Stirenin ısısal polimerizasyonunda lipoamidin etkisi

Abstract

ÖZET Günümüzde, polimerik malzemeler günlük hayatımızda artan bir oranda kullanılmaktadır. Buna paralel olarak polimer kimyası da önemli bir bilim dalı haline gelmiştir. Polimerizasyon sistemlerinde başlatıcı, modifiye edi ci, zincir sonlandırıcı, önleyici, geciktirici, iniferter, monomer ve aktif solvent olarak kullanılan kükürtlü bile şikler, bu dalda önemli bir yer teşkil etmektedir. Kü kürtlü bileşiklerden halkalı disülfitler de vulkanizasyon hızlandırıcısı, radikal başlatıcısı ve zincir transfer bi leşiği olarak endüstride, çeşitli amaçlar için biyokimya sal alanda, iniferter olarak blok kopolimer sentezlerinde ve daha birçok alanda kullanılmaktadır. Bu çalışma, halkalı disülfit olan lipoamidin, polime rizasyona ve kopolimerizasyona uğrayıp uğramadığının, v inil monomerlerin polimerizasyonlarında başlatıcı olarak davranıp davranmadığının anlaşılması amacıyla gerçekleşti rilmiştir. Bu amaçla, lipoamid miktarının, sürenin, stiren ile lipoamid toplam konsantrasyonunun, sıcaklığın, verime ve molekül ağırlığına etkisinin anlaşılması için değişik koşullarda, lipoamid varlığında stiren polimerizasyonları incelenmiştir. NMR, İR ve elementer analiz teknikleri ile karakterize edilen polimerlerin molekül ağırlıkları da GPC yöntemiyle belirlenmiştir. Ayrıca, mekanizmayı açıklayaca ğı düşünülerek, lipoamidin lazer flaş fotoliz çalışmaları gerçekleştirilmiştir. Sonuç olarak, lipoamidin kendi kendine polimerizasyo na uğramadığı halde stiren ile kopolimerizasyona uğradığı, yani monomer olarak değil de komonomer olarak davrandığı, ayrıca düşük konsantrasyon ( <^% 2.5 mol) ve kısa polime rizasyon sürelerinde (</v<46 saat) başlatıcı ve diğer koşul larda, çoğalan polimerik radikalleri kısmen sonlandırıcı, yani geciktirici olarak davrandığı gözlenmiştir.

Effect of Lipoamide on Thermal Polymerization of Styrene SUMMARY Polyrr.er science is a rather young domain of science, but it has developed very rapidly over the past 50 years. Synthesized polymers are utilized increasingly in our daily life, and manifold industrial applications have contributed to their expansion. Sulfur compounds are one of the important group to participitate in vinyl and related polymerization. Some of them function as initiators or as part of an initiator system, some as modifiers, chain-terminating agent, or inhibitors, and some as monomers or comonomers or as active solvent in the polymerization process. A large number of sulfur compound such as organic sulfides, di sulfides and polysulfides can initiate polymerization of vinyl and related monomers and act as modifiers or chain transfer agents under different conditions. Disulfides, for example (PhCH2S)2, are good oxidation inhibitors for commercial products like lubricating oils and polymer. By reducing the concentrations of peroxides, they have been trought to break the oxidative chain reac tion, perhaps they also trap free radical chain carriers. Unimolecular scission of some disulfides can be induced thermally in the neighborhood of 100-150'C to give RS`, a thily radical (sometimes called a sulfenly radical) as seen in equation (1). R'SSR* A or hv > R'S' + R'S* (1) Evidence for such dissociation is the initiation by some disulfides of vinyl polymerization with heat in the dark. Thus the commun vulcanization accelerator tetramethyl- thiuram disulfides (TMTD) [(CH, ) 2NC(S)S] 2, is nearly as efficient as a peroxide in initiating polymerization of methyl methacrylete (70`C). Ferrington and Tobolsky, however, studied the thermal polymerization of methyl methacrylate at 70`C, using tetramethylthiuram disulfide, as the initiator, and found it some what less reactive than benzoyl peroxide. A plot of the square of polymeri zation rate against TMTD concentration deviated markedly from linearity a rather low concentrations of the initi ator. Thus, besides bimolecular termination of the growing chains, other modes of termination are significant, particularly at low initiator concentration. This presumably involves termination by reaction of the growing chains with TKTD molecules. It has been established that VIthe retardation of polymerization in this system is due to the initiator molecules rather than their decomposition products. Tetraalkylthiuram disulfides have been found to be capable of initiating thermal polymerization of styrene and methyl methacrylate in the temperature range 