Grup transfer kopolimerizasyonu yöntemi ile hazırlanan çeşitli metakrilat kopolimerlerinde monomer reaktiflik oranları hesabı
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Abstract
DZET Grup Transfer Polimerizasyonu, oda sıcaklığında ya da yüksek sıcaklıklarda yaşayan polimerler veren yeni bir polimerizasyon tekniğidir. İlk olarak 19B3'te Eku Pont Araştırma Grubu tarafından dünyaya tanıtılmıştır. GTP ile, oda sıcaklığında heterojenlik indisi 1'e yakın polimerler elde edilmektedir. Bunun yanında mole kül ağırlığının denetimi de bu yöntemle mümkün olmakta dır. Monomer ağırlığı/Başlatıcı mol sayısı oranı ortala ma molekül ağırlığını vermektedir. Diğer yöntemlerdeki başlatıcı yanında bir de katalizör kullanılmaktadır. GTP'nda aktif merkez, büyüyen zincirin daima en so nunda yer alır. Yapılan incelemeler, zincir transferle rinin olmadığını göstermiştir. Büyümekte olan zincir an cak bir dış etkiyle sonlandırılabilir. GTP 'nda reaktif lik oranlarının hesaplanması, Anyonik ve Radikal polimerizasyonda bilinenlerle kıyaslanması me kanizmayı açıklığa kavuşturacaktır. Radikal kopolimeri- zasyonunda reaktiflik oranlarının 1 civarında olmasına karşılık, iyonik mekanizma ile yürüyen kopolimerizasyon- larda reaktiflik oranları 1'den çok farklıdır. G.T.P' nda da bu oranlar bize mekanizma hakkında fikir verecektir, Bu çalışmada, radikal yöntemle jel oluşturan iki ak tif merkez içeren Allilmetakrilat ' in, Metilmetakrilat, Etilmetakrilat ve Butilmetakrilat ile kopolimerleşmesi incelenmiştir. Monomer karışımları, önceden belirlenen değişik yüzde bileşim oranlarında reaksiyona sokulmuş^çö- zücü jbaşlatıcı ve katalizör varlığında polimerleştirilmiş- tir. Polimer analizlerinde infrared spektrof otometresi kullanılmıştır. AMA-MMA ve AMA-EMA kopolimarlsri için reaktiflik oranları Kelen Tüdös yöntemiyle hesaplanmıştır. MONOMER REACTIVITY RATIOS OF METHACRYLATES PREPARED BY GROUP TRANSFER COPOLYMERIZATION SUMMARY In this work Group Transfer Copolymerization is studied.. Statistical copolymers of aliyi methacrylate- methyl methacrylate, allyl methacrylate - ethyl methacrylate and allyl methacrylate- butyl methacrylate are synthesized by group transfer copolymerization. The monomer reactivity ratios of these copolymers are calculated according to Kelen Tüdös procedure. __ Group Transfer Polymerization (GTP) is a new type of living polymerization which is introduced by Du Pont in 19B3. This Polymerization technique is based on the 1:1 addition reaction of unsaturated esters, ketones and nitriles with the compounds containing activated Si,Sn,Ge, etc. With GTP, polymers with heterogenity indexes of about unity can be obtained at room temperature. Molecular weight control is also possible by using this technique. The molecular weight of the polymer produced by GTP can be determined by using the equation; n t t... Weight of monomer Average molecular weight = - =* Moles of initiator This allows one to control the average molecular weight in the 1 0DD to 2D. ODD range. However, it causes a problem when one tries to produce a polymer with a very high molecular weight because the amount of initiator required is so low as to approach the level of the impurities. It is possible to produce a polymer in the 1DD.DDD to 200. D00 molecular weight range with the use of very pure ingredients. VIMonoirters most suitable far use in GTP are the methacrylates, acrylates, acrylonitrile, methacrylo- nitrile and vinyl ketones. The monomers can contain any substituent groups based on oxygen, silicon, or nitrogen, however, active hydrogen atoms interfere with GTP. Active hydrogen compounds will stop the chain growth if they are present in concentrations higher than the initiator concentration. It is possible to produce polymers by GTP, with monomers containing active groups during the polymerization process if these groups are masked. This is not necessary if the active groups are hindered in some way and hence are rendered inert during the reaction. The most commonly used initiators for GTP are the ketene silyl acetals. Nitriles have also been reported in the literature to be efficient initiators. Unlike the other polymerization techniques which require the presence of only monomer and initiator; GTP also requires the presence of a cocatalyst. These can be either nucleophilic ; such as fluorides, bifluorides, azides and cyanides; or electrophilic, such as zinc chloride and zinc bromide. The solvents used for polymerization with a nucleo philic initiator are Tetrahydrof uran, Acetonitrile, and Toluene. Those used