Ultraviolet photopolymerization

Photopolymerization and Plasma Polymerization. The use of ultraviolet light alone (14) as well as the use of electrically excited plasmas or glow discharges to generate monomers capable of undergoing VDP have been explored. The products of these two processes, called plasma polymers, continue to receive considerable scientific attention. Interest in these approaches is enhanced by the fact that the feedstock material from which the monomer capable of VDP is generated is often inexpensive and readily available. In spite of these widespread scientific efforts, however, commercial use of the technologies is quite limited. 

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). 

Photopolymerizations were initiated with either ultraviolet or visible blue light of varying intensity (1-150 mW/cm2). In general, the high concentration of double bonds in the system and the multifunctional nature of the monomer (two double bonds per monomer molecules) led to the formation of a highly crosslinked polymer system in a period of a few seconds, depending on the initiation rate. 

Figure 3 Polymeriztion of 50/50 w/w methecrylic anhydride initiated with 0.5 wt% DMPA and 80 mW/em of ultraviolet light (a) Illustration of the temporal eontrol of the photopolymerization. Initiation is eeased at —30 s and reinitiated at —40 s (dashed line) eompared to eontinuous initiation (solid line), (b) Evaluation of the eompressive modulus as a funetion of exposure time.

Diels-Alder dimer or its reaction with styrene is the rate-determining step in initiation is not completely established. The dependence of Rp on [M] is closer to third-order than second-order, indicating that Eq. 3-63b is the slow step. The Diels-Alder dimer has not heen isolated, but ultraviolet spectroscopy of the reaction system is entirely compatible with its presence. There are indications that the photopolymerization of neat styrene proceeds by a similar mechanism. 

Convincing evidence for phase separation was obtained from the photopolymerization behavior of 6 in the mixed 6/DSPE monolayer films. Photopolymerization of diacetylenes is a topotactic process which requires the proper alignment of the 1,3-diyne moieties [35]. Thus diacetylenes typically polymerize rapidly in the solid state but not in solution. Polymerization is triggered by ultraviolet irradiation and proceeds via a 1,4-addition mechanism yielding a conjugated ene-yne backbone (Fig. 5). The reaction can be followed by the growth of the visible absorption band of the polymer. 

Sodium chloroaurate (NaAuCl4 2HaO) initiates polymerization of VCZ both thermally and photochemically. When the polymerization is conducted in nitrobenzene, photopolymerization under illumination of near ultraviolet to visible light proceeds with remarkable rapidity, whereas thermal polymerization starts after an induction period as shown in Fig. 8. This is a unique example of photoacceleration of cationic polymerization. Since the initiation mechanism is different from known cationic polymerization, the thermal system will be mentioned briefly before the discussion of the photochemical system. 

Dark Stability. The examples cited have demonstrated that dye sensitized photopolymerization is potentially rapid in solution, but that substantial speed losses result when polymeric binder compositions are used in an imaging fashion. Our work has shown that TBBS activated compositions sensitized by MB can exhibit absolute photospeeds to red light that are comparable to those of commerical photopolymer systems that are sensitive only to ultraviolet wavelengths (65). These systems can offer features not now available in UV-only sensitive systems. The major failure of all such systems, however, is a pronounced lack of stability on dark storage for the lengths of time required by industry standards. 

The irradiation of 8-thia-l-aza-bicyclo[4.2.1]nona-2,4-diene 8,8-dioxide 125 with 350nm ultraviolet (IJV) light in pure acetone resulted in the formation of 2-thia-6-aza-tricyclo[5.2.0.01,4]non-8-ene 2,2-dioxide 60 in 52% yield <20040L1313>. However, photopolymerization occurred as a side reaction, but this can be prevented by the use of acetone with acetonitrile (2 1) as a solvent, although the yield diminished significantly to 28%. The authors proposed a mechanism for this reaction (Scheme 20) <20040L1313>. 

After the template-monomer complexes have been formed, an azo initiator (usually azo-V,M-bis-isobutyro-nitrile, AIBN) is added to the polymerization mixture. Free-radical polymerization is initiated by heating at 40-60°C or by photochemical homolysis by ultraviolet (UV) radiation (0-15°C). MIPs prepared at lower temperatures (0°C) by photopolymerization have been found to exhibit better molecular recognition. It is theorized that the template-monomer complexes are more stable at lower temperatures thus, the imprints are more homogeneous and better defined in the resulting MIPs. 

The azonitriles may also be used as initiators in photopolymerizations, in which case free radicals are formed at temperatures in the neighborhood of O C upon irradiation with rays of short wavelength in the visible or neaiv ultraviolet region. 

Photopolymerization using ultraviolet light-activated initiators... 

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