Vinyl ester monomer radicals

VEC is a sluggish monomer having free radical polymerization toward free radical polymerization. Due to the electronic structure of the double bond, VEC copolymerizes with vinyl ester monomers over a wide compositional range. However, VEC cannot be completely incorporated into an acrylic copolymer. When copolymerization with styrene is attempted, VEC is barely incorporated. 

Polymers with pendant cyclic carbonate functionality were synthesized via the free radical copolymerization of vinyl ethylene carbonate (4-ethenyl-l,3-dioxolane-2-one, VEC) with other imsaturated monomers. Both solution and emulsion free radical processes were used. In solution copolymerizations, it was found that VEC copolymerizes completely with vinyl ester monomers over a wide compositional range. Conversions of monomer to polymer are quantitative with complete incorporation of VEC into the copolymers. Cyclic carbonate functional latex polymers were prepared by the emulsion copolymerization of VEC with vinyl acetate and butyl acrylate. VEC incorporation was quantitative and did not affect the stability of the latex. When copolymerized with acrylic monomers, however, VEC is not completely incorporated into the copolymer. Sufficient levels can be incorporated to provide adequate cyclic carbonate functionality for subsequent reaction and crosslinking. The unincorporated VEC can be removed using a thin film evaporator. The Tg of VEC copolymers can be modeled over the compositional range studied using either linear or Fox models with extrapolated values of the Tg of VEC homopolymer. 

Interestingly, the polymerization rates of vinyl esters are remarkably retarded by small amoimts of styrene. The highly reactive vinyl ester radicals readily react with the activated styrene monomer, which results in a relatively stable benzyl-type styrene radical. The vinyl ester monomer molecule is not activated enough for the addition of the styrene radicals and the reaction ceases (464). 

In general, acryUc ester monomers copolymerize readily with each other or with most other types of vinyl monomers by free-radical processes. The relative ease of copolymerization for 1 1 mixtures of acrylate monomers with other common monomers is presented in Table 7. Values above 25 indicate that good copolymerization is expected. Low values can often be offset by a suitable adjustment in the proportion of comonomers or in the method of their introduction into the polymerization reaction (86). 

In these equations I is the initiator and I- is the radical intermediate, M is a vinyl monomer, I—M- is an initial monomer radical, I—M M- is a propagating polymer radical, and and are polymer end groups that result from termination by disproportionation. Common vinyl monomers that can be homo-or copolymeri2ed by radical initiation include ethylene, butadiene, styrene, vinyl chloride, vinyl acetate, acrylic and methacrylic acid esters, acrylonitrile, A/-vinylirnida2ole, A/-vinyl-2-pyrrohdinone, and others (2). 

In general, acrylic ester monomers copolymerizc readily with each other or with most other types of vinyl monomers by free-radical processes. 

VINYL ESTER RESINS. The vinyl ester resins are a relatively recent addition1 to thermosetting-polymer-chemistry. Superficially, they are similar to unsaturated polyester resins insofar as they contain ethylmic lmsaturation and are cured throngh a free-radical mechanism, usually in the presence of a vinyl monomer, such as styrene. However, close examination of the chemistry and structure of the vinyl ester resins demonstrates several basic differences which lead to their unique characteristics. 

Alkoxylation with, for example, propylene oxide (PO). Preferred amine is triethanolamine, or ammonia and other alkanolamines used amine can also be quaternized Best example is triethanol with 14.9 PO units Contains vinyl ester acetal functionalities besides some unreacted vinyl alcohol monomer units. Preferred aldehyde is butyraldehyde Backbone, for example, polyalkylene glycol, polyalkyl-eneimine, polyether, or polyurethane, and active functional side groups made from grafting VP or VCap to backbone using radical initiators TBA (tributylammonium groups)... 

