Acrylic double bonds

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

When this resin was exposed as a thin film to the UV radiation of a medium pressure mercury lamp (80 W aiH), the crosslinking polymerization was found to develop extensively within a fraction of a second (18). The kinetics of this ultra-fast reaction can be followed quantitatively by monitoring the decrease of the IR absorption at 810 an-1 of the acrylic double bond (CHCH twisting). Figure 8 shows a typical kinetic curve obtained for a 20 pm thick film coated onto a NaCl disk and exposed in the presence of air to the UV radiation at a fluence rate of 1.5 x 10 6 einstein s-1 cm 2. 

From the cure profile recorded after a short and intense UV or pulsed laser exposure 18, one can evaluate the actual rate at which the polymer chains are growing. By calculating the ratio Rp/[R ], where [R ] is the number of initiating radicals generated by the UV exposure, we found that 5.104 acrylate double bonds have polymerized per second, for each initiating radical. From this value, the average time for the addition of one monomer unit was calculated to be 20 jlls. 

As the polymerization proceeds, the viscosity of the polymer system increases steadily to finally yield a solid and rigid highly cross-linked polymer in which the segmental mobility is quite restricted. Consequently, the encounter probability of the polymer radicals with the reactive acrylate double bonds is then sharply reduced, which accounts for the rate slowing down observed in the latter stages of the irradiation (Figure 2). 

In a more quantitative approach, the decrease of the IR absorption band at 810 cm-1, characteristic of the acrylate double bond, was followed as a function of the exposure time (Figure 3). This permits the precise evaluation how many acrylate functions have polymerized as a function of dose and then to deduce the degree of conversion of the polymer formed. Figure 4 shows the... 

The biosynthesis of artemisinin3 is of interest in that it provides clues to the chemical synthesis of artemisinin from its more abundant precursor in A. annua, artemisinic acid 2. Conjugate reduction of the acrylate double bond of 2 followed by singlet oxygenation leads,... 

In the case of an acrylic derivative, the primary reduction takes place at the porphyrin ring. The migration of the acrylic double bond to the reduced ring rapidly follows and the final product of the photoreduction is zinc(II) porphyrin propionate. The mechanism of the reactions can be schematized as follows... 

In this case, the free-radical reaction is a copolymerization between styrene and (meth)acrylate double bonds. The reactivity ratios are rs 0.5 and rM 0.5. 

Meth)acrylates are polymerized by free-radical polymerization. Because of their high reactivity, acrylate double bonds are preferred their polymerization rate is 10 times faster than that of methacrylate monomers. 

The same kind of addition reaction as described in Section IV 1) and 2) can be performed with substituted bisacryl amides such as bisacryloylpiperidide and N,N -dimethylethylenediamine. However, the difference in the reactivity between the two acrylic double bonds is much less pronounced than in the case of p-divinylbenzene, so that a mere polycondensation occurs. Upon using a calculated excess of 1,4-bis-acryloylpiperidine bifunctional macromonomers of a known average molecular weight have been obtained by Ferruti et al. 90) ... 

A similar mechanism was proposed when 1,5-dithiocin 838g underwent polymerizations with methyl methacrylate (MMA) and styrene (STY). The activated double bound of 838g was found to have a profound affect on reactivity. In fact, co-polymerization of 838g with MMA at 70 °C the 5-terminated sulfanyl radicals preferred to undergo homopropagation, while cross-propagation is favored for MMA-terminated radicals. Both monomers possessed an electron-deficient acrylate double bond with similar possibilities for conjugative stabilization of the adduct radical by the ester functionality, which would explain the apparent equal reactivity of the MMA radical to either monomer. 

In co-polymerization of 838g with STY at 80 °C, the cross-propagation is favored, consistent with electrophilic sulfanyl radicals adding rapidly to electron-rich STY, and nucleophilic styryl radicals adding rapidly to electron-deficient acrylate double bond (Scheme 169) <2006MI2475>. 

Enhanced diastereofacial discrimination of an acrylic double bond was observed when p-methylbenzyl (3-L-arabinopyranoside 85 bearing an T -chromium(0) tricarbonyl moiety reacted with dienes (e.g. isoprene) [67]. After decomplexation with pyridine, cycloadduct 86 was isolated as a mixture of diastereomers (90% de), whereas the uncomplexed counterpart showed inferior... 

