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

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14 B. Charleux, R. Faust preparation of the arms. The solubility properties of these star block copolymers were essentially determined by the nature of the outer block. 2.1.2.2 Poly(isobutylene-b-styrene) n In US patent 5,395,885 (1995) Kennedy et al. disclosed and specifically claimed star polymers with PS-PIB block copolymer arms, formed by linking with DVB and related diolefins. Examples however were not provided. Subsequently Storey and Shoemake [15] published the synthesis and characterization of multiarm star polymers based on PS-PIB block copolymer arms using essentially the same method. First S was polymerized with the cumyl chloride/TiCl 4 initiating system in the presence of pyridine in hexane/methylchloride (60/40 v/v) at –80 °C. At high (unspecified) S conversion the desired amount of IB was added and polymerized to obtain the PS-PIB block copolymer arm. SEC traces (presumably Differential Refractive Index, DRI) are given to show that the amount of homopolystyrene is small. This is unconvincing, however, in light of the small MW difference between the IB segment (M n=1900 g mol –1 ) and PS segment (M n= 28,500 g mol –1 ). The SEC UV trace, which would clearly show the extent of blocking, was not shown. The PS-PIB block copolymer arms were next reacted with DVB at [DVB]/[chain end]=10. After 72 h the linking was nearly complete. Further increase in the linking time led to star-star coupling with marginal increase in the incorporation of the arms. To suppress star-star coupling the temperature of the star-forming reaction was increased from –80 °C to room temperature after 0.5 h reaction with DVB at –80 °C. Star formation was more rapid at the higher temperature and reportedly the higher temperature suppressed star-star coupling. This is surprising in view of findings reported by Storey et al. [8] in the synthesis of multiarm star PIBs, namely that star-star coupling can be effectively frozen out by decreasing the temperature from –40 to –80 °C. Properties of the star block copolymers were not reported. Subsequently, Asthana et al. [16] published the synthesis, characterization, and properties of star polymers with PS-PIB block copolymer arms. In a one pot procedure S was first polymerized with the cumyl chloride/TiCl 4 initiating system in the presence of triethylamine in CH 2 Cl 2 /hexane (50/50 v/v) at –80 °C. At ~90% S conversion, IB was added and polymerized to ~95% conversion. Then DVB was added to effect linking of the arms. In a two pot procedure the PS-PIB diblock copolymer was isolated and purified. Then it was redissolved in CH 2 Cl 2 /hexane (60/40 v/v); triethylamine, TiCl 4 and DVB were added and linking was completed in the –56 to –25 °C range. It was reported that linking did not proceed below –56 °C. The SEC DRI trace of the product formed by linking PS- PIB (M n(PS)=8900 g mol –1 , M n(PIB)=30,000 g mol –1 ) for 72 h showed mainly higher order stars with ~15% unreacted diblock copolymer. Mechanical properties of this star polymer were compared to a linear PS-PIB-PS triblock copolymer of segment MWs of 8900–120,000–8900. It is not clear why the PIB segment was chosen to be twice the desired 60,000 for direct comparison with the star
Synthesis of Branched Polymers by Cationic Polymerization 15 block copolymer. Tensile strength of the star block copolymer (with 15% diblock contamination) was found to be 20.5 MPa, much higher compared to 10 MPa found for the linear triblock (with 7% diblock+homopolymers). However, it is still somewhat lower than the 25–26 MPa reported for well defined linear PS-PIB-PS triblock copolymers. The melt viscosity of star block copolymer was found to be close to an order of magnitude lower than the linear triblock copolymer over a wide range of shear rates. Again this comparison is ambiguous since the PIB middle segment had MW=120,000 g mol –1 and not the desired 60,000 g mol –1 . It is possible that the melt viscosity of a direct linear analog with 8900–60,000–8900 segment MWs would be more similar. 2.1.3 A n-Type Star Polymers with a Functionalized Core: Poly(vinyl ether) n Star copolymers of IBVE and AcOVE were prepared where the second monomer was added together with the cross-linking agent [17]. Typically, IBVE was polymerized first and after complete conversion the resulting living polymer (DP n=30–38) was allowed to react with a mixture of AcOVE and 1 in various proportions. Complete consumption of AcOVE and 1 ensued and soluble high MW star type polymers with 7–10 arms per molecule were obtained, as evidenced by SEC, light scattering, and NMR characterizations. In order to obtain stars with a true functionalized core and not the analogous (AB) n- or A nB n-type stars, the second block must be a random copolymer of the monofunctional vinyl ether and of the difunctional one. Although 1 was found slighly more reactive than AcOVE, experimental evidence based on 1 H NMR and 13 C NMR relaxation time supported the existence of a cross-linked core with incorporated segments of poly(AcOVE). Hydrolysis of the pendant esters of the microgel core was found to yield hydroxyl functions quantitatively and solubility properties of the final products were studied. 2.1.4 A n B n -Type Star Copolymers: Poly(vinyl ether) n -Star-Poly(vinyl ether) n Based on the same technique of core cross-linking, amphiphilic heteroarm star polymers of IBVE and hydrolyzed AcOVE or VOEM were prepared with independent arms of both homopolymers [18]. The first step consisted of the previously described synthesis of a star polymer of IBVE with a microgel core, using 1 as a cross-linker. The final polymer had M w =50,300 g mol –1 with an average of ten arms (each of DP n =30) per molecule. This initially formed star polymer still carried living sites within the core which were suitable for initiation of the second monomer, AcOVE or VOEM. Actually, the number of newly growing arms per molecule should be the same as that of the first series since each active site comes from one initial arm. This was verified with a second stage polymerization of IBVE and supported by experimental results (Fig. 2), although the number of living sites in the core after the first stage polymerization could not
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Page 3 and 4: Branched Polymers I Volume Editor:
Page 5 and 6: Volume Editor Dr. Jacques Roovers I
Page 7 and 8: Preface While books have been writt
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Page 11 and 12: Synthesis of Branched Polymers by C
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Synthesis of Branched Polymers by C
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72 N. Hadjichristidis, S. Pispas, M
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Author Index Volumes 101-142 Author
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Author IndexVolumes 101-142 231 Dun
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Author Index Volumes 101-142 233 Ka
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Author Index Volumes 101-142 235 Og
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Author Index Volumes 101-142 237 Su
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Subject Index Addition-fragmentatio
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Subject Index 241 Microphases 11z,
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