Methylene free radicals

Bohm, the methylene free radical, —CHoCCHL—, may be produced momentarily in polyethylene by eliminating molecular hydrogen during the irradiation. This process could replace or exist in addition to that represented by Equation 1. Such a biradical (or Lewis acid) would not be expected to be stable but could revert to the vinvlene group... 

Carbene. )Same as Cuprene, (C, SH,0)X, a yel-bm polymerization product made by heating acetylene in the presence of Cu(Ref 1) 2)A positive or negative ion involving two- valentcarbon and paired electrons as in H2C , R2C or RZC++. These ions are different from methylene free radicals in that the latter have unpaired electrons. Carbenes have only transitory existence and are difficult or impossible to collect as such, but... 

In both the following exercises assume that all the methylene groups in the alkane are equally reactive as sites of free radical chlorination... 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

The most common reaction of methylene chloride is its reaction with chlorine to give chloroform and carbon tetrachloride. This occurs by a free-radical process initiated by heat or light in the gas or Hquid phase. Catalytic chlorination to these same products is also known (see Chlorocarbons and Cm OROHYDROCARBONS, Cm OROFORM). 

At first, these highly reactive free radicals react with the antioxidant, but as the antioxidant is consumed, the free radicals react with other compounds. Hydrogens on methylene groups between double bonds are particularly susceptible to abstraction to yield the resonance stabilized free radical ( ) ... 

The peioxy free radicals can abstract hydrogens from other activated methylene groups between double bonds to form additional hydroperoxides and generate additional free radicals like (1). Thus a chain reaction is estabhshed resulting in autoxidation. The free radicals participate in these reactions, and also react with each other resulting in cross-linking by combination. 

Free radical (3) can rearrange, add oxygen to form a peroxy free radical, abstract a hydrogen from a methylene group between double bonds, combine with another free radical, or add to a conjugated double-bond system. 

A waterborne system for container coatings was developed based on a graft copolymerization of an advanced epoxy resin and an acryHc (52). The acryhc-vinyl monomers are grafted onto preformed epoxy resins in the presence of a free-radical initiator grafting occurs mainly at the methylene group of the aHphatic backbone on the epoxy resin. The polymeric product is a mixture of methacrylic acid—styrene copolymer, soHd epoxy resin, and graft copolymer of the unsaturated monomers onto the epoxy resin backbone. It is dispersible in water upon neutralization with an amine before cure with an amino—formaldehyde resin. 

FIGURE 4.20 The bond dissociation energies of methylene and methyl C—H bonds in propane reveal difference in stabilities between two isomeric free radicals. The secondary radical is more stable than the primary. 

The effectiveness of incineration has most commonly been estimated from the heating value of the fuel, a parameter that has little to do with the rate or mechanism of destraction. Alternative ways to assess the effectiveness of incineration destraction of various constituents of a hazardous waste stream have been proposed, such as assessment methods based on the kinetics of thermal decomposition of the constituents or on the susceptibility of individual constituents to free-radical attack. Laboratory studies of waste incineration have demonstrated that no single ranking procedure is appropriate for all incinerator conditions. For example, acceptably low levels of some test compounds, such as methylene chloride, have proved difficult to achieve because these compounds are formed in the flame from other chemical species. 

Ring-opening polymerization of 2-methylene-l,3-dioxepane (Fig. 6) represents the single example of a free radical polymerization route to PCL (51). Initiation with AIBN at SO C afforded PCL with a of 42,000 in 59% yield. While this monomer is not commercially available, the advantage of this method is that it may be used to obtain otherwise inaccessible copolymers. As an example, copolymerization with vinyl monomers has afforded copolymers of e-caprolactone with styrene, 4-vinylanisole, methyl methacrylate, and vinyl acetate. 

FIGURE 6 Synthesis of PCL by the free radical polymerization of 2-methylene-l,3-dioxepane. (From Ref. 51.)... 

Bailey, W, J., Ni, Z., and Wu, S.-R., Synthesis of poly-e-capralactone via a free radical mechanism. Free radical ringopening polymerization of 2-methylene-l, 3-dioxepane, J. Polym. Sci., Polym. Chem. Ed.. 3021-3030, 1982. 

When a reducible compound (that is not directly attacked by the free radicals) is present in the solution, the stored electrons may react with it. Stored electrons may be transferred pairwise to the solute. The reduction of the aqueous solvent and of the solute compete with each other. A typical example is shown in Fig. 3. Methylene dichloride was the solute here, the product of its reduction being the Cl ion. It is seen that the H2 yield decreases as that of Cl increases with increasing CH2CI2 concentration. For each H2 molecule not formed, one Cl anion is produced. As two electrons are necessary to produce one molecule, one has to conclude that methylene... 

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