Polisilanların kullanımıyla fotokimyasal blok kopolimer sentezi

Abstract

Polisilanların belli dalga boyundaki ışık altında, uygun monomerlerın varlığında, serbest radikal polimerizasyonunu başlattığı bilinmektedir. Bu çalışmada, polisilanların ışınlandırılması sonucu oluşan silil radikallerinin başlatıcı: etkisinden yararlanarak serbest radikal mekanizmayla polimerleşebilen Metil Metakrilat ve Stiren monomerlerinin blok kopolimerinin sentezi gerçekleştirilmiştir Ayrıca blok kopolimerizasyona aydınlatma süresinin ve ön polimer yapısının etkisi incelenmiştir. Sentezlenen blok kopolimerin yapısı ayırma teknikleri ve spektroskopik yöntemlerle belirlenmiştir.There is a continuous interest in developing new synthetic routes to block copolymers due to their novel physical and mechanical properties which are not offered by the corresponding homopolymers. High molecular weight soluble and formable polysilanes find application as photoresist materials [20], photoconducters [21] and photoinitiators of radical polymerization [22-2$. These applications are mainly based on their strong ultraviolet absorbtion in the region 300 - 350 nm and rapid photodegradation upon irradiation at this band. It has been reported by West et al. [22,23] that polysilanes are effective photoinitiators for the free radical polymerization and it was assumed that the initiating process consists of the reaction of silyl type radicals with vinyl monomers. Therefore, it appeared interesting to include this rather novel class of polymers in our current program concerning block copolymerization by using them as multifunctional initiators. As it will be shown below, radical routes are employed Polymethylphenylsilane was used to photoinitiate polymerization of methylmethacrylate. Polymethylmethacrylate (PMMA), obtained this way, possesses remaining polysilane chains. Photolysis of PMMA in the presence of vinyl monomers such as Styrene makes it possible to prepare block copolymers. Experimental Materials : Monomers, Styrene (St), Methylmethacrylate (MMA) were washed with aqueous NaOH solution, dried over CaH2 and distilled. Dichloromethane (Aldrich) was dried over CaH2 and distilled. Polysilanes were prepared as described by Zhang and West with the aid of the Wurtz coupling reactions using finely dispersed sodium and diphenylmethylsilane. Notation and molecular weights of poiymethylphenylsilanes (PMPSi), prepared and used in this study, are collected in Table 1. Table 1. Photoinitiator Polysilanes *:35C n=. ? -r law.1^ ?hctc initialed block copclymeris =o I'-Taethvlnethacrvla-he0 ion51- of stvrene vith a A: 350 nm ; [St]: -8,7 moll-1 b [PMMA] : 25 gl-1 mf inish-IT,-s~a-~ mstar~ A m= VI Photoinitiated Free Radical Polymerization : Appropriate solutions containing polysilanes and MMA or St in dichloromethane was degassed with nitrogen prior to irradiation. At the end of irradiation from an Annular photoreactor (Applied Photophysics) equipped with lamps emitting nominally at 350 nm, polymer was precipitated with methanol. Block Copolvmerization : Experimental conditions were the same as free radical polymerization, except that polymers obtained above were used as light absorbing molecules instead of PMPSi. Analysis H NMR measurements were performed in CDCI3 solution using a Varian instrument (model EM 390). UV spectra were recorded on a Perkin Elmer Lambda 2 UV - VIS spectrophotometer. Infrared spectra were recorded on a Jasco FT/IR 5300 spectrometer. Molecular weight of polymers were determined by gel permeation chromatography using polystyrene standart samples. A Knauer M64 instrument employed with a differential. Results and Discussion Block Copolymers by Free Radical Route : Photoactive polymethylmethacrylate (PMMA) was prepared by photoinitiated free radical polymerization of MMA initiated by PMPSi. The segment lenght of polysilane incorporated to PMMA was controlled by irradiation time. The purposed reaction mechanism is presented in reactions (1,2,3). VII R1 R1 R1 «mm» Si - Si - Si"***- hv Rj R2 Rj -^.2 ********* Si ' * *' (1) *S h Si - Si****** - ?- - - >. 2 ********* S\ R2 R2 (2) CH3 CH3 ( 3 ! 3 Si - SI****- O O CH3 ÇH3 ÇH3 CH, hv MMA >CH? - C ****Si - Si*w*CH2 - C OCHj OCH3 (3) Photoactive PMMA The primary silyl radicals from polysiiane decomposition initiate and propagate with MMA yielding polymer Si - Si bonds in the main chain. A typical UV absorbtion spectrum for a photoactive PMMA is displayed in Figure 1a. This spectrum exhibits the polysiiane absorbtion band between 310 and 360 nm. With longer irradiation, the absorption above 300 nm dissappears which indicates further cleavage of polysiiane segment attached to PMMA (Figure 1 b - c). Moreover, low molecular weight products are formed upon irradiation indicating photodegradation of PMMA (Figure 2). As seen from Table 2, conversion and molecular weight of PMMA, and the silicon content of PMMA can be adjusted by irradiation time. Photoactive PMMA were used to initiate polymerization of St. Obviously, the initiating radicals are derived from silyl radicals as a result of photodegradation of polysiiane moiety in PMMA backbone. Photoactive PMMA h v St >CH2- C ÖCH /n St - MMA Block Copolymer (4) VIM Typical results are presented in Table 3. As it can be seen, by adjusting the amount and position of polysilane segment in the initially - formed PMMA, higher block yields may be obtained. A different monomer sequence may also be used. When polystyrene, obtained with 6 min. irradiation under identical conditions to PMMA 2, Table 2, used as initiator for the polymerization of M MA, 48.3 % block yield was obtained after 10 minutes irradiation. In conclusion, these results indicate that polysilanes with appropriate absorption characteristics can be used to produce block copolymers of monomers that are prone to free radical polymerization. Preparation method pertains to attachment of Si - Si bonds to polymers by means of photodegradation of polysilanes and subsequent photolysis of these groups in the presence of monomers. Q 330 a (nm) Figure 1. UV spectra of photoactive PMMA obtained by using PMPSi a) Before photodegradation, b) alter 1 min., c) after 3 min. irradiation ix .'-*V.^ ; / 1'* IK -!~ Ve (ml) Figure 2. Gel Permeation chromatograms of photoactive PMMA 3) Before photodegradation b) after 0.5 min. c) 1.5 min. irradiation