Polymers derived using vinyl ester and allyl ester functionality

Swern et al.57 synthesized a series of allyl containing esters of oleic acid via direct esterification with 2-chloroallyl, allyl and, methally alcohol. As reported, homo-polymerization of these allyl eaters with 1% benzoyl peroxide resulted viscous insoluble materials rather than linear soluble polymers. Formation of crosslinked polymers was due to the participation of the 9, 10 double bond in oleic acid in olefinic copolymerization reaction. Allyl ester monomers were also copolymerized with vinyl acetate using 0.5% benzoyl peroxide. However, instead of linear soluble copolymers, soft gels were obtained when incorporation of allyl ester monomers was 30% or more below which the copolymers were insoluble hard, glassy material.

Toussaint et al.58 in early 1942 disclosed the synthesis of vinyl oleate by reacting vinyl acetate with oleic acid using mercuric acetate and sulfuric acid as catalysts, which mainly involved vinyl exchange (transvinylation). An excess of vinyl acetate at higher temperature (limited to the boiling point of vinyl acetate) was required for efficient vinyl exchange to the fatty acid.

Harrision et al.59 studied the free radical polymerization of vinyl and allyl esters of stearate, oleate and linoleate fatty acids using 2% benzoyl peroxide as initiator at 80°C. In the case of vinyl and allyl linoleate, the slowest polymerization rate was observed along the slowest conversion comparing to stearate and oleate due to the presence of easily abstractable bis-allylic protons by the peroxy radical. This observed trend in conversion was also confirmed from the intrinsic viscosity of the resultant polymers showing a decrease in viscosity of the polymer with increasing the fatty acid unsaturation of the vinyl or allyl eater monomer. Frank et al.60 and Wilson et al.61 also observed similar retarded polymerization behavior with unsaturated fatty acids, however they did not report any explanation for this behavior. As concluded by Harrison et al.59 the free radical initiator could add to the 9 and 12 double bonds of the linoleate and the vinyl or allyl groups in the monomer. In the scenario where the radical adds to the allyl or vinyl groups it readily propagates to another double bond. However, the radicals formed at the fatty unsaturation are relatively stable, thereby rather than propagating it undergoes termination by combination. In the case of linoleate, the propagation mostly occurs through the vinyl or allyl group which may react with the 9 or 12 double bonds, resulting in ultimate chain termination and therefore observed low conversion. Although this research attempted synthesizing linear chain growth polymers, they all had issues for the preservation of fatty unsaturation which is important for further crosslinking.

Lincoln et al.62 developed a ruthenium catalyst for trans-vinylation to synthesize vinyl esters and showed a performance advantage over previously reported mercury58 and palladium63

catalysts. Unlike the mercury catalyst which has toxicity and volatility issues and palladium catalyst which has low thermal stability resulting from inactive palladium (0) near the boiling point of vinyl acetate, the ruthenium catalyst showed desirable physical and chemical properties suitable for commercial purposes.64 _{Hideto et al.}65 _{disclosed the use of catalytic amounts of [Ir(cod)Cl]}

2 or

[Ir(cod)2]BF4 for the synthesis of a vinyl ester by reacting carboxylic acid with terminal alkynes

resulting in 1-alkenyl ester. Following this work, Gandini et al.29 used iridium catalyst [[Ir(cod)Cl]2] for the synthesis of fatty acid vinyl esters by the transvinylation of oleic and linoleic

acids with vinyl acetate. Fatty acid was reacted with 10 eq. excess of vinyl acetate (VAc) in the presence of catalyst ([Ir(cod)Cl]2, 0.01 eq), along with sodium acetate (0.03 eq), under magnetic

stirring at 100°C resulting in vinyl oleate (VO) and vinyl linoleate (VL) in 90% and 50% yield, respectively. Homopolymers and copolymers of vinyl oleate (VO) and vinyl linoleate (VL) with different ratios of vinyl acetate were carried out free radically using benzoyl peroxide as initiator at 85 °C. Homo-polymerization of vinyl oleate resulted in linear polymers only through the vinyl double bond and was confirmed by NMR via the disappearance of vinyl proton resonances (δ 4.5, 4.8 and 7.3 ppm) and appearance of the backbone protons CH and CH2 with the retention of fatty

chain double bond proton at δ 5.3 ppm. However, in case of vinyl linoleate the backbone proton resonance was not clear in addition to the low intensity signals from the vinyl proton, implying polymerization occurred both through the vinyl and fatty acid double bond. The yield for polyvinyllinoleate was lower (<10%) compared to polyvinyloleate (>50%) which confirmed the involvement of the fatty acid double bond during polymerization resulting in a higher termination rate. Copolymerization with vinyl acetate showed similar behavior for both vinyloleate (VO) and vinyllinoleate (VL). Thermal analysis showed that irrespective of the type/amount of fatty acid vinyl ether (FAVE) monomers and quantity of radical initiator, vinyl acetate copolymers showed

a two-step pathway degradation profile around 340–350 °C (Td1) corresponding to the deacetylation of the pendent group and 440–450 °C (Td2), associated with the depropagation of the polyolefin backbone. Oxidative drying of the copolymer films was confirmed by FT-IR study at different time intervals viz 1, 3, 19, 26, 68 and 140 h. at RT. FT-IR spectra of poly(VAc-co-VL) copolymer with 13% VL obtained over 140 h. of curing exhibited two new bands around 3430 cm-1 and 1633 cm-1 assigned to hydroperoxide moieties and conjugated double bonds. Also, peaks at 3010 and 723 cm-1 confirmed the disappearance of the fatty double bond and occurrence of oxygen induced polymerization. Similar drying behavior was observed by Lazzari et al.66, during the study of the drying behavior of linseed oil. In the case of the vinyl ester of fatty acids, it is difficult to synthesize linear chain growth polymers of highly unsaturated vinyl esters like linoleate or linolenate that carry bis-allylic protons due to the affinity of the radical towards both vinyl and fatty double bonds in addition to the increasing affinity of the fatty double bond radical to recombine with another fatty radical rather than attacking the vinyl double bond.