Fluorination of polymers via metal-free azide-alkyne click reaction

Abstract

Fluoropolymers are one of the best important materials from the family of fluorinated compounds. They are alluring macromolecules due to their specific properties. So, these specialty polymers exhibit aging, chemical, high thermal and weather resistance, excellent inertness, to hydrocarbons, to acids, to solvents low surface energy, low dielectric constants, low flammability, low refractive index, and moisture absorption. Hence, they can be related in High-Tech applications such as automotive, aeronautics, petrochemicals, optics, textile, microelectronics, chemical engineering, automotive, aerospace, and building industries. Among the fluorinated commercial polymers, poly(tetrafluoroethylene) (PTFE), poly (vinylidene fluoride) (PVDF), and poly(chlorotrifluoroethylene) (PCTFE) have become a prerogative and well-studied since the mid-1940s. However, despite their excellent properties, fluorinated polymers have some drawbacks such as contradiction with other hydrocarbon polymers, insolubility in common organic solvents, and hence chromatographic analysis problems. Trifluoromethyl (CF3)-containing polymers are the special class of fluorinated polymers and have received great interest in recent years since they can overcome the above-mentioned problems associated with the fluorinated polymers while preserving unique properties of fluorine-containing polymers. Polycarbonates (PC) are carbonate esters of alcohols or phenols but they cannot be synthesized by the condensation polymerization between diols and carbonic acid. Therefore, a derivative of carbonic acid, phosgene, is used. Sodium salts of diols are used to avoid evolution of gaseous HCl. As a result, sodium chloride is formed as a by-product which requires purification processes. As an alternative method, polycarbonates can be synthesized by the ring opening polymerization (ROP) of cyclic carbonates with five, six or more members. Condensation polymerization is a form of polymerization where a small molecule is released each time two or more functional groups come together and react. Polyester is one example. Acyl chloride and hydroxyl groups react in a basic environment, forming an ester bond with the release of salt as a by-product. 1,3-dipolar cycloaddition reaction takes place between azide and alkyne groups. The cyclization reaction forms a triazole ring which is aromatic in character. Cycloaddition occurs in two ways. It is either at room temperature in the presence of Copper (I) salts which forms 1,4 triazole with regioselectivity or the thermal method where the reaction temperatures are well above the room temperature. The latter has no regioselectivity and produces both 1,4 and 1,5 triazole rings. Polyurethanes are synthesized by the reaction of diisocyanates with diols where urethane linkages are formed without a release of a by-product. Since there are no by- products, it's technically not a condensation polymerization, but the functional group rearranges. In this study, modifications were done on synthetically and industrially important polymers with the use of metal-free azide-alkyne cyclocation reaction. PVC, polyepichlorohydrin (PECH), poly(styrene-co-4-chloromethylstyrene) polymer was azide functionalized by the nucleophilic substitution reaction with sodium azide prior to click reactions. Polyurethane, polyester and polycarbonate polymers were synthesized from azide functional monomers. Each azide-functional polymer was subjected to react with Ethyl 4,4,4-trifluoro-2-butynoate under thermal method conditions. The synthesized polymers were characterized by GPC, 19F-NMR and1H-NMR.