gradually formed. At last stage, branched polymers were further formed due to increased contribution of styrene unit during copolymerization and enhanced probability of biradical termination. For polymers obtained by RAFT copolymerization of VBPT with MA, the degree of branching was not very high, and the polydispersity could only reach up to 1.7 at high conversion. When divinyl monomer was added to the reaction mixture, however, the degree of branching could be further enhances, and the maximum polydispersity of polymers could reach up to 5 or more. The reaction conditions such as proportion of cross-linking agent, reaction time and temperature should be carefully controlled to avoid the formation of gel.When VTPT was used instead of VBPT, RAFT copolymerization could also afford branched polymers since styrene unit could participate in the copolymerization and trimethoxysilane could be subjected to hydrolysis and condensation polymerization. Moreover, when silica particles was added to reaction systems for RAFT copolymerization of VTPT with vinyl monomers, silica-polymer hybrids could be obtained by stepwise or one-pot method, and the weight grafting ratio of grafted polymer on silica surface was varied from 20% to 50%. In addition, it was found RAFT copolymerization of BVBD could only afford polymers with low molecular weight (typically less than 3000 g/mol) and polydispersity less than 1.5, so the reaction conditions need further investigation.In summary, we have developed and improved general approach to the synthesis of tricomponent star polymers and further understood detailed evolution to form branched polymers during RAFT copolymerization using vinyl-functionalized chain transfer agents.