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Polymerizations in Supercritical Carbon Dioxide Scheme 2. Cationic Synthesis of P(IBVE) in Supercritical CO2 was consistent with those determined for conven- tional solvent systems.124-128 An isobutylene-styrene copolymer was also syn- thesized in CO2 using MeCl as a cosolvent and the TMPCl/TiCl4 initiating system.129 A conversion of 15% was produced, but molecular weights and PDIs were not reported. The authors only noted that longer reaction times produced higher molecular weights. In the synthesis of phenol-terminated polyiso- butylene, the first example of electrophilic aromatic alkylation in supercritical CO2 was reported.130 TMPCl and polyisobutylene-Clt (Mn ) 2000 g/mol) were alkylated by phenol at 32.5 °C and 140 bar in the presence of BF3‚OEt2 for 24 h. Yields of 75% and 60% respectively were observed. The isomerization polymerization of 3-methyl-1- butene and 4-methyl-1-pentene has also been studied in supercritical CO2.131,132 The reactions were per- formed at 140 bar CO2 and 32.5 °C with residual water and AlCl3 as the catalyst system. As with the IB polymerization, a cosolvent (5% methyl chloride or 10% ethyl chloride) was used to obtain good results. For 3-methyl-1-butene, a 40% conversion, a molecular weight of 1000 g/mol, and a PDI of 1.41 were observed; for 4-methyl-1-pentene the results were 70%, 1700 g/mol, and 2.16. DeSimone reported a study of cationic polymeri- zation of vinyl ethers in supercritical CO2.133,134 Both precipitation and homogeneous polymerizations were reported. The initiation system was based on Higashimura’s living cationic polymerization method developed for hydrocarbon solvents. This method uses the Lewis acid ethyl aluminum dichloride and the acetic acid adduct of isobutyl vinyl ether (IBVE) as the initiator in the presence of a Lewis base deactiva- tor such as ethyl acetate (see Scheme 2).113 The polymerization of IBVE began homogeneously, but became heterogeneous as the polymer precipitated. Yields of polymer synthesized in CO2 were similar to results obtained in cyclohexane, but with broader PDIs (for example, 1.2 for cyclohexane and 1.8 for CO2 at 40 °C, 345 bar). At 60 °C, the polydispersity of polymers produced in CO2 increased to greater than 9, indicating no molecular weight control, perhaps due to increased chain transfer to monomer and lower CO2 density, which would allow for faster precipitation of polymer. The homogeneous polymerization of 2-(N-propyl- N-perfluorooctylsulfonamido)ethyl vinyl ether (FVE) was also performed (see Scheme 3).133,134 The poly- merizations were homogeneous throughout the reac- tion and gave molecular weights of, for example, 4 × 103 g/mol with a PDI of 1.6. The narrow PDIs Chemical Reviews, 1999, Vol. 99, No. 2 555 Scheme 3. Synthesis of Poly(FVE) Scheme 4. Ring-Opening Polymerization of Oxetanes in CO2 achieved with the CO2-soluble fluorinated polymer compared to the broad PDIs obtained with the CO2- insoluble poly(IBVE) suggest that solubility of the resulting polymer plays an important role in deter- mining polydispersity in these cationic polymeriza- tions in supercritical CO2. Ring-opening polymerization of cyclic ethers were initiated by BF3 in liquid CO2 and compared to reactions performed in methylene chloride (see Scheme 4).134 Bis(ethoxymethyl)oxetane (BEMO) was poly- merized at -10 °C in CO2 (290 bar). As expected from insolubility of the resulting polymer, the CO2 reaction was heterogeneous, but the same reaction performed in methylene chloride was homogeneous. The yields were comparable (about 70%), but the PDI was 1.9 for CH2Cl2 and 2.7 for CO2. A fluorinated cyclic ether, 3-methyl-3′-[(1,1-dihydroheptafluorobutoxy)methyl]- oxetane (FOx-7) was polymerized homogeneously in both CO2 (0 °C and 289 bar) and Freon-113 (-10 °C). The polymer synthesized in Freon-113 had a molec- ular weight of 3.9 × 104 g/mol, a PDI of 1.7 and a yield of 70%; the results for the polymer synthesized in CO2 were 2.0 × 104 g/mol, 2.0 and 77% yield. Thus, when homogeneous conditions were used, similar results were obtained for the BEMO polymerized in methylene chloride and the FOx-7 polymerized in either CO2 or Freon-113. Initial attempts to perform the first cationic dis- persion polymerization in CO2 were with BEMO and IB.134 The polymerizations were catalyzed by BF3‚ THF (for BEMO) or SnCl4 (for IB) and were con- ducted in the presence of CO2-soluble surfactants such as poly(FOx-7), poly(FOA), and poly(styrene)- b-PDMS. In the absence of surfactant, the polymer precipitated. Yields and molecular weights were not improved by the presence of surfactants, but the polymer products were stable dispersed colloids in CO2 which could be redispersed in Freon-113 after removal of CO2. Scanning electron microscopy (SEM), which examined polymer following removal of CO2, detected slight particle formation despite the semi-PDF Image | Polymerizations in Supercritical Carbon Dioxide
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