Radical polymerization dimeric model radicals

A comparison of the ESR spectra of the dimeric radical (Fig. 3a), model radicals with Pn=100, and radicals in a polymerization system at 150 °C is shown in Figure 4. The separation of the inner lines, Pn of the propagating radical indicate that the degree of polymerization is higher than 100. When the values of hyperfine splitting constants measured from these spectra, were plotted against chain length, they seemed to show... 

Comparison of ESR spectra of radicals with various chain length at 150 °C. Dimeric model radical, model radical with Pn = ioo, and radicals in a radical polymerization (propagating radical). Characteristic lines were enlarged on the right hand side. 

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. 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mostly owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (31). Application of frontier molecular orbital theory may be more appropriate to explain certain reactions (32,33). Radical reactivities have been studied by esr spectroscopy (34-36) and modeling based on general reactivity and radical polarity (37). Recent radical trapping studies have provided considerable insight into the course of free-radical reactions, particularly addition polymerizations, using radical traps such as 2,4-diphenyl-4-methyl-l-pentene (a-methylstyrene dimer, MSD) (38-44) and 1,1,3,3-tetramethyl-2,3-dihydro-liT-isoindol-2-yloxyl (45-49). 

In this chapter, three examples of the application of ESR to conventional radical polymerizations based on controlled/living radical polymerizations wUl be demonstrated. The first example is estimation of the effect of chain length on propagating radicals. The second example is the detection of chain-transfer reactions on the propagating radicals in polymerization of tert-butyl acrylate (tBA). The third example is investigation of penultimate unit effects using ESR analysis of dimeric model radicals of (meth)acrylates prepared by ATRA. 

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.)...

Equilibrium studies under anaerobic conditions confirmed that [Cu(HA)]+ is the major species in the Cu(II)-ascorbic acid system. However, the existence of minor polymeric, presumably dimeric, species could also be proven. This lends support to the above kinetic model. Provided that the catalytically active complex is the dimer produced in reaction (26), the chain reaction is initiated by the formation and subsequent decomposition of [Cu2(HA)2(02)]2+ into [CuA(02H)] and A -. The chain carrier is the semi-quinone radical which is consumed and regenerated in the propagation steps, Eqs. (29) and (30). The chain is terminated in Eq. (31). Applying the steady-state approximation to the concentrations of the radicals, yields a rate law which is fully consistent with the experimental observations ... 

A possible reductive role for veratryl alcohol oxidase is proposed in Figure 5. Laccases from C. versicolor can produce both polymerization and depolymerization of lignin (29). In phenolic lignin model dimers, laccase can perform the same electron abstraction and subsequent bond cleavage as found for lignin peroxidase (30). The phenolic radical is however likely to polymerize unless the quinoid-type intermediates can be removed, for example by reduction back to the phenol. Veratryl alcohol oxidase, in... 

Theoretical modeling was performed, based on the theiry that radical coupling caused polymerization among the monomers, as well as between the substrates of different DPs. The equal reactivity of these substrates was assumed, while the calculated model successfully reproduced most of the experimental results an increase of the polymer with a decrease of the monomer and a temporal accumulation of the dimer responded to the addition of H202. The observed yield of the dimer, however, was much lower (<5%) than the 15% calculated. Our model may be improved if the reactivity of each oligomer is taken into consideration. 

Chain initiation occurs when two monomer radicals are coupled to form a dimer biradical and proceeds further." This is an endothermic reaction requiring a heat of formation of 16 kcal/mol. Because of energetic concerns, chain initiation is unlikely to happen in the gas phase at low pressure. When the monomers are adsorbed onto the surface of the substrate, it is believed that, the high local concentration of monomers promotes the formation of biradicals assisted by van der waals forces. Models developed for vapor deposition polymerization of parylene-N indicate that initiation is a third order reaction with an activation energy of 24.8 kcal/mol. 

All the phenyl radicals, phenoxyl radicals and hydroxycyclohexadienyl radicals produced from phenols by various reactions react with each other and with other radicals to form, at least in part, new C—C or C—O bonds. As a result of these reactions, irradiation of phenols can lead to dimeric and polymeric products and irradiation of phenols in mixtures with other compounds can lead to crosshnking of the two materials. For example, irradiation of tyrosine or dopa with albumin in aqueous solutions leads to binding of these phenols to the protein . Similarly, irradiation of tyrosine and its peptides - or mixtures of tyrosine and thymine led to various dimerization products. The latter case was studied as a model for radiation-induced crosshnking between proteins and DNA. 

Figure 6 Radical stabilization energies (RSEs AH ) for some model unimeric (H-M ) and dimeric (H-M-M ) propagating radicals, relevant to the polymerization of CH2=CH2 (Et), CH2=CHPh (STY), CH2=CHC00CH3 (MA), CH2=C(CH3)C00CH3, CH2=CH0C0CH3 (VA), CH2=CHC0NH2 (AM), and CH2=CHC00H (AA). Uni meric radicals with cyanoispropyl chain ends (Init-M ) are also shown. Lin, C.Y. Coote, M.L. Aust. J. Chem. 2011,64,747-756. ...

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