Photosensitization polymerization photoinitiators

Oxime photoinitiators, (II), were prepared by Tanabe et al. (2) and used in preparing photosensitive polymeric materials. 

PCMS) 1 8 mol/mol mixture has also been checked (Table 16). Indeed, the above mixture is found to show a slightly lower activity with respect to PABOK, thus suggesting that polymeric photoinitiators based on photosensitive moieties undergoing homolytic fragmentation may also display an activity synergism, provided that different photoreactive groups are anchored to the same macromolecular chain. 

A further interesting feature of the polymeric photoinitiators is based on the possibility of anchoring different photosensitive moieties to the same macromolecule in order to provide a synergistic effect on activity due to their interaction along the polymer chain. In this context, the synthesis of several copoly-meric photoinitiators bearing side-chain thioxanthone and hydroxyalkylphenone or morpholino ketone moieties has been reported recently [156]. In particular, the free radical copolymerization of 2-[2-(acryloyloxy)ethylthio] thioxanthone (AETX) with either [4-(2-acryloyloxyethoxy)phenyl]-2-hydroxy-2-propyl ketone (HPA) or [4-(2-acryloyloxyethylthio)phenyl]-2-(iV-morpholino)-2-propyl ketone (APMK) [poly(AETX-co-HPA) and poly(AETX-co-APMK), respectively] and of 4-[2-(methacryloyloxy)ethoxycarbonyl] thioxanthone (METX) with APMK [poly(METX-co-APMK)] has been performed. 

Photosensitizers or Photoinitiators. The primary component in the UV-curable coating system is the photosensitizer or photoinitiator (a light sensitive catalyst that upon absorption of energy results in a free radical species that can initiate acrylic monomer polymerization). C2.)... 

Z. Gomurashvili, Y.J. Hua, and J.V. Crivello, Monomeric and polymeric carbazole photosensitizers for photoinitiated cationic polymerization. Macromol. Chem. Phys. 2001, 202(11), 2133-2141. 

The synergic effects which are generally invoked to account for the specific features of a system of two ketones used as polymerization photoinitiators are reconsidered. The increase of reactivity observed when mixing these two Initiators is reinterpreted in terms of a simultaneous energy and electron transfer in the pair. The relative efficiencies of these processes depends on the energy gap between the triplet states involved, which is known to be influenced by the polarity of the medium. A general discussian on the efficiency of various couples photoinitiator/photosensitizers is presented. 

GOM 01] Gomurasvili Z., Crivello J.V., Phenothiazine photosensitizers for photoinitiated cationic polymerization . Journal of Polymer Science Part A Polymer Chemistry, vol. 39, no. 8, pp. 1187-1197, 2001. 

GOM 02] Gomurasvili Z., Crivello J.V., Monomeric and polymeric phenothiazine photosensitizers for photoinitiated cationic polymerization . 

The polymerization of the monomer in acrylic engineering adhesives can be initiated by electron beam or ultraviolet (UV) radiation, provided that the adherends or the fillers in the adhesives are not barriers to radiation. Acrylic monomers are generally more reactive to radiation than methacrylates. Electron beams generate free radicals directly in the adhesive, whereas UV curing requires a photosensitizer or photoinitiator to provide free radicals. 

Dry-Film Resists Based on Radical Photopolymerization. Photoinitiated polymerization (PIP) is widely practiced ia bulk systems, but special measures must be taken to apply the chemistry ia Hthographic appHcations. The attractive aspect of PIP is that each initiator species produced by photolysis launches a cascade of chemical events, effectively forming multiple chemical bonds for each photon absorbed. The gain that results constitutes a form of "chemical amplification" analogous to that observed ia silver hahde photography, and illustrates a path for achieving very high photosensitivities. 

If (P ) is terminated by a chain transfer to a solvent or a monomer, a graft copolymer is formed, or, if the termination is from a combination, a crosslinked network polymer is formed. If the pre-existing polymer (B) contains an end group that itself is photosensitive (or can produce a radical by interacting with photoinitiator) and in the presence of a vinyl monomer (A), block copolymer of type AB can be produced if the photosensitive group is on one end of the polymeric chain. Type ABA block copolymer can be produced if the polymer chain (B) contains a photosensitive group on both ends. 

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 quantum yield of polymerization is 6.72 and for photoinitiation < / = 2.85 x 10 . The polystyrene produced with this initiator shows photosensitivity when irradiated with UV light (A = 280 nm). This polymer, which carries two photosensitive end groups of - SC(S) N(CH3)2, behaves as a telechelic polymer and it is useful for production of ABA block copolymer. 

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

Polymerization of styrene and p-isopropylstyrene could be photo-initiated with radiation from the ruby laser in the absence of photosensitizers and oxygen Since ordinarily no unsensitized photoinitiation of styrene is detected for wavelengths longer than 4000 A, the results of this experiments must be due to two-photon processes. 

The term photosensitizer was originally used to refer to the second pathway, especially when it involved energy transfer, but that distinction has become blurred. The mechanism for photoinitiation in a reaction system is not always clear-cut and may involve both pathways. Photosensitizer is now used to refer to any substance that either increases the rate of photo-initiated polymerization or shifts the wavelength at which polymerization occurs. 

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...

Photopolymerization induced by donor-acceptor interaction has several characteristic differences from conventional photopolymerization. Firstly, the initiation is very selective. Appropriate strength of donor and acceptor is essential since the CT interaction might bring about spontaneous thermal polymerization if it is too strong. Although most charge transfer processes must be photosensitive, practically important systems are limited to those which conduct thermal reactions with negligible rates. The photopolymerization of MMA by triphenyl-phosphine should be called photoacceleration rather than photoinitiation since the rate of spontaneous photopolymerization of MMA is about half of that of polymerization photosensitized by 4 x 10 4 M of triphenyl-phosphine. Secondly, an ionic mechanism is expected. Thirdly, when both donor and acceptor are polymerizable monomers, the polymerization mixture is entirely solid and clean after polymerization. There is no initiator and no solvent. 

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