Radical polymerization monomeric model radicals

However, some of the differences may be explicable in terms of an effect of molecular size. For many of the model systems at least one of the reaction partners is monomeric (/.e. 2, 5, 8-10,15). Since combination is known to be more sensitive to steric factors than disproportionation (Section 2,4.3.2), kJk may be anticipated to be higher for the corresponding propagating species. The values of A td/Aic reported for 3 or 4 arc significantly greater than those for 2. Similarly, 6 gives much more disproportionation than 5. Thus, values of seen for systems involving monomeric model radicals (2, 5, 8-10, or 15) should be considered only as a lower limit for the polymeric system. 

It has been known since 1980 that the terminal model for free-radical copolymerization sometimes fails, due to the penultimate unit effect. Direct detection of the penultimate unit effect by ESR has been unsuccessfully attempted many times. In this section, direct detection of the penultimate unit effect using dimeric model radicals generated from dimeric model radical precursors prepared by ATRA is discussed (Fig. 19). The structures of the dimeric model radicals studied are summarized in Fig. 20. For a detailed discussion of the penultimate unit effect, dimeric, monomeric, and polymeric model radicals were examined. The radicals were generated by three methods homolytic cleavage of carbon-bromine bonds of alkyl bromides with hexabutyldistannane, photodecomposition of an azo-initiator, and radical polymerization performed directly in a sample cell in a cavity. 

Fig. 18. Experimental and simulated ESR spectra of fBA radicals with various chain lengths observed at 30°C (a) H-fBA (monomeric), (b) H-fBA-fBA (dimeric), and (c) H-(fBA)n-fBA [polymeric model radicals, 15 (DP = 15)]. (Center lines indicated in dashed squares are due to radicals of tin compounds). (Erom Ref. 50, with permission.)...

Fig. 22. Generated monomeric, dimeric, and polymeric model radicals of (meth)acrylates, (From Ref. 50, with permission.)...

From these results, and from the reported extrapolated values of 276 °C (syndiotactic) and 126 °C (isotactic) for the glass transition temperatures of PNVC it has been concluded that rotational eclipsing in isotactic NVC sequences is very much more favourable than in corresponding syndiotactic sequences. This contusion is strongly supported by examination of space filling molecular models. Such considerations must be borne in mind when considering the optical and chiroptical properties of carbazole containing polymers and it has to be expected that copolymers of NVC will have fluorescence emission properties that can be related to both main chain tacticity and distribution of monomeric units. Free radically polymerized mixtures of NVC and acrylic derivatives of (—)-menthol have fluorescence spectra... 

Degradation of Larger Silicon Catenates. Although it seems clear that polymeric disilane derivatives photodecompose by bond homolysis to produce silyl radicals, model studies on larger silicon catenates indicate that their photochemistry may be more complex (Scheme IV). Cyclic silane derivatives seem to extrude monomeric silylenes upon irradiation to produce smaller cyclic silanes (52). The proposed silylene intermediates have been identified spectroscopically 49, 53), and trapping adducts have been isolated in solution. Exhaustive irradiation ultimately results in acyclic silanes, which... 

An optically active polystyrene derivative, 40 ([a]25365 -224° to -283°), was prepared by anionic and radical catalyses.113 The one synthesized through the anionic polymerization of the corresponding styrene derivative using BuLi in toluene seemed to have a high stereoregularity and showed an intense CD spectrum whose pattern was different from those of the monomer and a model compound of monomeric unit 41. In contrast, polymer 42 and a model compound, 43,... 

Ito s model [68] bears resemblances to the model of Ref. [35], but is different by two aspects. Firstly, it assumes that the constant rate of the chain termination depends on the number of monomeric units (so-called polymerization degree) of tn and n radical chains taking part in the termination reaction and represents the sum of the independent contributions of m and n. Secondly, the dependence of the chain termination constant on the length of chains under two types of conditions is described the first condition is < n, controlled by segmental diffusion, and the second one is m > controlled by the reptation diffusion. In the reptation chemical mechanism of diffusion in the deep states of conversion the macroradicals move snake-like between the network joints. De Gennes connected a reptative moving of macroradicals with the dynamic properties of the medium with the use of scaling ratios [37-40] as applied in Refs. [41-46] for the description of constant chain termination in the late conversion state. 

A number of mechanisms and models have been proposed for latex particle formation in emulsion polymerization systems [18-20]. These include particle formation by entry of a free radical into a micelle [6,7] or by homogeneous nucleation of oligomeric free radicals in the aqueous phase [18,21-23] or within microdroplets of the monomeric emulsion [24]. After their formation, these primary particles may simply grow by conversion of monomer into polymer within these particles, or undergo coagulation [22,25]. 

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