CO/ethene copolymerization at zirconocene centers?

The alternating copolymerization of CO and ethene at the zirconocene centers CP2Zr, Me2SiCp2Zr, and rac-Me(2)SiInd(2)Zr was investigated by DFT methods. CO coordinates much more strongly than ethene but has a rather high insertion barrier. Thus, propagation is slowed dramatically but the growing chains will not necessarily all incorporate CO. Secondary alkyls insert CO more efficiently than primary alkyls. After the first CO insertion, subsequent olefin and CO insertions will alternate. Olefin insertion will be very slow at high CO pressures, but at low [CO] the olefin and CO insertion barriers are comparable and are lower than that of the first CO insertion. Use of CO as a quenching and chain-counting method appears to be safe, provided a high pressure of CO is employed and the quenched reaction is not worked up at low temperature.