MECHANISM OF EXCHANGE IN PBT PC AND PET/PC BLENDS - COMPOSITION OF THE COPOLYMER FORMED IN THE MELT MIXING PROCESS/

Mechanisms operating in the exchange reactions occurring in the melt m ixing processes of Bisphenol A polycarbonate (PC) with poly(butylene t erephthalate) (PBT) and poly(ethylene terephthalate) (PET) blends have been investigated making use of appropriate polymer samples, capped o r containing reactive chain end groups. The exchange process may proce ed by two different mechanisms: a direct exchange reaction between inn er functional groups located inside the polymer chains, i.e., inner-in ner, or by attack of reactive chain ends functional groups (outer) on inner groups, i.e., outer-inner. It is shown that the distinction betw een the two processes can be conveniently made by determining the comp osition of the copolymer formed in the exchange reaction. The inner-in ner mechanism occurs only in the reaction between end-capped or high m olar mass PET/PC or PET/PC samples, and it was found that the molar co mposition of the copolymer formed is always equal to the feed ratio of the two homopolymers and independent from the reaction time. The oute r-inner mechanism occurs in the presence of hydroxyl or carboxyl react ive chain ends in PBT and PET samples. The reaction proceeds by the at tack of the reactive end groups on the PC chains, originating block co polymers of PC and PBT and low molar mass PC with phenol end groups wh ich are unreactive. The reaction stops right after the reactive end gr oups are consumed. The amount and the composition of the copolymers ge nerated in the reactions are found to be constant as a function of tim e. The copolymer composition shows an excess of PBT or PET units with respect to the feed molar ratio. These results indicate that monitorin g the composition of the copolymer formed in each case is diagnostic f or establishing the mechanism of the reaction. The approach used here allows control of the composition and yield of the copolymer to be pro duced, and it is applicable to other systems where exchange reactions occur.