Another example of controlling the polymer microstructure was reported by Rieger and coworkers, by varying the CO2 pressure in the reaction vessel (FIG. ). A zinc compound is capable of catalyzing the copolymerization of cyclohexene oxide (CHO) and CO2 to produce polycarbonate, and the homopolymerization of (rac)--butyrolactone (BBL) to generate polyester. However, the latter process could be reversibly stagnated by the presence of CO2. When using CO2 as the external stimulus to control the terpolymerization of a mixture of CHO/BBL/CO2, only CO2/CHO copolymerization is observed when the CO2 pressure is up to 40 bars, whereas a statistical copolymer of CHO/BBL/CO2 can be obtained when the CO2 pressure is 3 bars, and only BBL homopolymerization proceeds when CO2 is released. The microstructure of the copolymer can thus be modulated by adjusting the CO2 pressure in the system.