Vinyl decay

Possibly the most convincing evidence for positive ion-molecule reactions in polymers is the high rate of decay of vinyl unsaturation during the radiolysis of polyethylene, as recently discussed by Dole, Fallgatter, and Katsuura (13). The ideas of these authors with respect to the carbonium ion mechanism for vinyl decay by means of a dimerization reaction were largely suggested by the mechanisms proposed by Col-linson, Dainton, and Walker (5) for vinyl decay (polymerization) in the radiolysis of n-hexa-l-decene, Reactions 3 and 4 of Table I. 

When the tritium (half-life 12.26 years) decays it is converted to the helium-3 isotope, which, of course, does not form covalent bonds, and so immediately departs, leaving behind the alkynyl cation. When this was done in the presence of benzene, RC CCgHs was isolated. The tritium-decay technique has also been used to generate vinylic and aryl cations. 

Fang, L Brown, W, Decay Time Distributions from Dynamic Light Scattering for Aqueous Poly(vinyl alcohol) Gels and Semidilute Solutions, Macromolecules 23, 3284, 1990. 

The results presented in this report correspond to systems where reactions 2 and 3 account for the decay of carbonyl triplets A series of copolymers of phenyl vinyl ketone and o-tolyl vinyl... 

The vinyl radical (C2H3) decays to acetylene primarily by... 

Although no reported work is available on vinyl acetylene oxidation, oxidation by O would probably lead primarily to the formation of CO, H2, and acetylene (via an intermediate methyl acetylene) [37], The oxidation of vinyl acetylene, or the cyclopentadienyl radical shown earlier, requires the formation of an adduct [as shown in reaction (3.142)]. When OH forms the adduct, the reaction is so exothermic that it drives the system back to the initial reacting species. Thus, O atoms become the primary oxidizing species in the reaction steps. This factor may explain why the fuel decay and intermediate species formed in rich and lean oxidation experiments follow the same trend, although rich experiments show much slower rates [65] because the concentrations of oxygen atoms are lower. Figure 3.13 is a summary of the reaction steps that form the general mechanism of benzene and the phenyl radical oxidation based on a modified version of a model proposed by Emdee et al. [61, 66], Other models of benzene oxidation [67, 68, which are based on Ref. [61], place emphasis on different reactions. 

Many valuable reviews of the chemistry of these species are given in the new book Dicoordinated Carbocations An introduction by Grob " is followed by reviews of various theoretical studies of vinyl cations, their gas-phase chemistry, their generation by nuclear decay, and their NMR spectroscopic characterization. Vinyl cation production by addition to acetylenes and allenes, by solvolysis, and photolytically are covered, together with the chemistry of the species generated in these various ways. The next chapter deals with the synthetic applications of vinyl cations,and alkynyl and aryl cations are covered in the last chapter. A review of the NMR spectroscopic and quantum-chemical investigation of vinyl cations in superacid media (also of dienyl and 1-cyclopropylvinyl cations) is published separately,as is a review of alkynylcar-... 

A study on mechanistic aspects of di-ir-methane rearrangements has been published recently [72]. The kinetic modeling of temperature-dependent datasets from photoreactions of 1,3-diphenylpropene and several of its 3-substituted derivatives 127a-127d (structures 127 and 128) show that the singlet excited state decays via two inactivated processes, fluorescence and intersystem crossing, and two activated processes, trans-cis isomerization and phenyl-vinyl bridging. The latter activated process yields a biradical intermediate that partitions between forma-... 

Various authors—for example, Dole, Milner, and Williams (15) and Lyons (25)—have suggested that the decay of vinyl groups initially present in some types of high density polyethylene involves an end-linking process, these authors disagreeing only about the mechanism involved. If such were the case, some difference in solubility or elastic behavior above 140 °C. would be expected between low and high density poly-... 

Figure 9. Decay of vinyl umaturation in Surhjn A exposed to ionizing radiation...

Huffman (87) studied the transient emissions from terbium in a vinylic resin matrix. His compound was Tb tris-[4,4,4-trifluoro-l-(2-thienyl)-1,3-butaneodione] in polymethylmethacrylate. This may be conveniently abbreviated as TbTTA in PMMA. The compound EuTTA in PMM A had previously been reported by Wolff and Pressley (99) to give laser oscillation. Working with small fibers at 77°K, Huffman found distortions from the normal fluorescent decay curves when the optical pumping was large. He interprets this as evidence for stimulated emission. A comparison of these distorted decays with EuTTA in PMMA indicated a similar behavior, thus tending to substantiate his hypothesis. 

The conformational statistics of asymmetric vinyl chains such as P2VN are well-known 126). The rotational conformers of isotactic (meso) dyads are entirely different from those of syndiotactic (dl) dyads. Frank and Harrah132) have described each of the six distinct conformers for meso and dl dyads, using the t, g+ and g nomenclature of Flory 126). Excimer-forming sites (EFS) are found in the tt and g g+ meso states, and in the degenerate tg , g t dl state. Because the rotational conformers of compounds such as l,3-bis(2-naphthyl)propane do not match those of either the iso-or syndiotactic dyads of P2VN, the propane compounds make poor models of aryl vinyl polymers. However, the rate constants of fluorescence and decay of the intramolecular excimer in polymers can usually be determined from the propane compounds (but see the exceptional case of PVK and its models133)). 

This is about 1/100 the experimental value of ID/IM = 0.9 recorded for 0.3% P2VN (70,000)/PS (2200) blends. 3<> Thus, there must be some mechanism that increases the probability that an absorbed photon decays from the excimer state, since there are not enough intramolecular EFS to cause such a large value of ID/1M by direct excitation alone. Similar observations were made for dilute miscible PS/Poly(vinyl methyl ether) blends 140). 

Free radicals are also produced by chain scission during deformation of polyethylene and FT-IR has been used to follow this process 237). The polyethylene samples were unaxially drawn and the resultant spectra corrected for orientation. An increase in the vinyl and methyl end groups created by decay of the free radicals occured in going from draw ratios of 5 to 20 44). A similar study involving deformation was made of polystyrene 246) and a comparison demonstrated between the results of thermal and mechanical degradation 24S. ... 

Glusker (37, 38) attempted to prove that these processes are absent by an estimation of active chains by reaction with C14 labelled C02 or H8(T) labelled acetic acid, followed by measurements of the radioactivity of the polymer isolated. Most of the experiments were carried out with fluorenyllithium as initiator in toluene containing 10% diethyl-ether at —60°. At —78° at least 80% of the polymer chains were found to be active at the end of polymerization. The lowest fraction was appreciably less active. Similar results were obtained at —60° although no examination was made of the fractions of lowest molecular weight. Kinetic experiments indicated a first order decay of monomer concentration after an initial rapid consumption of about 3 molecules of monomer per initiator molecule. The mechanism suggested to explain these results involves rapid addition of fluorenyllithium across the vinyl double bond followed by the rapid addition of three monomer units. At this stage it is... 

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