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

The authors have examd. the influence of reaction environment on the solid-state polymn. (SSP) of thin (180 mm) poly(ethylene terephthalate) (PET) chips at 250 Deg 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 the authors still detect the acid and the hydroxyl end-groups at concns. of 3 and 5 mequiv./kg, resp. This suggests a role of crystn. in limiting the approachability of the end-groups to each other, thereby temporarily rendering them inactive. At this stage, raising the temp. to 270 Deg 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 cryst. restraints in the melt allowing some these inactive end-groups to approach each other. They 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 mol. wt. build-up are somewhat lower compared to SSP under vacuum. A sublimate is collected during SSP under vacuum, and the authors 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. [on SciFinder (R)