50-90°C, but not of vinyl acetate and acrylonitrile. Benzothiazolyl disulfide initiates with the methacry late also (at 95°C), although it fails with certain mono mers where radicals of growing polymer chain attack the S-S bond in preference to monomer. As indicated, tempera tures vary at which thermal homolysis of disulfides occurs, a further illustration is that methyl ethyl disulfid (CH3- S-S-CH2-CH3) will cause acrylonitrile to polymerize at 150°C in the dark, though not at 100°C. Photolytic condi tions, on the other hand, can lead to initiate of vinyl polymerization by most disulfides at 25°C. Indeed, disulfides can be even more effective initiators than benzoyl peroxide under such conditions. Dibenzoyl disulfide, JZf-CO-S-S-CO-JZf, decomposes slowly to radicals that induced vinyl polymerization at 70°C. Many other aromatic disulfides, such as diphenyl, dibenzyl, dithio benzyl, dinitrodiphenyl, dibenzothiazyl, and dialkyl xanthogene disulfide were reported to be ineffective as thermal initiators of polymerization, but some of them were found to act as retarders of polymerization. Ferrington and Tobolsky, however, reported that diphenyl disulfides exhibited a combination of initiating and retarding abi lity at 100°C. It appears that the confusion as to whether some of the above-mentioned disulfides initiate or retard polymerization can be resolved by considerations at which they are present in polymerization systems. At low con centrations, they may behave as initiators, but at rela tively high concentrations their role as retarders becomes very prominent because of extensive chain, transfer (possibly of the degredative type). Most of organic di sulfides mentioned above, as well as certain monosulfides, are effective photoinitiators of vinyl polymerization. Pearson et al. studied the polymerization of styrene in the presence of a varietry of disulfides and showed that they have little effect on the over-all polymeriza tion rate and that the polymers incorporated two RS-end groups per molecule. This would indicate the generation of only RS* radicals from disulfides as a result of chain transfer. Parallel observations were made by means of polymerization in presence of ring disulfides; incorpora tion of large amounth of sulfur into the polymer molecules in this case is explained by consideration of chain exten sion due to copolymerization at each instance of chain transfer involving a ring disulfides (2). Mn* + S-S > Mn-SRS' (2) Mn-SRS` + M > Mn-SRS-M', etc vnCyclic disulfides are easily polymerized to polydi- sulfides. Precautions must be taken to perevent polymeri zation during their synthesis and in some cases (four-and five-membered ring compounds) they are unstable and spon taneously form polymers. Tobolsky and coworkers, reported that l-oxa-4,5-dithiacyclo heptane and l,3-dioxa-6,7-di- thiacyclo nonane are polymerized under the influence of cationic catalysts such as sulfuric acid, iron (III) chloride (with a trace of water), aluminium chloride, boron trifluoride, and tin (IV) chloride. Cyclic disulfides have been copolymerized with vinyl monomers such as styrene, butyl acrylate, vinyl chloride, and acrylonitrile under the influence of radical initi ators such as azobisisobutyronitrile. That the incorpara- tion of cyclic disulfide is really due to copolymerization and not to chain transfer reactions has been established by comparing the thermal polymerization of styrene in the presence of a simple dialkyl disulfide (dibutyl disulfide) and in the presence l-oxa-4,5-dithiacycloheptane. Lipoic acid and lipoamide are important coenzymes involved in the acyl transfer reaction in living system. Lipoic acid is immobilized on crosslinked polystyrene beads to generate a polymeric reagent. Here, lipoamid molecule is attached to polystyrene backbone via amid linkage and cyclic disulfide sturucture is present. o CH2-NH-C-(CH2)4~^ S- S In this work, thermal polymerization of styrene in the presence of lipoamide was investigated. It was found that the resulting polymer containing both lipoamide and styrene segments. The polymerization yield reached to a maximum of 52.5 % at 120`C of polymerization temperatu res. The effect of temperature on lipoamid content of the polymer and its number avarage molecular weight was signi ficant. Increasing temperature resulted a decrease in molecular weight and the lipoamide content of the polymer. This result may be due to