Question

How would you prepare (a) a very high molecular weight PET and (b) a low molecular weight PET terminated with hydroxyl groups?

Answer 1

a)

The number-average molecular weight of PET obtained by the ordinary melt polymerization method does not normally exceed 30,000. PET of number-average molecular weight of the order of 120,000 was prepared with a Gas Chromatograph apparatus through solid-state polymerization technology. PET particles were packed in the column, heated to a temperature somewhat below the melting temperature, and flushed with a carrier gas until the desired molecular weight was reached. Parameters like catalyst, the particle size, the molecular weight of the starting material, the reaction temperature and time, and the flow rate and nature of the carrier gas play a vital role in achieving this.

Ester interchange catalysts such as tetraisopropyl titanate, zinc acetate antimony trioxide, and cobaltous acetate are capable of increasing the molecular weight of PET. Small particle size and high molecular weight of the starting material are essential for obtaining products of very high molecular weight. The polymerization of PET in the solid state starts around 175° C (448 K); the molecular weight of the final product increased with increasing temperature for the period of time studied. The most preferable polymerization temperature is between 200° and 250° C (473 and 523 K). The optimum reaction time depends on other polymerization variables, including the column size of the Gas Chromatograph. Molecular weights are also increased by increasing the carrier gas flow rate to a certain practicable value, which varies with other polymerization conditions. Carrier gases used included nitrogen, carbon dioxide and helium.

b)

Generally, polyesters are insoluble in common organic solvents, even more so when in their crystalline form. The most successful solvent systems include chlorinated phenols and strong organic acids such as trifluoro-acetic acid (TFA) and dichloroacetic acid. In many applications a mixed solvent system, typically dichloro- methane (DCM) and an organic acid, is used as it is often found that only a small amount of the acid is required for solubility. Indeed for many systems once there is sufficient acid to dissolve the polymer, often only 10% by volume, further addition has little effect on solubility.

Usually TFA and DCM are chosen as a solvent system. An inherent complication in this system is the possible esterification reaction between the acid solvent and hydroxyl end groups. Initially, 19F nuclear magnetic resonance (n.m.r.) is used to provide evidence of the reaction between the acid and end groups, referred to as end-capping. Later, kinetics are studied in more detail using flourine n.m.r and proton n.m.r.