High molecular weight polyethylene terephthalate resins in which there is used a chain extending agent having the formula: ##STR1## wherein A is any hydrocarbon group having the necessary number of available valences to bound itself to all of the acyl groups of the compound, Y is at least 1 and x is an integer of from 6 to 12.Preferably, the chain extending agent is a bisacyllactam, such as terephthaloyl bislaurolactam.The chain extending agent can be used in combination with a carboxyl reactive compound.