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SYNTHESIS OF MACROMOLECULAR PHOTOINITIATORS AND THEIR EFFECTS ON PHOTOINDUCED FREE RADICAL POLYMERIZATION
Photoinduced free radical polymerization is a widely used technology with a wide range of industrial applications such as curing of coatings on various materials, adhesives, printing plates, inks, electronics and photoresist and has recently been recognized as also having great potential in the biomedical fields [1]. In the development of photopolymerization, photoinitiator systems play a very important role since even the most reactive acrylate monomers hardly polymerize when exposed to the pure form of UV light [2]. Free radical photoinitiators can be classed as α-cleavage (Type I) and H-abstraction (Type II) initiators. Type II photoinitiators are the most studied free radical photoinitiators. The most widely used free radical Type II photoinitiators include benzophenone and derivatives, thioxanthones, benzyl, quinines while alcohols, ethers, amines and thiols are used as hydrogen donors [3-6]. Thioxanthones are among one of the most widely used bimolecular photoinitiators in vinyl polymerization because of their absorption characteristics at near UV-vis range and whose triplet excited states readily react with hydrogen donors such as amine, alcohol, ether, acid or thiol functional compounds (Scheme 1) thereby producing initiating radicals [49].
Low molecular weight photoinitiators have a main drawback in that their photolysis products might migrate onto the coating surface and may create yellowing and unpleasant odors with serious problems of contamination. Therefore, much effort has been spent on the development of polymeric photoinitiators, which have some advantages such as low migration, reduced yellowing, higher reactivity and low volatility with respect to low molecular weight analogues.
Scheme 1. Photoinitiation mechanism of thioxanthone in the presence of a coinitiator.
Polymeric Photoinitiators: Polymeric photoinitiators have attracted much attention in the past years, for they combine the properties of polymers with those of low molecular weight photoinitiators [10-26]. Solubility and miscibility problems, often observed with coatings containing low molecular weight photoinitiators, do not occur with the polymeric ones since polymers are easily miscible with the resin to be cured as well as with the final cured film. Moreover, odor and toxicity problems do not occur with macrophotoinitiators owing to the low volatility of the large molecules. The low migration tendency of polymeric photoinitiators and of photoproducts means that cured coatings are less prone to yellowing [27-31].
Macrophotoinitiators possessing chromophoric groups either in the main chain or as pendant groups can be prepared in two ways: (i) synthesis and polymerization of monomers with photoreactive groups or (ii) introduction of photoactive groups into polymer chains (Scheme 2). In the latter case, macrophotoinitiators were synthesized either by using functional initiators and terminators in a particular polymerization or by reacting functional groups of a preformed polymer with other functional groups of low molecular weight compounds also possessing photoreactive groups. Macrophotoinitiators, analogues to the low molecular weight photoinitiators, are divided into two classes, according to their radical generation mechanism, namely cleavage type (type I) and hydrogen abstraction type (type II) macrophotoinitiators.
Scheme 2. Preparing the “Side Chain” and “In Chain” polymeric photoinitiators according to different pathways.
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