Etil metakrilat`ın polimerizasyon kinetiği

Abstract

- I - ETÎL METAKRİLAT' IN POLÎMERIZASYON KİNETİ?İ ÖZET Vinil radikal polimerizasyonunda, başlatıcı konsantrasyonu düşük ise veya başlatıcı yan reaksiyonlarla zincir başlatamayacak gruplar vererek parçalanıyorsa, tüm monomerin polimerleşmediği, reaksi yonun düşük bir dönüşme değerinde durduğu gözlenmiştir. Jel olayının görülmediği ve termal polimerizas- yonun etkin olmadığı bu tür sistemlerde, kısa zaman da sınır dönüşmeye ulaşılabilir, ilk kez Tobolsky tarafından incelenen bu reaksiyonlara `dead-end` polimerizasyonu adı verilmiştir. Bu çalışmada `dead-end` polimerizasyonu yön teminden faydalanılarak, etil metakrilat monomerinin 2,2' -azobisizobütironitril başlatıcısı ile polimerizasyon kinetiği incelendi. Etil metakrilat ' in blok polimerizasyonunda jel olayı görülür. Etil me- takrilat'm, benzendeki çözeltisinde, 60°C'nin üs tündeki sıcaklıklarda, `dead-end` davranışı gösterdiği bulundu. Bu sistemin 60-110°C sıcaklık bölge sinde, limit dönüşmeye ulaşıncaya kadar sürdürülen- ıı - polimer izasyon reaksiyonlarında, çeşitli zaman aralıklarında, % dönüşme değerleri gravimetrik yöntemle belirlendi. Etil metakrilat monomeri için termal polimerizasyon verilerine kaynaklarda rastlanmadığından 60-110 C sıcaklık bölgesinde bir seri termal polimerizasyon deneyi yürütülerek, termal dönüşme değerleri belirlendi. Etil metakrilat için termal aktivasyon enerjisi 23.1 Kcal olarak hesaplandı. Başlatıcılı ve termal veriler beraberce değerlendirilerek, `dead-end` hesaplarının uygulanması ile baş- latıcı bozunma hız sabiti için, i, _ ı i cı ml5, -30.5 Kcal », -1* kd = 1.157 x 10 exp ( - -) (sn ) ve etil metakrilat'ın polimerizasyon hız sabitleri için, v fX/Vfe - Ln 07,-4094 cal,, İt, * kp f /kt - 40.97 exP (-) C^-^) k^fkj = 5.73 x 10`4 exp (^^) (.^BR) bağıntıları elde edildi. 90°C'de elde edilen polimer örneklerinin molekül ağırlığı sayı ortalamaları viskozimetre yöntemi ile belirlendi. Bu değerlerin- III - etkinlik katsayısının fonksiyonu olan kuramsal değerler ile karşılaştırılmasından 90 C'de etkinlik katsayısı, f = 0.412 a 0.4 olarak hesaplandı. Bu değer ortalama bir değer kabul edilerek hız sabitleri için, t _ o n-iK ` ın~4,,,8240 calx -mol.sns - 2 - 2.215 x 10 exp (-^) ( - ^ - ) K. bağıntısı bulundu. `Dead-end` kuramından sonra ileri sürülen bazı yaklaşık hesap yöntemlerinin deneylerimize uygulanabilirliği tartışılarak `dead-end` yöntemi ile karşılaştırıldı. Jel olayını da içeren polimerizasyon reaksiyonları için önerilen hesap yöntemleri tartışıldı. Jel olayının görülebileceği blok polimerizasyonu reaksiyonlarında izlenmesi gereken yöntemler gösterildi.

