Modified PET polymers and copolymers suitable for extrusion blow molding

An improved modified polyethylene terephthalate polymer or copolymer of the type having high zero shear rate melt viscosity and shear sensitivity, comprising end-capped and branched polyethylene terephthalate polymer or copolymer molecules comprising terephthalic acid structual units and isophthalic acid structural units, of high molecular weight, formed by reacting end-capping agents and branch agents with low molecular weight polyethylene terephthalate polymer or copolymer molecules and thereafter polycondensing the endcapped and branched low molecular weight molecules to form high molecular weight molecules, wherein the improvement comprises:

• the end-cap consists of the reacted component of a difunctional acid or alkali metal salt of a difunctional acid having the formula:
• ##STR3## wherein:
• X has a valency of 6 and is selected from sulfur, selenium, and tellurium;
• M is an alkali metal or hydrogen;
• and,
• Y is hydrogen or an aliphatic group containing from 1 to 18 carbon atoms.