These dual UV/thermally curable conformal coatings are based on the ability to rapidly transform a liquid composition into a cross-linked solid by the free-radical polymerization of a polyfunctional acrylate in the presence of a low concentration of a polyfunctional thiol (1). Under these conditions, the polythiol functions partly by adding across the acrylic double bond and partly as a chain transfer agent in the acrylate homopolymeriza-... 

Photoinitiators as Iniferters The availability of iniferters that behave as an initiator, a transfer agent and a terminator is rather limited [98,99]. A renewal of interest is noted for the development of efficient compounds usable in controlled radical photopolymerization reactions [100-104]. Examples of cleavable photoiniferters are shown in (10.42) the particular tetrazole derivative shown here is noticeably attractive due the generation of a tetrazoyl radical that presents a low selectivity and a high efficiency for the addition to acrylate double bonds [105],... 

The first step is the generation of a A /T-butoxyl radical through the photochemical decomposition of ferf-butylperoxide the second step corresponds to a a(C-H) hydrogen abstraction reaction from triethylamine as proposed many years ago [291]. The originality of the present procedure lies in the third step that corresponds to the addition of the aminoalkyl radical to a (meth) acrylate double bond. Indeed, upon addition of, for example, methyl acrylate MA or methyl methacrylate MMA, new RM transients are observed with absorption maxima at 470 and 425 nm for MA and MMA, respectively, as shown in Fig. 10.8. 

There are several interesting polymerization schemes intermediate between a sequential IPN and an SIN. For example, in in situ prepared sequential IPNs, both monomers are polymerized via free radical reaction [He et ai, 1993 Rouf et ai, 1994]. The two monomers must have quite different reactivities towards the free radicals. This situation arises with vinyl or acrylic double bonds and aUylic double bonds. The allylic double bonds react about 100 times slower than acrylic or methacrylic bonds. Often, two initiators are used, one reacting at a lower temperature, and the other at a higher temperature. In one of the systems studied, based on methyl methacrylate and diallyl carbonate of bisphenol-A (DACBA), first, crosslinked PMMA was formed at moderate temperatures. Then, by just increasing the temperature after completion of the first polymerization, the synthesis of the allylic network followed. 

With allyl methacrylate (99) and ally acrylate (100),r-butoxy radicals give siibslanlially more addition to the acrylate double bond than to the allyl double bond (see Figure 3.4). 

Analysis. The kinetics of the light-induced cross-linking of EPI-based coatings was studied by infrared spectroscopy, by following the decrease upon UV exposure of the absorption bands characteristic of the related functional groups epoxy ring at 877 cm- vinyl ether and acrylate double bonds at 1628 and 812 cm- respectively. The degree of conversion (x) was calculated from the equation ... 

A similar study has been performed on EPI blends in which the vinyl ether was replaced by an acrylate monomer (HDDA) to produce, by different mechanisms, two interpenetrating polymer networks. With the onium salt as sole photoinitiator, the cationic polymerization of the EPI epoxy groups occurred as fast in the formulation containing 20% of HDDA by weight as in the EPI/DVE-3 blend, to reach nearly 100% conversion within 0.6 s (Fig. 11). The polymerization quantum yield was found to be similar to that measured in the EPI/vinyl ether blend Op 650 mol E. By contrast, the acrylate double bonds were found to polymerize at a much slower pace, most probably because of the low reactivity of the free radicals generated by the cationic-type photoinitiator. 

Amine Acrylate Synthesis. During this study a number of amine acrylates and model compounds were synthesised. The syntheses were based on the Michael type addition of a secondary amine to an activated acrylate double bond. Our procedure is as follows. 

One of the early theoretical studies of the polymerization of allyl acrylate considered the homopolymerization of this monomer to be an intramolecular copolymerization of the allyl and the acrylic double bonds. In this process it was calculated that only about 3% of the allyl groups participated in the process. Upon extended heating, the residual allyl groups served as cross-linking sites [69]. 

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