occurs the short chain ester homopolymer or copolymer can be prepared by ester interchange from either the dimethyl esters and low molecular weight diols as above or from the free acids with the diol acetates alternatively the short chain ester copolymer can be prepared by direct esterification from appropriate acids anhydrides or acid chlorides for example with diols or by other processes such as reaction of the acids with cyclic ethers or carbonates obviously the prepolymer might also be prepared by running these processes in the presence of the long chain glycol ester interchange polymerizations are generally run in the melt without added solvent but inert solvents can be used to facilitate removal of volatile components from the mass at low temperatures this technique is especially valuable during prepolymer preparation for example by direct esterification however certain low molecular weight diols for example butanediol in terphenyl are conveniently removed during high polymerization by azeotropic distillation other special polymerization techniques for example interfacial polymerization of bisphenol with bisacylhalides and bisacylhalide capped linear diols may prove useful for preparation of specific polymers both batch and continuous methods can be used for any stage of copolyester polymer preparation polycondensation of prepolymer can also be accomplished in the solid phase by heating finely divided solid prepolymer in a vacuum or in a stream of inert gas to remove liberated low molecular weight diol this method has the advantage of reducing degradation because it must be used at temperatures below the softening point of the prepolymer the major disadvantage is the long time required to reach a given degree of.