142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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28 B. Charleux, R. Faust quence order with a complete shift of the peak to higher MW after the second step. The products, obtained after quenching with methanol, were analyzed by 1 H NMR spectroscopy to determine DPn of both segments, which were in good agreement with the calculated values. However, F n was not given and no experimental evidence of the three arm block copolymer structure was provided. Hydrolysis of the acetate groups was found to be quantitative according to 1 H NMR analysis and gave amphiphilic stars with solubility properties essentially determined by the nature of the outer segments. 2.2.2.2 Poly(isobutylene-b-styrene) n Radial three arm star poly(isobutylene-b-styrene)s have been prepared by many groups. The synthesis invariably involved the living polymerization of IB with the tricumyl chloride (14) or tricumyl methyl ether (15)/TiCl 4 initiating system in CH 3Cl/methylcyclohexane (or hexane) (40/60 v/v) in the presence of a Lewis base at –80 °C followed by the sequential addition of S. For instance, tricumyl methyl ether was used as initiator by Kaszas et al. [39] in CH 3Cl/methylcyclohexane in the presence of dimethylacetamide (DMA). The tensile strength of the star block copolymer, which was rather low (13.7 MPa) due to unoptimized conditions, was similar to that of a linear triblock copolymer with comparable composition and MW. For linear triblock copolymers better results were obtained (18.7 MPa) in the combined presence of DMA and DTBP. Star blocks have not been prepared under these conditions, but expectedly they should exhibit similar tensile strength. Storey et al. [40] prepared three arm star block copolymers of poly(isobutylene-b-styrene) by slightly modifying the above procedure using tricumyl chloride as initiator in the combined presence of pyridine and DTBP. Interestingly, the three arm star block copolymer exhibited tensile strength of 16 MPa, about twice that of a linear triblock copolymer with similar block segment lengths. This is probably due to the fact that incomplete crossover from PIB to S resulted in the formation of diblock copolymer in the synthesis of linear triblock copolymer, whereas in star block synthesis incomplete crossover would only result in dangling ends. It is well documented that even small amounts of diblock copolymers substantially decrease the mechanical properties of triblock copolymer thermoplastic elastomers. There was no clear difference between the mechanical properties of star block and linear diblock copolymers prepared in CH 3 Cl/hexane mixture in the combined presence of pyridine and DTBP at – 80 °C. The synthesis, characterization, and mechanical properties of a novel star block copolymer thermoplastic elastomer with eight poly(isobutylene-b-styrene) arms radiating from a calix[8]arene was recently reported by Jacob et al. [41]. The process involved the synthesis of eight arm star PIB by a method essentially identical to that described above, followed by sequential addition of S after the IB conversion has reached 95%. To minimize alkylation and to obtain high MW PS blocks, moderate TiCl 4 concentration (0.059 mol l –1 ) and a 2- to
Synthesis of Branched Polymers by Cationic Polymerization 29 2.5-fold excess of S relative to the targeted MW was used. The produced star block copolymer was contaminated by 3–5% homoPS and ~10% linear diblock copolymer. The mechanical properties of selected star blocks have been investigated. All products investigated exhibited excellent tensile strengths up to 26 MPa. 2.2.2.3 Poly(isobutylene-b-p-methylstyrene) n Well defined three arm star block copolymers were prepared by sequential block copolymerization of IB with p-methylstyrene (p-MeS) [42]. First IB was polymerized by the 14/TiCl 4 initiating system in CH 3 Cl/hexanes (40/60 v/v) at –80 °C in the presence of the proton trap DTBP. When the polymerization was complete the living PIB chain ends were capped with 1,1-diphenylethylene. Subsequently, titanium(IV)isopropoxide was added to decrease the Lewis acidity and p-MeS was introduced. The mechanical properties of the star block copolymers were determined and were found to be similar to linear triblocks with the same p-MeS segment length and composition. The best star block copolymers exhibited ~22 MPa tensile strength. 2.2.2.4 Poly(isobutylene-b-THF) n The synthesis of three arm star block copolymers of IB and THF was described by Gadkari and Kennedy [43]. First, three arm star PIB with hydroxyl termini was obtained by dehydrochlorination of three arm star PIB carrying terminal tert-chlorine, followed by hydroboration and oxidation. Quantitative conversion of the primary hydroxyl end groups was achieved with triflic acid in the presence of pyridine at 0 °C. The resulting triflate functional PIB was used to induce living polymerization of THF. At room temperature, low initiation rates were observed, which could be increased by increasing the temperature to 60 °C. The star block copolymer which contained considerable amounts of unblocked PIB was purified by column chromatography with hexane/THF mixtures as eluent. The polymer fractions were analyzed and the blocking efficiency was calculated to be >70%. These block copolymers carried an HO- functionality at the polymer end of each arm and thus could be used to prepare polyurethane networks. 2.2.2.5 Poly(isobutylene-b-methyl methacrylate) n Star block copolymers of IB and methyl methacrylate have been prepared very recently by the combination of living cationic and anionic polymerizations [44]. First, three arm star PIB (M n=30,000 g mol –1 ) was prepared by living cationic
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Page 3 and 4: Branched Polymers I Volume Editor:
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Author Index Volumes 101-142 Author
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Author IndexVolumes 101-142 231 Dun
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Subject Index Addition-fragmentatio
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Subject Index 241 Microphases 11z,
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