Polymerization of Alkenes under Supercritical Conditions

The copolymerization of ethene with a variety of other alkenes or dienes was also studied. The copolymerization of supercritical mixtures of ethene and propene (120-220 °C and 1000-1500 bar) was catalyzed by the silyl-bridged bis-(tetrahydroindenyl)zirconocene catalyst 19 and MAO at a metallocene concentration of 6 X 10 mole fraction and an Al Zr ratio of 22000 [92]. With a 50 50 mixture of SCC2H4 and scCsHg, the resulting polymer had only 8% incorporation of propene. Increasing concentrations of propene resulted in 

Compressed liquid or supercritical carbon dioxide has been recognized as a useful alternative reaction medium for radical and ionic polymerization reactions (see Chapter 4.5). Many of the benefits associated with the use of SCCO2 in these processes apply equally well to polymerizations relying on a metal complex as the chain-carrying species. However, the solubility of the metal catalyst and hence the controlled initiation of chain growth add to the complexity of the systems under study. Furthermore, many of the environmental benefits would be diminished if subsequent conventional purification steps were needed to remove the metal from the polymer. Nevertheless, the interest in metal-catalyzed polymerizations is increasing, and some promising systems have been described. 

One interesting aspect of applying metal catalysts to polymerizations in SCCO2 is the incorporation of CO2 into the polymer. The copolymerization of epoxides with CO2 is an example of such a process, and has been studied 

In contrast, the carbene complexes 15 and 16 could be used very efficiently in pure liquid or supercritical CO2, giving high yields of polynorbomenamer with very similar cis trans ratios of the double bonds to those obtained with the same catalyst in CH2CI2 [6]. More recent studies showed that the robust complex 16 can be used at substrate to catalyst ratios up to 5300 without deactivation [100]. Fairly broad molecular weight distributions were observed when 16 and norbomene were placed together in the reactor, but narrow distributions could be achieved if the catalyst was injected as a solid or in solution to the supercritical mixture of alkene and CO2. The polymer morphology was very similar to samples prepared with 16 under conventional conditions. 

Collectively, these studies illustrate the broad range of possible applications of SCFs in metal-mediated organic synthesis, and the challenge is now to make efficient use of these methodologies. Investigations towards the understanding of coordination chemistry in SCFs (Chapter 4.2) will stimulate the elucidation of metal-complex-catalyzed reactions. However, it should be evident from this chapter that Ae field of complex-catalyzed SFRs is far from being complete, and much remains to be done. The potential of the technique has been hinted at, but many new ways to exploit the special properties of SCFs in metal complex catalysis are still to be discovered. 

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