- IV - POLYMERIZATION KINETICS OF ETHYL NETHACRYLATE SUMMARY In the absence of the retardation of termina tion reaction, the radical initiated polymerization of vinyl monomers ceases a short of complete conver sion. That phenomenon is termed as dead-end polyme rization and appears either in a very low initiator concentration or the catalyst decomposes by side reactions which do not start polymer chains. In or der to apply the dead-end theory to a given monomer initiator system, the thermal conversion values must also be negligible compared with the initiated values. A complete theoretical treatment of the theory was first developed by Tobolsky for the case of sty- rene polymerization initiated with 2, 2 '-azobisizo - butyronitrile (AIBN). Following the proposed proce dure in short, the rate of radical initiated vinyl polymerization, assuming a steady-state radical con centration is,,rM, f k. [I], -'Sr1- Vır - } [,M] ' (I)- V - In this equation [M] and [I] represents the concent rations of the unreacted monomer and catalyst, f the catalyst efficiency, that is the fraction of radicals produced by the primary cleavage of catalyst that actually start polymer chains. The quantity k, is the specific rate constant for the first order primary cleavage of catalyst, k is the specific rate of propagation, k. = k. +k. j is the sum of specific rate constants t tC tu r of combination and disproport ionation respectively. In case of the azo catalyst the concentration varies with time according to the equation -k,t [ I J = [I] e d (2) o As the polymerization proceeds the monomer converts to a polymer of lesser specific volume. This results in an increase of the initial catalyst and monomer concentrations. At a fractional conversion x, x = ([M 1 - [M])/[M 1 (3) o o the effective unreacted catalyst and monomer concent rations are,- VI - IM] IM1= (T-Hb (1 - x) (5) where [M] and [ I] are the initial monomer and ca- o o talyst concentrations and a is the total volume shrinkage for the complete conversion of monomer to polymer. Substituting eq. (4) and (5) into eq. (1) and integrating in terms of the fractional conver sion x, one obtains successively, ` y+a _ ` 1+a L K,, cr`/,,. Jin ^ - = An ?=- - + - - (1-e ) (6) y-a l~a a where U 1/2 Wo 1 1 2 = (1 - ax)1'2 2k f [I] K = a - (I - a)112 k1/2 kj/2 At t-H» the conversion reaches a limiting value x o c so that; I ym+a ` 1+a K,-,, n -^-oa - = £n t^ - + - (') y-a 1-a a oo Equations (6) and (7) may be combined to give; _ LnUniLAl) - to(Z!«) ] = - in I + ^ t (8) y«ra.y-a a 2 This equation may be used with experimentally deter mined conversion time data simultaneously to calcu- late the specific rate of initiator decomposition k.- VII - and the kinetic constant f*'2k /k?'2. If the quantity P t k /k is known for the monomer, the initiator ef- p t ficiency may be found. If k, and f1'2 k /k1'2 for the J J a p ' t system are known the course of conversion with time may be accurately predicted. In this thesis the polymerization kinetics of the 2, 2 ' -azobisizobutyronitrile-ethyl methacrylate system was investigated by means of the dead-end theory in order to find the kinetic parameters of the system. When compared with other methacrylates EMA was not so highly investigated monomer. First of all the determination of the reaction conditions which would not meet the gel effect was necessary. To satisfy this restriction expriments were held in benzene solution and at temperatures higher than 60 C. Ön the other hand the effect of thermal poly merization had to be determined so that thermal polymerization reactions were performed concurrent with the initiated polymerization reactions. All of the observed time-conversion data were corrected for the thermal results and the absolute conversion values of the initiated polymerization were obtained.- VIII - The whole time-conversion profile of thermal and initiated polymerizations at 60-110°C were obta ined. Using the thermal data, the thermal activation energy of EMA was found as 23.1 Kcal. The initiated conversion-time data and limiting conversion values were used to determine the k, and k f1'2/k1'2 values d p t at 60-110 C as outlined before. The Arrhenius equa tions which gives the temperature dependence of k, and k f1'2/k1'2 over a 60-110 C range were calculated P t as follows,, n c,,«15,-30.5 Kcalx, -I. k = 1.157 x 10 exp (-) (sn ) d RT k f^/k^O 97 exp r4094 cal) ( U )1/2 p /Kt ^u*y/ exP <? rt ' Vl.sn'' i i*, 2 _ c,~ iri-4 /8217 cal%,-mol.snv kt/fk = 5.73 x 10 exp ( - ^-?) ( - ^ - ) In 90 C the molecular weight of the polymer samples were determined by viscometry. The theoreti cal molecular weight values were also calculated as a function of f. Comparing the theoretical and expe rimental values f =0.412 was found as a mean effi ciency factor for 90 G. ' '- IX - In order to apply the dead-end theory to a chosett* monomer-initiator system it is necessary to find the limiting conversion values in addition to the time-conversion data. In recent research conduc ted by Hill & O'Donnell, Ng & Chee and Stickler on polymerization kinetics new methods are proposed to find the k, and k f1/2 /Is.1'2 values without recourse d p t to the measurement of the conversion at infinite time. In this worki*h.ose theories were discussed in short and the time-conversion data in hand applied to the Stickler's method where applicable. A compa rison of dead-end and that newly proposed simplif ying techniques were included. Up to now, the vinly polymerization kinetics and controlling mechanisms on each step have been extensively studied. They may be put together in two main headings first, trying to find the control ling mechanisms -of the reaction steps and second trying to derive empirical equations for the rate constants. On these subjects researches of North & Reed, Otsu & Ito & Imoto and Mahabadi & O'Driscoll were briefly summarized.- X - The most recent approach is to find a general mathematical concept capable of describing the obser ved rate phenomena. This concept must be independent of the type of monomer, solvent and initiator. O'Dris- coll, Soh & Sundberg and Stickler et.al. developed different mathematical models for certain systems which were capable to fit the experimental data by means of some parameters. Having those models and the special parameters of a system in hand, one can predict the course of conversion and molecular weight of the same system in different conditions. A brief discussion of those mathematical models and their basis were included in order to encourge the forego ing work. A great deal of research was examined on vinyl polymerization kinetics. Compared to the most recent investigations the dead-end theory was found to be capable of describing the rate and molecular weight data where applicable. In the investigation of new monomer-initiator systems dead-end polymerization method was a valuable tool to determine the decompo sition rate constant of initiator and the rate- XI - constants of monomer. The method was especially help ful for investigations of very fast initiators where it was not possible to obtain data easily from the initial rates of polymerizations.