UV-initiated synthesis of novel urethane polymers

Abstract

The Barton reaction involves the photoinitiated rearrangement of nitrite esters and is used primarily in the synthesis of complex steroidal structures. The rearrangement results in the introduction of active hydrogen atoms into the molecule and a novel use of this reaction for the production of polyurethane coatings through a photopolymerisation reaction with isocyanates has been discovered.
The nitrite esters were synthesised from nitrous acid produced in situ from sodium nitrite and sulfuric or nitric acid. Yields of 79.5 and 76.4% were obtained in the synthesis of the mononitrite esters butyl- and hexyl-nitrite, however lower yields of 55.1
and 53.4% were obtained in the synthesis of butane-1,4- and hexane-1,6-dinitrite respectively. It was found that the nitrite esters are very rapidly hydrolysed and loss of product through hydrolysis occurs during the synthesis because of the aqueous environment in which it is carried out therefore a position of equilibrium is reached.
The kinetics of the hydrolysis reaction was studied by monitoring the change in pH over time for both butyl- and butane-I,4-di-nitrite. The sensitivity of the nitrite esters to hydrolysis not only makes the synthesis of nitrite esters problematic but it also means that storage above the critical pH of 6.8 and at a temperature of 2-8°C is crucial to maximise the storage stability of the samples.
The stability of the nitrite esters mixed with isocyanates was determined, the results indicated that the two reagents did not react and therefore the compounds could be stored together prior to irradiation. An extensive study on the reactions occurring after the photolysis of a series of nitrite esters was undertaken. The optimum wavelength range of 3 10-385 nm was confirmed for the occunence of the Barton reaction and results showed that the competing reaction of disproportionation also occurred after irradiation. The structure of the starting nitrite was found to have a significant effect on the reactions occurring after photolysis. A chain length of greater than 4 carbon atoms was found necessary for the Barton reaction to dominate over disproportionation because the intramolecular hydrogen abstraction 111 step involved a secondary hydrogen atom, whereas when a nitrite ester with only a 4 carbon chain was subjected to photolysis the disproportionation reaction was the dominant reaction because a primary hydrogen atom was abstracted. However this
trend was not observed during the photolysis of dinitrite esters, the Barton reaction occurred to a greater extent than the disproportionation reaction even when a primary hydrogen atom was abstracted due to the second nitrite group which facilitates its removal.
A series of coatings were produced from solutions of nitrite esters and isocyanates. It was found that a catalyst was essential to drive the reaction towards the Barton reaction and prevent the occurrence of the disproportionation reaction in order to ensure the formation of highly cured coatings. An excess of nitrite was required to compensate for the loss of nitrite ester through evaporation that prevented the production of highly cured coatings. The occurrence of a post-curing reaction was observed due to the continuation of the reaction of the isocyanate with residual active hydrogen compounds.
The film thickness, nitrite ester, isocyanate and catalyst used were all found to have an effect on the rate of the polymerisation reaction. The photolysis reaction was also affected by the conditions under which the photopolymerisation
reaction was undertaken. In a heated system an oxime compound was formed after photolysis whereas when a cooled curing oven was used a dimer was produced. The formation of the dimer also resulted in coatings with inferior properties; this is because the absence of oxime groups and the lower temperature reduces the rate of the polymerisation reaction resulting in increased formation of by-products.
The properties of the coatings produced were tested and were found to have good solvent resistance (over 100 MEK double rubs), pencil hardness (7H), resistance to deformation (3mm pass in the bend test), adhesion (cross-cut value of 0) and weathering resistance. The properties of the coatings were also comparable to standard coatings indicating that they may have commercial importance.