Photoinitiator-free polymerization

Photoinitiator Free Polymerization of Maleimides and Vinyl Ethers... 

Lapin [33] suggested that photoinitiated cationic polymerization can proceed through reactions of free radicals, as shown below for benzophenone sensitized photoinitiation ... 

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 ... 

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. 

The nature and type of initiation scheme plays an important role in the performance of the adhesive [194,202-204]. Stresses due to polymerization shrinkage lead to the creation of a gap between the adhesive and tooth material. In the case of bulk chemical initiation, shrinkage stresses tend to create gaps at all interfaces, drawing material inward isotropically. With a photoinitiation scheme, polymerization begins at the free surface and pulls the material away from the dentin towards the free surface [194]. Thus the gap is created at the... 

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... 

Photoinitiation of polymerization can be obtained through a variety of photochemical reactions which produce reactive free radicals. These radicals then lead to the formation of the polymer chains through the addition of further monomer units to the end of a chain in a sequence of radical addition reactions (Figure 6.10). A photoinitiator of polymerization is therefore a molecule which produces free radicals under the action of light. Benzo-phenone and other aromatic ketones can be used as photoinitiators, since a pair of free radicals is formed in the hydrogen abstraction reaction. Some quinones behave similarly, for example anthraquinone in the presence of hydrogen donor substrates such as tetrahydrofuran. 

Photolysis of polysilanes produces silyl radicals, which can be used to induce free-radical reactions. Because the silyl radicals can add to carbon-carbon double bonds and begin the formation of polymer chains, polysilanes can be used as radical photoinitiators. In early studies, (PhSiMe) and its copolymers (PhQIUSiMe), and (cyclo-HexSiMe) were shown to photoinitiate the polymerization of styrene and several acrylate... 

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