Synthesis of Low-Temperature Stable Polymers. I. Chain-Extended Poly(tetrahydrofuran–urethane)

Abstract

ABSTRACT:Tetrahydrofuran was polymerised with boron trifuoride etherate (BF3 · Et20) and terminated with hydrobromic acid to give the bromide-terminated prepolymer, which, on further reaction with ketal derivative of glycerol and subsequent hydrolysis, gave tetrahydroxylterminated polytetrahydrofuran. The prepolymer was chain-extended with tolylenediisocyanate and hexamethylenediisocyanate in six different solvents to give cross-linked polyurethane. These polymers were characterised by infrared, viscosity and thermogravimetric analysis. The glasstransition temperature, the tensile strength at break, and elongation were also determined.KEY WORDS The diisocyanate polymer chain-extension technique has been utilised in many fields of polymer technology to produce materials such as foams, coatings, castable elastomers/ and cryogenic adhesives.2 The coreactants that have been used are hydroxyl-terminated polyesters, polyethers, and some naturally occurring products such as castor oil. For many reasons, such as mix-fluidity and improved low-temperature flexibility, polyethers are preferred in urethane technology. The three principal types of polyethers used in work up to the present are poly(ethylene glycol)s, poly(propylene glycol)s and poly(tetrahydrofuran glycol)s, and the glycols obtained from the copolymerization of cyclic ethers. 3 The tetrahydrofuran(THF)-based polyurethanes (PU) were found to have the best physical and low-temperature properties. 4 Cross-linking was normally introduced into the urethane-extended polymers through the use of intermediates possessing more than two functional groups or through secondary reactions of the isocyanate group with -NH-gr,oups derived from the primary reactions of the isocyanate group. The polyfunctionality could also be introduced in the glycol intermediates by polymerising the cyclic ethers with initiators having more than two functiona! groups. 5 • 6 The introduction of cross-linking into the polymers results in a polymer having a low glass-transition temperature, a decrease in elongation; the cross-linking has an insignificant effect on tensile strength. EXPERIMENTAL Materials and MethodsBoron trifluoride etherate (Fluka AG), tolylenediisocyanate (TDI) (2: 4 isomer Reidel), and hexamethylenediisocyanate (HMDI) (Fluka AG) were all distilled prior to use in the experiments. The tetrahydrofuran, triethylamine, and dioxane (AR samples) were purified as usual. 7 The polymerisation solvents, acetonitrile, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), and dichloromethane in analar qualities were dried by being kept overnight with phosphorous pentoxide, filtered, and fractionated in the reported boiling ranges. The ketal derivative of glycerol was prepared from dry glycerol and acetone with ptoluenesulfonic acid as catalyst. 8 The infrared spectra of polymers were taken by a Perkin-Elmer 257 spectrometer (KBr pellets and Nujol mulls). The intrinsic-viscosity measurements 99