Block Copolyesters
Two polyester homopolymers can react and form block copolymers in a molten state at temperatures high enough for ester interchange [414]. As the reaction mixtures are stirred and heated, the interchanges initially involve large segments. With time, however, smaller and smaller segments form as the transesterifications continue. To prevent eventual formation of random copolymers, the reactions should be limited in time. Ester interchange can be retarded, particularly when esterification catalysts like zinc or calcium acetate are present by addition of phosphorous acid or triphenyl phosphite [415]. This improves the chances of forming block copolymers. The procedure can be applied to preparation of block copolymers of poly (ethylene terephthalate) with poly (ethylene maleate), poly (ethylene citraconate), and poly (ethylene itaconate) [416]. With ester interchange catalysts, like titanium alkoxides or their complexes, melt randomization may be inhibited by adding arsenic pentoxide that deactivates them [417].
Block copolyesters also form in reactions between hydroxy and acid chloride-terminated prepolymers [419]. This can take place in the melt or in solution in such solvents as chlorobenzene or o-dichlorobenzene [418]. For relatively inactive acid chlorides, like terephthaloyl chloride, high reaction temperatures are required. Phosgene also reacts with hydroxy-terminated polyesters to form block copolymers. The reactions must be carried out in inert solvents. Block copolyethers also form readily by ester interchange reactions with low molecular weight diesters [348]:
Acetates of tin, lead, manganese, antimony, and zinc as well as esters of orthotitanates catalyze the reactions [421]. Optimum temperatures for these reactions are between 230 and 260C at 0.03–1 mm Hg pressure [421]. Block copolymers can also form by ring opening polymerizations of lactones, when carboxyl-terminated macromolecular initiators are used [422]:
