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The weight average molecular weight of the copolymers can be significantly increased by including a catalysts with the prepolymers in the melt polymerization. Any catalyst used for transesterification, ring opening polymerization and related polymerizations are useful. In the preferred embodiments acid, base and coordination catalysts, such as Cd(CH3COO).sub.2, earth metal oxides including CaO, BaO, CaCO.sub.3, and ZnEt.sub.2 H.sub.2 O are used. These catalyst were found to have a minor effect on the number average molecular weight. Table 1 compares the viscosity and weight average molecular weight for CPP:SA (20:80) polymers, melt polymerized at 180 degrees C. using various 2 mole percent coordination catalysts. Significantly higher molecular weights in shorter times were Cd(CH.sub.3 COO).sub.2, earth metal oxides, calcium carbonate and ZnEt.sub.2 H.sub.2 O. As can be seen in FIG. 2 the molecular weight of CPP:SA (20:80) increased up to 240,133 with a catalyst, in comparison to a weight average molecular weight of 116,800 without a catalyst. These catalysts are preferred since the reaction type is heterogenic, resulting in easy separation of the catalyst, which is a requirement for use of the copolymer in vivo or other medical applications. TABLE 1 ______________________________________ Melt Polymerization of CPP-SA (20:80) Using Coordination Catalysts* molecular polymerization viscosity** weight*** Catalyst time (min) [n] (dl/g) Mw ______________________________________ no catalyst 90 0.92 116,800 Barium oxide 30 0.96 185,226 Cadmium acetate 31 1.15 240,133 calcium oxide 20 0.88 140,935 calcium carbonate 28 0.81 128,763 ZnEt.sub.2 --H.sub.2 O (1:1) 60 1.18 199,060 ______________________________________ *2 mole % **Chloroform, 23 degrees C. ***GPC calibrated with polystyrene standards The molecular weights of CPP:SA (20:80) polymerized with 2 mole percent catalyst are depicted in FIG. 3 as a function of time and polymerization. Calcium oxide, barium oxide, calcium carbonate and cadmium acetate are used as the catalysts. The molecular weight and polymerization time for the polymerization of polyanhydrides carried out at 180 degrees C. with the catalyst cadmium acetate are listed in Table 2. TABLE 2 ______________________________________ Polymerization of polyanhydrides using cadmium acetate as catalyst polymerization Polymer time (min) Mw ______________________________________ P(IPh:SA) (20:80) 40 178,100 P(IPh:SA) (50:50) 35 87,850 P(CPP:DD) (20:80) 35 184,900 P(CPP:DD) (50:50) 40 61,050 DD 60 134,200 SA 60 138,500 ______________________________________ These results demonstrate that high molecular weight polyanhydrides can be synthesized only when critical parameters are observed. In the above examples the conditions which must be optimized to achieve the high molecular weights include the reaction time, the reaction temperature, high vacuum and the use of purified prepolymers. The molecular weight of these copolymers can further be significantly increased by including a catalyst, preferrably a coordination catalyst, an earth metal oxide, calcium carbonate or a complex of ZnEt hydroxylated compound. |
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