Azo bağı içeren politetrahidrofuran sentezi ve karakterizasyonu

Abstract

}IET Bu çalışmada yumuşak palieter zinciri (soft segment) içeren blok kapolimerlerin eldesi için yeni bir yöntem geliştirilmiştir. Bu amaçla tetrahidrofuranın azo bağı içeren bir diosit klorür 4-4. azobisiyanopentanailklorür) ve gümüş tuzları yardımıyla başlatılmış `katyonik polimerleşmesi değişik koşullarda incelenmiştir. Gravimetrik yöntemler yardımıyla tetrahidrofuranın katyonik palimerizasyonu için farklı sıcaklıklardaki hız sabitleri hesaplanmıştır. Ayrıca polimerizasyonun termodinamik parametreleri belirlenmiştir. Palimerler, U.V., I.R., 'H-NMR gibi spektroskopi k yöntemle rin yanı sıra G.P.C. ve viskozite yöntemleri kullanılarak da karakterize edilmiştir. Polimerizasyonun öne sürülen azo oksokarbenyum mekanizması doğrultusunda gerçekleştiği ve dönüşümün kullanılan karşı iyondan bağımsız olduğu saptanmıştır. Bu yöntem ile elde edilen azo bağı içeren politetrahidrofuran polimerlerinin blok kopolimer sentezinde kullanılabilirliğini belirlemek amacıyla yapılan ısısal bozunma deneyleri sonucunda herhir politetrahidrofuran zincirinin ısısal yada fotakimyasal aktif azo bağı içerdiği ortaya çıkmıştır. -v-

