Preparation And Evaluation Of Novel Initiators For The Thermally Mild Living And Controlled Free Radical Polymerization Of Methacrylates

Author : Mustafa Değirmenci
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (85 download)

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Download or read book Synthesis and Characterization of Novel Well-defined Macrophotoinitiators Prepared by Atrp and Rop Methods and Their Use in Block Copolymerization written by Mustafa Değirmenci and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Macrophotoinitiator is the abbreviation of molecular photoinitiator. A macrophotoinitiator is generally referred to a linear macromolecule possessing side- or main- chain photo reactive groups capable of initiating polymerization reaction. These materials are of great scientific and technological interest because of their application in UV-curable coatings and as precursors for graft and block copolymers depending on the position of the photoinitiator moiety incorporated into the polymer chain. The advantages expected from the polymeric photoinitiators include good compatibility , low migration, and low volatility which reduces odor problems associated with the low molar-mass photoinitiators. The present work describes the synthesis and characterization of novel well-defined end- and mid-chain functional macrophotoinitiators of polystyrene (PSt) and poly( e- caprolactone) (e-CL) prepared by two different controlled/living polymerization methods namely , Atom Transfer Radical Polymerization (ATRP) and Ring-Opening Polymerization (ROP) and-their subsequent use in block copolymerization. For this purpose, mono- and dihydroxy functional photoinitiators namely, benzoin (B), 2-hydroxy-2-methyl-l-phenyl propan-l-one (HMPP), Darocure 1173, and 2- hydroxy- 1 -[ 4-(2-hydroxyethoxy)phenyl]-2-methyl propan-l-one (HE-HMPP), Irgacure 2959, respectively were used as initiators. New mono and bifunctional ATRP initiators were synthesized by the condensation of 2-bromopropanoyl bromide with B, HMPP and HE-HMPP , respectively and characterized (reaction 1). The ATRP of styrene (St) in bulk at 110 0C by means of ATRP initiators in conjunction with a cuprous complex Cu(I)Br/bipyridine and the ROP of ( e- caprolactone) (g-CL) in bulk at 110 0C by means of ROP initiators in conjunction with the stannous-2-ethylhexanoate (Sn(Oct)2) catalyst yielded well-defined macrophotoinitiators of polystyrenes and poly(c-caprolactone)s with photoactive alkoxy phenylketone groups. The GPC, IR, H-NMR, uv and fluorescence spectroscopic studies revealed that low-polydispersity polystyrene and poly(c- caprolactone ) with desired photoinitiator functionality at the end or in the middle of the chain were obtained. The synthetic strategy followed for the preparation of macrophotoinitiators is described in Scheme 1. These prepolymers (macrophotoinitiators) were used to induce radical polymerization of methyl methacrylate (MMA) upon irradiation via a.-cleavage of the incorporated phenyl ketone groups. The type of macrophotoiniator influences the polymerization products. While end-chain functional macrophotoinitiators led to the formation of both homo and block copolymers, only block copolymers of St or c-CL and MMA were formed with mid-chain functional macrophotoinitiator (reactions 3, 4). Successful blocking has been confirmed by a strong change in the molecular weight of the prepolymer and the block copolymer as well as by NMR spectral measurements. As an alternative usage of the macrophotoinitiators, transformation polymerization approach was also applied. For this purpose, transformation reaction was carried out in order to convert the polymeric radicals into initiating cations with the aid of oxidizing agents such as iodonium and pyridinium salts. With this method block copolymers of monomers which do not polymerize with the same mechanism were prepared. For transformation reactions, macrophotoinitiators with benzoin end chain units (B-PSt and B-PCL) were used to induce free radical promoted cationic polyrnerization of cyclohexene oxide (CHO) in the presence of an onium salt. The reason for using benzoin containing macrophotoinitiators is the strong electron donating efficiency of hydroxybenzyl radicals stemming from such prepolymers. Schernatic representation for the preparation of PSt ( or PCL )- PCHO block copolyrners is depicted in Scheme 3. It should be pointed out that the benzoyl radicals formed instantaneously do not participate in the redox process, due to their electronic structure, i.e. possessing electron withdrawing carbonyl group. This way selected redox process is achieved.