slower decomposition rate of lipoamide compared to polymerization rate of styrene. The lipoamide content of the recovered polymer was calcuted from both elemental analysis results and NMR spectrums and similar result were observed. Lipoamide content of the polymer increased with increasing initial lipoamide con centration in the solution. Similar results were also observed with increasing polymerization time. At the early stage of polymerization, the yield of both styrene and lipoamide/styrene systems was similar. However, at vmlater stage of the polymerization, the yield was reached to a plato and with increasing lipoamide content, the yield decreased with increasing polymerization time. Compared to thermal polymerization, lipoamide content up to about 2.5 % caused to molecular weight to reach to a higher value while above 5 %, increasing lipoamid content resulted a decreasing molecular weight when the polymeri zation time was ca. 46 hours. However, further increase in the polymerization time resulted a decrease in molecu lar weight. From these results, the thermal polymeriza tion of styrene in the presence of lipoamide may proceeds as in the scheme (I). The yield of photochemical polymerization was lower compared to thermal polymerization and lipoamide content was higher. This suggests that lipoamide acted as a co- monomer. On the other hand, lipoamide was found to act as an initiator at low concentration (2.5 %) if polymeri zation time was about 46 hour while it acted as a retarder at higher concentration and for longer period of polymeri zation time. Laser flash photolysis results also support the above conclusions. Decomposition of lipoamide via S-S bond formed dithiyl radical with a life time of appx. 300 ns. In the presence of styrene, the concentration of dithiyl radical decreases with increasing styrene concentration. This suggested that dithiyl radicals react with styrene. As a conclusion, lipoamid, does not homopolymerize but can be copolymerized thermally with styrene. At the low concentration (2.5 %) lipoamide acts as an initiator if the polymerization time is kept for 46 h. other condi tions, i.e., higher lipoamide concentration and longer polymerization time, results a lipoamide/styrene copolymer. The polymer produced by this way is assumed to have follow ing structure. fS-CH0-CH0-CH-Sl- -[CH0-CHl u 2 2-Jm L 2 ı Jn c=o v Ah2 Here lipoamide molecules present in the polymer molecule have free amide groups. On the contrary to lipo amide supported polystyrene, the polymer does not contain cyclic disulfide groups. IXX o m CM X o -M CO CM X 0=0 M 0> E >. o D. ro.* <r m co.3- oo -M V3 OL C0 E tu sı o CO cc X.o I CM X a H§> 5^> CM X o CO X X o 1 1 r®BÖLÜM 1. GİRİŞ VE AMAÇ Bazı kükürtlü bileşiklerin, polimerizasyon sistemle rinde, başlatıcı, mcoifiye edici, zincir sonlandırıcı, ön leyici, geciktirici, monomer ve aktif çözücü gibi değişik amaçlarda kullanımı Ghosh [l] tarafından geniş olarak ince lenmiştir. Lastik sanayiinde, vulkanizasyon amacı ile bir çok kükürtlü bileşik, vinil polimerizasyonlarında aktifle- yici (sensitizer) olarak uzun süredir kullanım alanına sa hiptir. Bu alanda, diazotioeterler ısısal [2] ve disülfit ler fotokimyasal aktifleyici [3] olarak tül (S*) radikali üretimi için kullanılmıştır. Organik polisülfitler, vinil polimerizasyonlarında başlatıcı olarak kullanılmış [4,5,6] ve bazı polisülfitlerin de [7] önleyici etkisi gösterdiği gözlenmiştir. Organik disülfitler kondenzasyon polimeri zasyonlarında monomer olarak kullanılarak kükürt içeren po limerler [8,9] elde edilmiştir. Disülfitlerin, vulkanizas yon hızlandırıcısı [10], radikal başlatıcısı [11,12] ve zincir transfer bileşiği olarak [13] polimerizasyon siste lerinde endüstriyel amaçlar için kullanımı incelenmiştir. m- Disülfitler, teorik çalışmaların yanı sıra S-S bağ ya pısının ve bölünme mekanizmasının önem kazandığı çeşitli a- lanlarda da [14] kullanılmaktadır. İnsülindeki polipeptit zincirleri ile disülfitlerin çapraz bağ (cross-link) oluş turması [15], oksitosin ve vasopresin gibi laktojenik ve diüretik hormonların fizyolojik etkinliklerinin belirlenme si [16] albümin serumu gibi proteinlerin denatürasycnu ve daha sonra pıhtılaşmasında disülfit halkalarının [17] ve o(-lipoik asidin, piruvik asidin dönüşüm sistemindeki etki sinin incelenmesi [18] biyokimyasal alanda yapılan çalışma lara örnek olarak gösterilebilir. Tetrametiltiuram disülfitin (1.1), vinil monomerlerin