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554 Chemical Reviews, 1999, Vol. 99, No. 2 Kendall et al. bocationic polymerization is found to have solvent- separated ion pairs, while living systems have contact ion pairs. Solvent choice plays an important role in cationic polymerizations because it affects the equi- librium between contact pairs and solvent-separated ion pairs and the activation energy of transfer and termination reactions. Nonpolar solvents are gener- ally desirable for cationic polymerizations because they suppress ion separation. Because of the tunability of the solvent properties such as dielectric constant, supercritical CO2 should make for an interesting medium for studying cationic reactions. One disadvantage is the critical tempera- ture of CO2 (31.1 °C). Since cationic polymerizations are usually performed at low temperatures (often -70 to -30 °C) to diminish side reactions, cationic polymerizations in supercritical CO2 are inherently problematic. In fact, much of the early experiments using CO as a solvent for cationic reactions were 2 performed in liquid CO2 at low temperatures. How- ever, it has been shown that good results can be obtained in liquid and supercritical CO2. Further, CO2 has been shown to be inert to cationic polymer- ization conditions. The earliest work in cationic polymerizations in CO2 was aimed at preparing industrially important hydrocarbon polymers in CO2. These initial experi- ments utilized chain-growth polymerization mecha- nisms to produce polymers which were relatively insoluble in the CO2 continuous phase at the reaction conditions employed. While these early experiments often resulted in a low yield of low molecular weight products, this work was fundamental in demonstrat- ing the compatibility of cationic chain-growth mech- anisms with CO2. A 1960 report by Biddulph and Plesch explored the heterogeneous polymerization of isobutylene in liquid CO2 at -50 °C.116 Two catalyst systems were shown to be effective: AlBr3 and TiCl4 (using ethyl bromide and isopropyl chloride, respectively, as cosolvents). The AlBr3-catalyzed reactions proceeded very fast, but were incomplete and gave molecular weights of about 5 × 105 g/mol. The low conversion was at- tributed to catalyst becoming embedded in the white polymer precipitate. The TiCl4 reaction was slower, but proceeded to completion and gave molecular weights of about 3 × 104 g/mol. A 1970 patent covering the precipitation polymer- ization of vinyl compounds in liquid CO2 included the polymerization of ethyl vinyl ether at room temper- ature.68 This heterogeneous reaction was catalyzed by either SnCl4 or BF3‚OEt2. A yield of 57% was reported, but no other characterization was given. The first systematic study of cationic polymeriza- tions in compressed liquid was a series of papers in the late 1960s, reporting the precipitation polymer- ization of formaldehyde in liquid and supercritical CO2. A carboxylic acid, such as acetic or trifluoro- acetic acid, was added to catalyze the polymeriza- tion.117-119 The polymerizations were performed at 20-50 °C and gave conversions of 50-60%. By infrared spectroscopy, it was shown that CO2 was not being incorporated into the polymer backbone. This Scheme 1. Synthesis of -Cl-Terminated PIB spectroscopic measurement confirmed that CO2 is inert to the propagating cationic species. It was also noted that some polymer was produced in the absence of added catalyst. The authors speculated that an impurity was causing the polymerization in the absence of added catalyst because the degree of polymerization increased linearly with conversion. In 1969, the authors elucidated their impurity as formic acid (formed from the reaction of formaldehyde with water) by careful control of monomer synthesis to either repress or increase acid formation.120 Kennedy, building on earlier work by Plesch,116 reported the polymerization of isobutylene (IB) in supercritical CO2 using 2-(2,4,4-trimethylpentyl) chloride (TMPCl) as an initiator and a Lewis acid catalyst such as BCl3, TiCl4, or SnCl4 as the co- initiator.121 Polymerizations were conducted at 32.5- 36 °C and 75-135 bar. Methyl chloride was added as a cosolvent (3%) (presumably to solubilize the ionic species), and its presence gave higher conversions and narrower polydispersity indices (PDI ) Mw/Mn). Conversions of up to 30-35% and polymers with Mn of 1 × 103 to 2.5 × 103 g/mol and PDIs of 1.5-3.1 were produced. Because 1H NMR results showed significant amounts of unsaturated end groups, chain transfer to monomer likely limited molecular weights, as expected from such a high reaction temperature. The mixed initiating system of TMPCl/(TiCl4/BCl3) was used to form well-defined polyisobutylene with terminal chloride (-Clt) end groups (see Scheme 1).122 Reaction conditions similar to those used previously were employed: 32.5 °C, 140 bar CO2, and 5-10% MeCl. After 3 h, conversions of 40-45%, molecular weights of 1.8 × 103 to 2.4 × 103 g/mol, and relatively low PDIs of 1.3-1.5 were obtained. The same experi- ments performed in hexane gave essentially no polymer and a conversion of only about 0.5%. Thus, although low molecular weight material was pro- duced in CO2, the molecular weights were higher than in hexane. 1H NMR spectra showed no evidence of olefinic end groups (in contrast to the TiCl4 or SnCl4 initiated polymers). Dehydrochlorination fol- lowed by 1H NMR spectroscopy determined each polymer chain was terminated by a chloride, indicat- ing an absence of chain-transfer side reactions. Kennedy studied the temperature effects of IB polymerization in CO2.123 Over the range of 32-48 °C, conversion dropped from 40% to 4% and the molecular weights dropped from 2 × 103 to 7 × 102 g/mol. In addition, although polymers synthesized at 32 °C had greater than 99% terminal chloride groups, that value fell to 60% for 38 °C and 44% for 48 °C, consistent with a higher probability of chain transfer to monomer at the higher temperatures. For this reaction, a ceiling temperature of 88 ( 9 °C, calcu- lated by linear extrapolation of molecular weight to 56 g/mol (mass of IB) as a function of temperature,PDF Image | Polymerizations in Supercritical Carbon Dioxide
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