The radical polymerization in aqueous solution of a series of monomers—e.g., vinyl esters, acrylic and methacrylic acids, amides, nitriles, and esters, dicarboxylic acids, and butadiene—have been studied in a flow system using ESR spectrometry. Monomer and polymer radicals have been identified from their ESR spectra. fi-Coupling constants of vinyl ester radicals are low (12-13 gauss) and independent of temperature, tentatively indicating that the /3-CH2 group is locked with respect to the a-carbon group. In copolymerization studies, the low reactivity of vinyl acetate has been confirmed, and increasing reactivity for maleic acid, acrylic acid, acrylonitrile, and fumaric acid in this order has been established by quantitative evaluation of the ESR spectra. This method offers a new approach to studies of free radical polymerization. 

Van Der Meer et al. [113] have found that vinyl ester reactivity increases with the electron-withdrawing ability of the ester group. All the measured ester radicals prefer addition of their own monomer to that of ethylene. The observed relative reactivity is mostly affected by polar factors resonance stabilization plays only a minor role. Vinyl ester reactivity grows in the series vinyl iso-butyrate < vinyl butyrate < vinyl propionate < vinyl acetate < vinyl formate vinyl pivalate. 

The most common polymer of a vinyl ester is poly(vinyl acetate), CAS 9003-20-7, with the formula [-CH2CH(OC(0)CH3)-]n. Other vinyl esters also are known, such as poly(vinyl butyrate), poly(vinyl benzoate) CAS 24991-32-0, and poly(vinyltrifluoroacetate), CAS 25748-85-0. Poly(vinyl acetate) is typically obtained from the monomer with radical initiators, either by emulsion or suspension polymerization. The polymer Is used in water-based emulsion paints, adhesives [22], gum base for chewing gum, etc. Also, poly(vinyl acetate) is used as a precursor for the preparation of other polymers such as poly(vinyl alcohol) or poly(vinyl acetals). Thermal decomposition of poly(vinyl acetate) starts at a relatively low temperature, around 200° C, some of the reports regarding its thermal decomposition being given in Table 6.5.8 [13]. The same table includes references for poly(vinyl butyrate) and poly(vinyl cinnamate), CAS 9050-06-0. 

On the other hand, since poly(vinyl ester) radicals are reported to be electron-rich104, they are electron donors with respect to solvent. The poly(vinyl acetate) radicals is more likely to form a complex than the poly(vinyl benzoate) radical, since the methyl group increases the electron density of the propagating radical as compared to the phenyl group. If the complexed radical is assumed to be less reactive than the free radical or inactive, the concept of the 7r-complex along with the experimental evidence of similar methyl affinity47 of both monomers explains the fact that kp for vinyl acetate is smaller than that for vinyl benzoate. 

When we combine this observation with the autoaccelerating tendencies of the system, the chain-transfer reactions to both the monomer and the polymer on one of the several positions which leads to branched-chain formation, and the possible reactivation of dead polymer molecules by hydrogen abstraction with monomeric free radicals [78], the complexity of the kinetics of vinyl acetate polymerization may be appreciated. Similar factors may be involved not only in the polymerization of other vinyl esters, but also in the fiee-radical polymerization of other types of monomers. 

When photosensitizers such as benzophenone are added to polymer, absorption of ultraviolet radiation results in excitation of the sensitizer followed by hydrogen abstraction fi-om the polymer to yield radical sites (Equation 1.39) available for cross-linking by combination reactions. Copolymers of vinyl esters and fluorinated monomers that can be cross-linked by ultraviolet radiation have been developed for use as weather-resistant wood coatings. In this application, the vinyl ester constitutes about 10% of the copolymer and benzophenone is added as a photosensitizer. The vinyl ester polymer also undergoes a-cleavage reaction (Equation 1.40) with subsequent cross-linldng. 

In monomers with two or more polymerizable sites the structure of the resulting polymer depends on the initiator. Vinyl isocyanate, CH2=CHNCO, polymerizes via the vinyl group free radically, but via the nitrogen/ oxygen double bond in anionic polymerization. Diketenes polymerize to polyesters, polyketones, or poly(vinyl esters) according to what initiator is used ... 

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