New catalysts for poly(butylene terephthalate) synthesis: 1. Titanium–lanthanides and titanium–hafnium systems

A complete study of the catalytic activity of lanthanide- and hafnium acetylacetonate catalysts in PBT synthesis was conducted in order to investigate any improvement in the process and/or in the properties of the final polymer with respect to the industrially used titanium tetrabutoxide (TBT) catalyst. Small scale polymerization and subsequent scale up in higher capacity reactors showed that TBT-Hf(acac)4 and TBT-La(acac)3 mixed catalysts were more active with respect to TBT as single catalyst. Decreases in polymerization time and THF formation were also observed, which in turn can improve the productivity of the whole process. Furthermore, for similar values of molecular weight, a lower melt viscosity (and thus better processability and crystallizability) was obtained by using mixed catalysts, presumably due to weaker interactions of the polymer terminal groups to lanthanum and hafnium metals with respect to titanium.