Free-Radical-Initiated Systems

There are many possibilities, but only a few systems are being used for practical UV curable formulations using free radical or cationic initiation. These will be discussed in this section. 

The most widely used UV curable radical-initiated systems are based on acrylate unsaturation with the general formula H2C = CR-COOR (if R = H, the monomer is an acrylatel if R = CH3, it is a methacrylate). Methacrylates are less reactive than acrylates,but are less toxic and cause less skin irritation than acrylates. 

The curing reaction of acrylates is typical of vinyl monomers. Therefore, the degree of double-bond conversion is the measure of the degree of cure. The best results are obtained when using oligomers as binders and monomers as reactive thinners. Examples of difunctional and polyfunctional acrylates are in Table 4.4. A partial list of the most common acrylate oligomers is below. -  

Urethane acrylates Urethane acrylates are formed by the reaction of isocyanates with hydroxy-functional acrylate monomers. After UV cure, they produce tough, flexible materials, which exhibit a good abrasion resistance. 

Polyester acrylates Acrylated polyesters are prepared by reacting the OH group of polyesters with acrylic acid or hydroxy acrylate with acid groups of the polyester structure. Polyester acrylates are often low-viscosity resins requiring little or no monomer ll They produce coatings and adhesives dominated by the polyester structure used in the oligomer. They are used for pressure-sensitive adhesives and 

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The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

Having made the prepolymer, a reactive diluent, usually styrene, is added, together with an appropriate free-radical initiator system in order to bring about crosslinking. This cure reaction can be made to occur at either ambient or elevated temperatures and, depending on conditions, is complete in anything from a few minutes to several hours. 

CEC capillary columns filled with hydrophilic polymer gels mimic those used for capillary gel electrophoresis [91]. Typically, the capillary is filled with an aqueous polymerization mixture that contains monovinyl and divinyl (crosslinking) acrylamide-based monomers as well as a redox free radical initiating system, such as ammonium peroxodisulfate and tetramethylethylenediamine (TEMED). Since initiation of the polymerization process begins immediately upon mixing all of the components at room temperature, the reaction mixture must be used immediately. It should be noted, that these gels are very loose, highly swollen materials that usually contain no more than 5% solid polymer. 

Most kinetic studies on copolymerizations using coordination catalysts have been restricted to the determination of monomer reactivity ratios. There are problems both experimentally and in interpretation since the major simplification assumed to hold for most free radical initiated systems, namely that monomer incorporation is determined only by the monomer concentrations and the four rate coefficients, cannot be taken for granted. Further, catalyst activity and selectivity are influenced by the conditions of catalyst preparation including the manner and order of... 

While the detailed mechanistic understanding of many cationic polymerization systems has not as yet been clearly established, there are some that are reasonably well understood. Let us consider the detailed progression of the cationic polymerization of isobutylene using a TiCl4/H20 catalyst system as an example. This system cannot be used as a general model for all cationic polymerizations as is possible for free radical-initiated systems, since there is a wider variety of mechanisms seen for cationic, than for free radical, systems. 

With the present series of monomers, described in Table I, the polymerization methods studied were the same as those described in previous work (2). Again, polymerization of concentrated aqueous solutions of the amine hydrochlorides using the titaneous chloride/ hydrogen peroxide initiation system provided the best polymerization method. Where this technique did not yield polymers, other free-radical initiation systems were equally unsuccessful. Table II gives some examples of the results of these polymerization experiments. 

In the case of free radical-initiated systems, the basis for this post-cure change in properties is less clear. Both the initiation and propagation steps proceed rapidly, even at ambient temperature. Furthermore, free radicals generally are short-lived species, and are subject to a number of reactions with atmospheric species which consume them. Therefore, the changes in physical properties observed in acrylated epoxies and other free radical-initiated coating systems must be accounted for by some other mechanism, since the changes in properties occur at ambient temperature, it is unlikely that a chemical mechanism will account for them. Enthalpy relaxation, a... 

The polymerization mechanism of anaerobic adhesives is similar to that of other free-radical initiation systems except for the special ways in which the inhibiting effect of oxygen is used to delay the polymerization, and in the chemical activation that occurs... 

Reagents i, IM-NaOH (exact equiv.), then removal of arylamldoxime (ether extraction), acidification (HiSO ) of aqueous layer and ether extraction ii, PhNHg iii, NaOH(aq) iv, 210—280°C, in vflc o tv, boilingKMnOi(aq) vi, unsuccessful attempts were made to homopolymerize or copolymerize (with CF2=CFj or CH =CFa) the propenes where x = 3, Ar = Ph, p-MeCjHi, or m-MeC,Hi using free-radical initiation systems. 

Whereas many free radically initiated systems have been investigated, ionic initiation has received less attention. Dispersionsof polypropylene initiated by an aluminium/titanium alkyl catalyst were produced on a commercial scale for a short period but failed to find significant uses (21)=, The stabilisation of anion-ically initiated polystyrene with poly(t-butyl styrene) has also been described (l)o In one variant of this system the soluble component is formed first, the disperse phase polymer is then grown from the living polymer until precipitation occurs and polymerisation is continued within the particle by the addition of further styrene. 

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