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556 Chemical Reviews, 1999, Vol. 99, No. 2 Scheme 5. ROMP of Norbornene in Supercritical CO2 crystalline nature of poly(BEMO) and low Tg of poly- (IB). However, since an in situ technique was not used to measure the particles, the absence of well- defined particles in SEM images does not mean the particles were not stable and dispersed in the CO2. To overcome the problems associated with SEM analysis of poly(BEMO) and poly(IB), a more conclu- sive example of cationic dispersion polymerizations in CO2 was reported with styrene.135 First, a suitable surfactant for polystyrene was synthesized in CO2, using the vinyl ether system previously reported.134 A block copolymer was synthesized from FVE and methyl vinyl ether (MVE) (9) using the EtAlCl2/FVE acetic acid adduct/EtOAc initiating system. The poly- (FVE) block serves as the soluble block for steric stabilization, and the poly(MVE) block serves as the anchoring unit due to its miscibility with PS. The surfactants were employed in the cationic polymer- ization of styrene initiated by TiCl4 at 330 bar CO2 in the temperature range of 0 to 25 °C. The results of these reactions were quite sensitive to temperature effects. No improvement in yields or molecular weights and no stable colloids were observed for reactions performed at 0 °C. At 15 °C, the presence of 4 wt % stabilizer leads to increased yields (from about 50% to 95-97%), increased molecular weights, and de- creased PDIs. The reaction had a milky-white ap- pearance, indicative of a stable polymer colloid. SEM analysis showed well-defined PS particles with a broad distribution of sizes that ranged from several hundred nanometers to one micrometer in diameter. At 25 °C, yields for the reaction in the presence or absence of surfactant are lower than at 15 °C, and particles appear coagulated by SEM for reactions performed in the presence of surfactant. The lower yields are probably due to higher rate of chain transfer at the higher temperature. C. Transition Metal-Catalyzed Polymerizations Metal-catalyzed polymerizations have been per- formed in supercritical CO2. The ring-opening metathesis polymerization (ROMP) of bicyclo[2.2.1]- hept-2-ene (norbornene) in CO2 was catalyzed by [Ru- (H2O)6(tos)2] (tos ) p-toluenesulfonate) (see Scheme 5).136,137 The reaction was performed at 65 °C with pressures ranging from 60 to 345 bar. The insoluble polymer precipitated, and there was no obvious correlation between pressure and molecular weight (which ranged between 104 and 105 g/mol), yield (30- 76%), or PDI (2.0-3.6). The [Ru(H2O)6(tos)2] catalyst is insoluble in CO2, but can be solubilized by the addition of methanol. When the polymerization was performed with up to 16 wt % methanol as a cosolvent, the Mn and PDI were in the same range as the polynorbornene produced in the absence of methanol, but the yields Kendall et al. Figure 4. Catalysts for ROMP of norbornene in super- critical CO2. increased with increasing methanol content. For example, the reaction with 16 wt % methanol gave a similar yield in 5 h as the reaction without methanol gave in 16 h. Therefore the reaction was much faster in the presence of methanol. A profound effect on polymer microstructure was found with increasing methanol content. The presence of methanol de- creased the cis-vinylene content in the resulting polymer (83% cis for no methanol and 33% cis for 16 wt % methanol). Presumably the addition of a polar cosolvent favors the trans-propagating species at the metal center and allows for control of polymer micro- structure by control of cosolvent content. This hy- pothesis could be confirmed by observing the effect of other polar cosolvents on the polymer trans content. However, these experiments have not been conducted by the authors. Higher activities for the ROMP of norbornene were observed with ruthenium and molybdenum carbene catalysts reported by Grubbs138-140 and Schrock,141 respectively (see Figure 4).142 While the Ru catalyst appeared insoluble in CO2, the Mo catalyst was partially soluble. These catalysts gave up to 94% yield of precipitated polynorbornene in CO2 (97% using toluene as a cosolvent) and molecular weights in the range 105-106 g/mol at much milder reaction condi- tions of 25-45 °C and about 100 bar. The Ru catalyst gave about 25% cis content with no apparent depen- dence on density while the Mo catalyst gave 66% cis content at a reaction density of 0.57 g/mL and 82% cis at 0.72 g/mL. The ruthenium carbene catalyst was also used to polymerize cis-cyclooctene in 50% yield and a molecular weight of 105 g/mol. The polymers produced in CO2 were similar in molecular weight and microstructure to those produced by conventional means in dichloromethane. There have also been a number of reports of polycarbonate synthesis from the copolymerization of CO2 and epoxides. The precipitation copolymeri- zation of CO2 and propylene oxide in supercritical CO2 has been reported using zinc(II) glutarate as a heterogeneous catalyst143 (Scheme 6). The polycar- bonate, with a molecular weight of about 104 g/mol, was formed at 60 or 85 °C with both sub- and supercritical pressures (21-83 bar). Polymerizations performed in CO2 above the critical pressure had an increased percentage of carbonate linkages relative to the ether linkages (over 90% vs less than 75%). Propylene carbonate was formed as a byproduct, and its production was increased with increasing tem- perature. Yields were generally around 10-20%, but the addition of acetonitrile or hexane as a cosolventPDF Image | Polymerizations in Supercritical Carbon Dioxide
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