Solid-state polymerization of PET: influence of nitrogen sweep and high vacuum

We have examined the influence of reaction environment on the solid-state polymerization (SSP) of thin (180 mum) poly(ethylene terephthalate) (PET) chips at 250 degreesC by following the intrinsic viscosity (IV) increase and the end-group depletion. When the SSP reaction is carried out in vacuum, IV increases from 0.58 to 2.4 dl/g in 2.5 h of reaction. The initially rapid reaction slows considerably with time, and IV rise nearly stops at 2.75 dl/g at 6 h, though we still detect the acid and the hydroxyl end-groups at concentrations of 3 and 5 mequiv./kg, respectively. This suggests a role of crystallization in limiting the approachability of the end-groups to each other, thereby temporarily rendering them inactive. At this stage, raising the temperature to 270 degreesC to melt the PET in vacuum again increases the IV to 2.97 dl/g in 1.5 h, perhaps due to the release of crystalline restraints in the melt allowing some these inactive end-groups to approach each other. We find that accounting for these temporarily inactive end-groups is a must for a good kinetic description of SSP to IV > 1.3 dl/g. When nitrogen is used as a carrier gas, the reaction rate and the extent of molecular weight build-up are somewhat lower compared to SSP under vacuum. A sublimate is collected during SSP under vacuum, and we find it to be made up of terephthalic acid, monohydroxyethyl terephthalate, bishydroxyethyl terephthalate and cyclic oligomers. This indicates the presence of a new condensation mechanism during SSP under vacuum. (C) 2003 Elsevier Science Ltd. All rights reserved.