Synthesis and characterisation of some alkyds for unsaturated polyester resins

In an attempt to improve the physical properties of products from unsaturated polyester resins, three new compounds have been synthesised. These were designed to be useful, first stage intermediates in the two stage synthesis of some novel unsaturated polyester alkyds.

Bis (2-hydroxypropyl) terephthalate was successfully synthesised from propane-1,2-diol and terephthaloyl chloride, and isolated from its isomers. Subsequent analysis also provided evidence for inter- and intra- molecular hydrogen bonded isomers.

An interesting low temperature, interfacial polymerisation technique was later developed to prepare a low molecular weight prepolymer from bisphenol A and terephthaloyl chloride. Unfortunately, this product must be endcapped with propylene oxide before it can be polymerised with fumaric acid.

The third intermediate, 2,5-dimethyl-2,5-bis (4-hydroxy-3,5-dimethylbenzyl) hexane, was prepared by the Friedel-Crafts alkylation of 2,6-dimethylphenol with 2,5-dichloro-2,5-dimethylhexane. Once again this material must be endcapped with propylene oxide before it can be polymerised with fumaric acid. Unfortunately it also has the disadvantage of being rather unstable as it tends to eliminate 2,6-dimethylphenol and produce 1,1,4,4,5,7-hexamethy1-6-hydroxytetralin.

When bis (2-hydroxypropyl) terephthalate was polymerised with maleic anhydride, GPC and NMR analysis proved that transesterification also occurred and that the rate of maleate/fumarate isomerisation was very slow (cf the synthesis of ICI Impolex Alkyds). Therefore, as Impolex alkyds are made from a low molecular weight propane-1,2-diol/terephthalic acid copolymer (first stage intermediate) and maleic anhydride, it was decided to investigate this variation in the rate of maleate/fumarate isomerisation. An attempt was also made to determine if transesterification also occurred in this commercial system. Studies of a propane-1,2-diol/ maleic anhydride polymerisation and a bis (2-hydroxypropyl) terephthalate/ propane-1,2-diol/maleic anhydride polymerisation revealed that propane- 1, 2-diol catalysed maleate/fumarate isomerisation. They also suggested that some propane-1,2-diol underwent an addition reaction with maleate groups to produce some alkoxysuccinate groups. Later, 220 MH_Z ¹H NMR and ¹³C NMR analysis of some Impolex alkyds revealed that transesterification had occurred and that some alkoxysuccinate groups were present.

These results prompted a study of the reaction between diethylmaleate and propane-1,2-diol. When an equimolar mixture of these reactants was heated under nitrogen, transesterification and isomerisation occurred. There was also evidence of alkoxysuccinate groups in the end product.

However, when diethylfumarate was heated with propane-1,2-diol no reaction occurred and the mixture distilled as an azeotrope. NMR analysis revealed that an acceptor/donor complex may have formed between the maleate group (acceptor) and propane-1,2-diol (donor). Subsequent studies revealed that this complex could be detected as an "acid" with a pH meter. These studies also provided evidence for a possible family of similar complexes. However, no evidence for a fumarate/propane-1,2-diol complex was found. Therefore it appears that a maleate/propane-1,2-diol complex may be responsible for maleate/fumarate isomerisation and the addition of propane-1,2-diol to the maleate group. It may also assist the transesterification reaction. Novel systems designed to make stronger unsaturated polyesters should therefore be based on fumaric acid instead of maleic anhydride, especially if propane-1,2-diol (or a similar glycol) is not present in the formulation.