SYNTHESIS AND CHARACTERIZATION OF AZO-LINKED POLYTETRAHYDROFURAN SUMMARY In the applications-oriented polymer research the trend Is to concentrate more and mare an the improvement of established polymerization techniques and to develop composites from already available polymers. Such composites can be produced either by simply mixing two or more polymers together (Polymer Blends) or by joining the appropriate polymer via covalent bonds. The former is easier, most direct and cheaper way of producing new materials. However, the properties of such mixtures are very dependent on the mlscibility of the components. Immiscible polymer blends generally display disappointing mechanical performances due to lack of interfacial adhesion and to uncontrolled phase-separation processes. The latter leads, principally to either block or graft copolymers which because of the large number of possible structural variations, represent an enormous scientific challenge to understand the details of such systems. The other way of obtaining a polymer consisting of two different monomers is to copolymerize the monomers. Depending on the reactivities of two monomers, the copolymerization tends to yield either statistical or altenating copolymers. However, since the properties of products, such as Tg, gas permeabality and mechanical strength are determined by avarage composition, any improvement made by increasing the amount of monomer (M. ) leads to a consequent loss in the desirable properties of the comonomer (M2). The preparation of block copolymers was greatly stimulated by the development of living (i.e. terminationless) anionic polymerization technique. Preparing block copolymers using an anionic mechanism is certainly the best way of obtaining monodisperse copolymers, free of homapolymers, with well defined and predetermined structure and molecular weight. However, the living anionic polymerization technique although attractive, has some limitations. If the mechanism of propagation can be transformed from one best suited to the first monomer to that best suited ta the second monomer so on, then all monomers may be polymerized. for the preparation of block copolymers. Further consideration of this concept indicates that each transformation requires three distinct steps; the first introducing the functional group into polymer by living polymerization of first monomer using appropriate mechanism, the second involving termination of polymer, isolating the polymer and then dissolving it in an appropriate solvent -VI-containing the second monomer. The third step is an reaction on the functional group of the polymer to transform it into an active specie capable of polymerizing the second monomer. Palytetrahydrofuran has many desirable properties which have been utilized by industry in polyuretnane and polyester thermoplastic elastomers. In these applications; slow crystal iza- tion tendency of polytetrahydrofuran at room temperature is overcome by copolymerization, and difficulties in cross-linking are circumvented by introduction of copolymeric segments capable of forming networks. As a homopolymer, polytetrahydrofuran is currently too expensive to use with general-purpose or even specially elastomers, but as the soft segment of a thermoplastic elastomers, polytetrahydrofuran is widely used industrially. In these applications it is valued as a precursor leading to elastomers with outstanding hydrolytic stability, good low-temperature flexibility, outstanding thermal stability at elevated temperatures. The aim of this study is to investigate' a new method to prepare block copolymer of polytetrahydrofuran by cation to radical and reverse transformation polymerization. The cationic polymerization of tetrahydrofuran proceeds as a living system if the conditions are properly selected. In such a system each polymer chain has an oxonium ion as a chain end. If a bifunctional initiator is used, a bifunctional living pTHF is obtained. It has been shown that oxoca'rbenium ions with low nucleophilic counterions can easily be prepared by reacting an acid chloride with a stoichiometric amount of silver salt, such as silverhexofluarDantimonate (AgSbFg), silvertetrafluoroborate (AgBF),, and.silverhexafluorophosphate (AgPFg). R - CDC1 + AgSbF6 ~R - CH0+SbF6+ AgCl (I) The diacid chloride was prepared by reacting k.k*- aznbisCcyanopentanoic acid) with phosphorus pentachloride. ChL CH, I 3 j 3 H0DC-CH`-CH`- C - IV = N - C -. CH`-CH`-CDDH (II) I CN 0 DHL If I 3 I CN PC1.- 3 I 3 H CI - C -CH--CH-,- C-IM = I/I-C- CH`-CH`-C - CI (III) <L <L d. c UN CIM. The method used in this study involves a two-stage procedure. First, synthesis Df polytetrahydrofuran containing one azo linkage in the main chain by the use Df difunctiDnal azo-oxocarbenium initiator and then the decomposition of the azo linkage produces -vii-3k %, and for silvertetrafluoroborate at -20, DD respectively. and +1DUC In order to obtain the propagation rate constant, the data were plotted l/[lJ.Ln ([Mj-Dl]) / ([Mj- [M ] ) versus time. The straight lines obtained in all cases. The propagation rate constant were determined from the slopes of straight lines. Following k values were obtained: P k (L.molTİ sec`-1-) T(DC) -2D D +10 AgSbF, AgBF,; 4.7x10 1.6xlD -k -k 3.69xlD 1.27x10 -3 -3 1.16x10 4.55x10 -2 -3 These values are quite comparable to that previously founded values for the cationic polymerization of tetrahydrofuran using oxocarbenium salts. The results for the silverhexafluoroantimanate and silverhexafluoroborate indicated that (1) no termination occurs (upto 25 % conversion) (2) initiation takes place quickly and quantitatively. Arhenius plots give activation energies of E=57.6 K.J.mol` far silverhexafluoraantimonate and of Ea = 65 H.J. mal far silvertetraf luoroborate respectively. The equilibrium concentration Me is related to temperature according to the equation of Dainton an Ivin as shown belDw; Ln [%] Ah As0 E E_ RT R Cuii) The heat of polymerization (AH ) and corresponding entropy values (ASD) were calculated; ^ AH (K.J.mol`1) AS°(J.mol. ^degree`1) AgSbF, ; AgBF^ -2k -29.7 -86.7 -61.2 It is interesting to note that the reported values of equilibrium monomer concentration [thfJ (3.1 mol.L-1 ) for the polymerization of tetrahydrofuran initiated by CgH5C0+SbFg~ at 25DC also fits with our values which implies that the azo-oxocarbenium initiation shows the general behaviour of oxocarbenium type of initiation. When silvertetrafluoroborate was used in the system a decrease in the rate of polymerization and broadening of molecular weight distribution were observed. The monomer conversion and polymer viscosity increased with -ix-increasing concentration of initiator system. B.P.C. chromotagrams, indicate that the avarage molecular cueight increases with both increasing reaction time and temperature and molecular weight distribution becomes broder. Conformation of the structures of the polymer was obtained from the detailed characterization of polytetrahydrof uran end-capped with phenoxy groups. For this purpose polymerization of tetrahydrofuran was initiated by 4.A-'-azobis(cyanopentanoil chloride) / silverhexafluaroantirnanate initiation system, at DDC. After few minutes, propagating chain end were converted into phenyl ether by treating with sodium phenoxide. SbF~ f VvwvwwHM = /^wwa~~-q ShFğ+2Nad/0/ ^.^-V /^ °r-^ V-y (VII) (O/-0 (CH^ C-*WM/I = l/H^vwH]- (CH2)it-0-/O) +2I/laSbF6 The end-standing phenoxy groups exhibit a strong U.W. absorbtion at 272 nm. Knowing the extinction coefficient and assuming that a polymer chain contains two phenoxy groups the value of number avarage molecular weight can be calculated. 'H-NMR is another efficient method of determination of number avarage molecular weight. There is a satisfactory agreement between the number avarage molecular weight obtained by all methods mentioned above. This clearly indicated that the every polytetrahydrofuran contains one azo linkage in the main chain and two phenoxide function at the both chain ends. Even mare convincing evidence for the presence of the azo linkage was obtained from the thermal degradation of polymers produced by means of azo oxocarbenium initiation. The thermolysis of azo group was almost completed after 6 hour heating at 60 C. The agreement between number of chain scission per molecule and expected azo content is remarkable. It is concluded that block copolymerization by cationic and radical routes is versatile and two-stage method applicable to most nucleophilic and vinyl monomers. The overall structure of the black copolymer is determined by a well-defined structure of prepolymer which itself requires knowledge of the reactivities of the components and of the reaction kinetics. The cationic `Living` polymerization technique of tetrahydrofuran can be used for the synthesis of well defined prepolymers. The azo functionality can be introduced efficiently, upon initiation by using an aza-oxocarbenium salt as initiator. -x -