X-, W dihidroksi politetrahidrofuran sentezi ve poliüretan eldesinde kullanımı

Abstract

Bu çalışmada, yumuşak polieter zinciri içeren blok poliüretanların eldesi için yeni bir yöntem geliştirilmiştir. Bilindiği gibi, piridinyum tuzlan fotokimyasal katyonik polimerizasyon başlatıcısı olarak kullanılmaktadır. Bu polimerizasyonlarda, piridinyum tuzu doğrudan fotokimyasal parçalanmaya uğrayarak aktif katyonları oluşturduğu gibi, dolaylı olarak da aktive edilebilir. Bu özellikten yararlanılarak piridinyum sonlu poHtetrahidrofuranlar sentez edilmiştir. 300 ninnin üzerinde aydınlatma yapılarak, piridinyum uç grupları hidroksi uç gruplarına dönüştürülmüştür. Elde edilen hidroksi sonlu polimerler, di ve tri fonksiyonel diizosiyanatlarla kondenzasyona sokularak poliüretanlar elde edilmiştir. Elde edilen poliüretanların THF içersindeki çözünürlüğü kullanılan izosiyanat miktarlarına bağlı olduğu gözlenmiştir. îzosiyanat konsantrasyonunun artmasına bağlı olarak allofonat ve biuret yapılan oluşup, çapraz bağlı çözünmeyen malzemeler elde edilmiştir.

Polymers with hydroxy, carboxyl, amine functions are of increasing interest due to their use as crosslinker, chain extender precursors for block and graft copolymers. These telechelics can be obtained by a variety of procedure. The most convenient way to prepare telechelics is the termination of living polymerization with appropriate reagents. For instance hydroxy functional polytefrahydrofuran(PTHF) was obtained by deactivation of living polymerization ofTHF with excess water. More recently we have prepared PTHF-pyridinium salts by pyridinium N-oxide deactivation as precursors for block copolymers. Upon direct and sensitized irradiation of these photoactive PTHFs alkoxy radicals at both chain ends capable of initiating the radical polymerisatlon of methyl methacrylate. In this way, ffiblock copolymers were formed. CF3S03 CF3S03 MMA PMMA-b-PTHF-b-PMMA The present study was primarly conducted to uncover whether photolysis of the polymers terminated by p-phenyl pyridinium ions can yield hydroxy end functional polymers. During our investigation on photoinitiated cationic polymerization using pyridinium salts it soon became evident that ethanol is formed upon photolysis of pyridinium salt in hydrogen donor solvents such as THF according to the following sequence ofreactions. hv OEt vi o o EtO + • + EtOH (iii) It seemed therefore appropriate to use alkoxy pyridinium terminated polymers as precursors for hydroxy-functional polymers. For this purpose PP+PTHF was prepared by living polymerization of TI--F initiated by triflic anhydride as bifunctional initiator and terminated by N-oxides. Irradiation of this polymer THF at 320 nm resulted in the shift of the characteristic pyridinium ion absorbtion band (max=320 nm) to shorter wavelength (figure I a and b) and in the formation hydroxyl end grups. The observed blue-shift is due to the absorbance of p-phenyl pyridine formed according to the reactions 4-6. Indeed, this new band dissapeared when the polymer was reprecipated in to methanol (figure I c). CFS CF3X)3 (v) (vi) OOA (vii) (3 200 300 400 R (nrn) Figure 1. UV spectra of PP+-PTHF (a) before (b) after irradiation at 320 nm in THF and (c) after precipitation into methanol Confirmation of the presence hydroxyl groups was obtained from the characterization of PTHF end capped with phenylcarbamate groups. Hydroxyl chain ends were converted into phenylcarbamate by treatment with phenylisocyanate in the presence of catalytic amount of dibutyltin dilaurate. (viii) O o + R-NCO ....................-.....-..> (ix) NH (x) The phenylcarbamate end-groups exhibit a strong absorption at 235 nm (figure 2). Knowing the extinction coefficent (e23s=15600) and assuming that each polymer chain contains two phenylcarbomate groups, the value of the Mn can be calculated.The data are collected in table 1. As can be seen, there is a remarkable agreement between the number average molecular weights obtained by both methods. This clearly indicates that polymer chains carry two hydroxyl functions after the photolysis. Even more convincing evidence for the presence of the hydroxyl functionality was obtained from the in situ polyurethane formation upon photolysis of PP+-PTHF at 320 nm in THF containing toluen diisocyanate and catalyst. As the irradiation proceeds an insoluable polyurethane formed possesses urethane carbonyl bands at 1730" and 1540 cm-I in addition behavior may be due to the formation of allophanate and biuret linkages from an isocyanate functions and already existing urethane link since hydroxyl ffinctions are available only when the pyridinium ions are decomposed by light. Table 1. Number average molecular weights of polytetrahydrofuran Method of Determination GPC 12600 UV hen I carbamate 12000 In conclusion it has been shown that alkoxy pyridinium terminated polymers are suitable to produce hydroxy-functional telechelics via photoinduced process. This process is particularly atrictive in applications where hydroxyl groups are needed to be protected. Further works on the application of resulting polymers in other reactions as well as the nrepa.ra. ' of chain -oxy pyrö alium san as precv Jrs for v' )lymers with *dant hY 'xyl grov now progress Wavelength (nm) Figure 2. UV Spectrum PTHF with a phenylcarbamate end groups (1.66 mol.l l, Imax=235 nm) ix ao.o 40 . 0 4600 WAVENUMEäS cm-2 aCO Figure 3. l.r. spectrum of the cross-linked polyurethane