Styrene photoinitiated copolymerization

An additional class of photo initiated radical polymerizations in wide use is composed of a combination of a styrenic monomer together with a polyester oligomer bearing unsaturated double bonds arrayed along the backbone. Such systems are, in fact, photoinitiated copolymerizations that have a strong tendency toward alternation. A graphical representation of the mechanism involved in these chemistry systems is Shown in Scheme 12. 

The best evidence for the photolytic decomposition of mercaptans and disulfides into free radicals involves photoinitiation of polymerization of olefins. Thus, photolysis of disulfides initiates the copolymerization of butadiene and styrene,154 as well as the polymerization of styrene207 and of acrylonitrile.19 Thiophenol and other thiols promote polymerization upon ultraviolet irradiation.19 Furthermore, the exchange of RS-groups between disulfides and thiols is greatly accelerated by light. Representative examples are benzothiazolyl disulfide and 2-mercapto-thiazole,90 tolyl disulfide and p-thiocresol, and benzyl disulfide and benzylmercaptan.91 The reaction probably has a free radical mechanism. Similar exchange reactions have been observed of RS-groups of pairs of disulfides have been observed.19... 

An inverted sequence of the same procedure has also been used [139] to prepare the same three-block copolymers. Indeed, thermal polymerization of MMA by ABME gives rise to a polymer mainly containing only one benzoin methyl ether moiety per macromolecule, since growing MMA radicals terminate mostly by disproportionation. Thus, terminally photoactive poly(MMA) is used to obtain the photoinitiated block copolymerization of styrene. In this case, a 90% yield of block copolymers is obtained, appreciably higher than in the preceding method, fully consistent with the usual assumption that the termination in styrene polymers occurs by combination. In fact, coupling of the growing styryl radicals with the less reactive poly(MMA)-bound methoxy benzyl radicals also contributes to the formation of block copolymers. 

The role of sulphur-containing compounds in photopolymerization appears to have attracted some interest. Bis(j -methylpyridazinyl)-3,3 -disulphide has been found to initiate the photopolymerization of styrene but inhibits the thermal polymerization. The role of thiyl radicals (PhS-) in photoinitiated polymerization of vinyl monomers by aromatic thio-compounds has been postulated by several workers. In one study, flash photolysis was used to identify the nature of the radical. Sulphur-containing monomers such as 4-methyl-2-(vinylthio)thiazole and thiocyclanes have been photopolymerized and copolymerized with other vinyl monomers. Luca et al. have devised a mathematical model for the photopolymerization of 2,3-dimethylbutadiene and thiourea. 

Kinetic analyses indicate that DHb is consumed during polymerization about twenty times slower than DHa. The main consumption pathway for DHb appears to be copolymerization. From kinetic analysis of photoinitiated polymerization of styrene [64], they conclude that chain transfer to monomer is negligible and that most of the chain transfer that takes place during spontaneous styrene polymerization is due to DHa (chain transfer constant 100). 

A common approach to the preparation of polymeric photoinitiators is to attach photoinitiator molecules, like benzophenone, or benzoin ether, or thioxanthones to some polymerizable groups For instance, acryloyl chloride was reacted with hydroxyfliioxanthone and flien copolymerized with styrene ... 

In some cases, a monomer may function as a photoinitiator and become incorporated into a copolymer chain. This has been shown for styrene and other conjugated monomers when exposed to deep-UV light however, the initiation efficiency in these systems is substantially less than when a photoinitiator is present (p. 223 of Ref 33, and Ref 34). A better polymerizable initiator scheme is illustrated by the acceptor-donor chemistry of maleimide-donor systems (35-37). Maleimide acts as both photoinitiator and comonomer in the presence of hydrogen donors such as vinyl ethers or vinyl esters (38,39) an example of this copolymerization is shown in equation 5. It shows the molecular structure of the acceptor er -butylmaleimide (left), the donor 4-hydroxy-butyl vinyl ether (right), and their corresponding copolymer repeat unit. 

Unsaturated Polyester Systems. Coatings in the furniture industry rely heavily upon resin formulations containing unsaturated polyesters, styrene, and photoinitiator (3,4,13,43,44). The imsaturated polyesters are synthesized using step-growth polymerization (see Polyesters, Unsaturated). Upon illumination, the carbon-carbon double bond in the imsaturated polyester and st5Tene copolymerize to form a cross-linked network (eq. 6). Equation 6 shows a generalized reaction scheme for an imsaturated polyester system. 

The UV-initiated copolymerization of the monomer pair, in the absence of a photoinitiator, has received some study.In the case of high concentrations of MA in the feed, the copolymerization of the vinyl acetate-MA and styrene-MA pairs were comparable. The degree of polymerization (DP) decreased with increasing MA concentration. 

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