with an electrophilic initiator include dichloromethane. GTP bears a strong resemblance to anionic polymeri zation initiated by enolate ions. The mechanism of addition of the monomer to the active chain is very similar in both cases. Both techniques are useful for the same monomers, they have almost identical enthalpies and entropies of activation. The main advantage of the GTP, compared to the anionic technique, is that it requires much less demanding conditions. Anionic poly merization is restricted by temperature and solvent, where' as GTP operates over a temperature range of -1 DO C to 150DC. The most effective temperatures are between D°C and 5D°C. By choosing two (or perhaps more) suitable monomers, A and B, chains incorporating both can be prepared and Vllmany of the products exhibit the better qualities of the parent homopolymers. This is known as copolymeri- zation. Copolymerization enables us to obtain polymers which have very usefull properties. Even in the simplest case, that of copolymerization involving two monomers, a variety of structures can be obtained, and four important types exist: `Statistical copolymers` are formed when irregular pro pagation occurs and the two units enter the chain according to their reactivity ratios. i.e. ( ABBAAAABAABBBA). This is the most commonly encountered structure. `Alternating copolymers` are obtained when equimolar quantities of two monomers are distributed in a regular alternating fashion in the chain.(ABABABA). Many of the step-growth polymers formed by the condensation of two (A-A), (B-B) type monomers could be considered as alternating copolymers but these are commonly treated as homopolymers with the repeat unit corresponding to the dimeric residue. `Block Copolymers`. Instead of having a mixed distribution of the two units, the. copolymer may contain long sequences of one monomer joined to another sequence or block of the second. This produces a linear copolymer of the form AA~~~AABBB B, i.e. an {a} {b} block or sometimes an Aj /b/ {aJ- type block copolymer. ` Graft Copolymers`. A non-linear or branched block copolymer is formed by attaching chains of one monomer to the main chain of another homopolymer. In general block and graft copolymers possess the properties of both homopolymers, where'as the random and alternating structures have characteristics which are more of a compromise between the extremes. The following group of homo-and hetero-polymerization reactions were proposed by Dostal in 1936 for a radical copolymerization between two monomers M,, and M~, and ultimately extended and formalized by a number of workers who established a practical equation from the reactions: M1. + M1 - ^->^~M1. V + M2^~M2. - nz. + m2 k22> - m2. - M2. + M1 ^~~ M1. VXllwhere k-.. and k`2 are the rate constants for the self- propagaxing reactions and k.. ` and k21 are the corres ponding cross- propagation rate constants. Under steady-state conditions, and assuming that the radical reactivity is independent of chain length and depends only on the nature of the terminal unit, the rate of consumption of M. from the initial reaction mixture is then, -d p-g/dt = k^^]^.] + k^frgpig.] (D and M` by -d[M2]/dt = k22[M2][Mr] + k12[M2][Mv] (2) The copolymer equation can then be obtained by dividing equation 1 by 2 and assuming that k2* [M7O CMi]^ k>, 2 [M*.`] /3Ö] ^aT sxeady-state conditions, so that d[Mn] [M^ r^M,] + [M2] (3) d[M2] [M2] [M^ + r2 [M2] The. quantities r^. and r2 arB tne manDroer reactivity ratios uhich belong to Monomer -1 (M,, ) and Monomer-2 (M`) respectively. The monomer reactivity ratio is the ratio of the rate constant for a given radical adding to its own monomer to the rate constant for its adding to the other monomer (r = k11 _ k22 >, In radical copolymerization, the reactivity ratios far various methacrylates do not significantly differ from unity. In contrast for anionic and group transfer polymerization those ratios are quite different from unity and show a high similarity. This is a very convincing argument that the GTP follows an ionic rather than a radical mechanism. IXIn this uıork Allyl Methacrylate-Methyl Methacrylate, Allyl Methacrylate-Ethyl Methacrylate and Allyl Be^ha- crylate- Butyl Methacrylate statistical copolymers are synthesized. Allyl contents of AMA-MMA and AMA-EMA copolymers are determined by IR spectroscopy and monomer reactivity ratios are calculated by Kelen-Tüdös Method as r1 = 1.B54 r2 = 0.671 (AMA/MMA) and r^ = 4.7B1 r2 = 0.971 (AMA/EMA).
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