Process for preparing polyethylene terephthalate

Claim 1

IN A PROCESS OF MAKING HIGHLY POLOYMERIC POLYETHYLENE TETRHALATE HAVING AN INTRINSIC VISCOSITY OF AT LEAST 0.55 WHEREIN AN ALKYL ESTER OF TEREPHTHALIC ACID HAVING 1-7 CARBON ATOMS IN THE ALKYL GROUP IS REACTED WITH A POLYMETHYLENE GLYCOL HAVING FROM 2-10 CARBON ATOMS IN THE PRESENCE OF AN ESTER INTERCHANGE CATALYST TO EFFECT ESTER INTERCHANGE AND THE RESULTING BIS GLYCOL ESTER IS SUBSEQUENTLY POLYMERIZED, THE IMPROVEMENT WHICH COMPRISES POLYMERIZING SAID BIS GLYCOL ESTER UNTIL A FILM-FORMING POLYMER IS PRODUCED IN THE PRESENCE OF A CATALYST SYSTEM COMPRISING CATALYTIC AMOUNTS OF (1) AN ALKALINE METAL FROM THE GROUP CONSISTING OF LITHIUM, SODIUM, CALCIUM AND MAGNESIUM AND THEIR HYDRIDES, ALCOHOLATES, CHLORIDES AND GLYCOL-SOLUBLE SALTS OF MONOCARBOXYLIC ACIDS; (2) A GLYCOLSOLUBLE SALT OF A MONOCARBOXYLIC ACID AND A METAL FROM THE GROUP CONSISTING OF ZINC, MANAGANESE AND CADMIUM; (3) AN ANTIMONY COMPOUND FROM THE GROUP CONSISTING OF ANTIMONY TRIOXIDE, ANTIMONYL POTASSIUM TARTRATE, ANTIMONOUS OXYCHLORIDE, ANTIMONY TRIFLUORIDE AND SODIUM ANTIMONYL HYDROXY ACETATE; AND (4) AT LEAST 0.05%, BASED ON THE WEIGHT OF THE ALKYL ESTER OF TEREPHTHALIC ACID, OF A PHOSPHORUS COMPOUND FROM THE GROUP CONSISTING OF MONOAMMONIUM PHOSPHATE, DIAMMONIUM PHOSPHATE AND MONOAMMONIUM PHOSPHITE.