Free radical polymerization photoinitiation

A comparison was made of the activity of xanthene dyes as free-radical polymerization photoinitiators that included succinylfluorescein and some of its halogenated derivatives as well as an ester of Eosin with an orrAo-benzoyl-a.-oxooxime group (shown below) under different experimental conditions. The polymerization studies were carried out on 2-hydroxyetiiyl methacrylate with ethylene glycol dimethacrylate as cross linkers using visible light irradiation. N,N,- dimethylaniline was the co-initiator. The polymerization rate obtained with xanthene dyes was found to be similar to or as much as 4.5 times higher (in the case of the Eosin ester) than that reached with Eosin alone. ... 

Chimica e Vindustria 79, No.5, June 1997, p.597-603 FREE RADICAL POLYMERIC PHOTOINITIATORS FOR UV CURING... 

Photoinitiation. Since photolysis of polysdanes generates sdyl radicals, which can add to carbon—carbon double bonds, these polymers have been used for the free-radical polymerization of unsaturated organic monomers (135,136). Though about one-tenth as efficient as other organic photoinitiators, polysdanes are nevertheless quite insensitive to oxygen effects, which somewhat compensates for their lower efficiency. 

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Photoinitiation of Free Radical Polymerization by Organometallic Compounds... 

Photoinitiation is an excellent method for studying the pre- and posteffects of free radical polymerization, and from the ratio of the specific rate constant (kx) in non-steady-state conditions, together with steady-state kinetics, the absolute values of propagation (kp) and termination (k,) rate constants for radical polymerization can be obtained. 

Thus, a mixture of simple carbonyls Me(CO)n and halides should behave as a photoinitiator of free radical polymerization. Many such systems have been found to function in this way. Complexes formed by irradiation of Fe(CO)5 in the presence of a vinyl monomer (M) (such as MMA, styrene, vinyl acetate, propylene, and vinyl ether) have been studied by Koerner Von Grustrof and colleagues [12,13] and shown to have the chemical struc-... 

Bamford and coworkers [24] also investigated the kinetics and mechanism of free radical polymerization of bulk MMA photoinitiated by Mn2(CO)io or Re2(CO)io in the presence of a series of fluoro-olefms such as ... 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

Mn2(CO)io in a simple photoinitiated free radical polymerization. The antiplatelet activity of these polymers were compared with that of poly(ether-Wrethane) carrying the hydantoin residues in side chains. 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

In this paper, we report efforts to find donor/acceptor systems, comprised of at least one multifunctional monomer, capable of sustaining rapid free-radical polymerization without the need for external photoinitiators. Although we will include in this report comonomer systems which form ground state CT complexes, we stress that the primary mechanism for generating free-radical in each case may not be via excitation of ground state CT complexes. 

Ajayaghosh A, Francis R (1999) A xanthate-derived photoinitiator that recognizes and controls the free radical polymerization pathways of methyl methacrylate and styrene. J Am Chem Soc 121 6599-6606... 

Fig. 2 Various types of photoinitiators (1) peroxides, (2) azo compounds based on AIBN, (3) benzoin ethers, (4) triplet photosensitizers, (5) onium salts for cationic polymerization, and (6) controlled free radical polymerization with photoiniferters...

Photoinitiated free radical polymerization proceeds via three main steps ... 

There are several differences between the photoinitiated cationic polymerization and the photoinduced free radical polymerization ... 

Primary, and Secondary Reactions in Photoinitiated Free-Radical Polymerization of Organic Coatings... 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

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... 

Vmyl ethers can also be formulated with acrylic and unsatiiraterl polyesters containing maleate or fumarate functionality. Because of their ability to form alternating copolymers by a free-radical polymerization mechanism, such formulations can be cured using free-radical photoinitiators With acrylic monomers and oligomers, a hybrid approach has been taken using both simultaneous cationic and free-radical initiation. 

Photopolymerization using polysilanes can also be carried out to yield block copolymers containing runs of silicon atoms, for instance polysilane-methyl methacrylate. The resulting copolymer can then be used as a macro-photoinitiator for further free-radical polymerizations.144145... 

Compounds capable of generating free radicals on exposure to light and thereby inducing free radical polymerization are known as photoinitiators. However, the utility of many of these compounds is limited to activation by ultraviolet light only. The addition of a compound that absorbs incident light more efficiently may activate the photoinitiator to produce free radicals using visible light. Such compounds are known as photosensitizers. 

In contrast, photoinitiated free radical polymerization of glycidyl methacrylate and trimethylolpropane trimethacrylate in the presence of porogenic solvent affords a monolithic plug within the column that serves as a frit. This procedure represents a simple approach to reproducible fabrication of frits even in capillaries with large inner diameters. 

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