Monomer radicals

Table 7.2 Values of the Cross-Propagation Constants k 12 for Four Monomer-Radical Combinations...

The presence of stable free radicals in the final polycondensate is supported by the observation that traces of (11) have a strong inhibiting effect on the thermal polymerization of a number of vinyl monomers. Radical polymerization was inhibited to a larger extent by a furfural resin than by typical polymerization inhibitors (34). Thermal degradative methods have been used to study the stmcture of furfural resinifted to an insoluble and infusible state, leading to proposed stmctural features (35). 

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). 

Sodium naphthalene [25398-08-7J and other aromatic radical anions react with monomers such as styrene by reversible electron transfer to form the corresponding monomer radical anions. Although the equihbtium (eq. 10)... 

S = excited sensitizer, R = chain-transfer agent radical, M = monomer, M = monomer radical, and P = polymer. 

The most widely accepted mechanism for the anodic polymerization of pyrroles and thiophenes involves the coupling of radical cations produced at the electrode (Scheme l).5 The oligomers so produced, which are more easily oxidized than the monomer, are rapidly oxidized and couple with each other and with monomer radical cations. Coupling occurs predominantly at the a-positions (i.e., 2- and 5-position),5 and so pyrroles and thiophenes with substituents in either of these positions do not undergo anodic polymerization. The reaction is stoichiometric in that two... 

The physical picture of emulsion polymerization is complex due to the presence of multiple phases, multiple monomers, radical species, and other ingredients, an extensive reaction and particle formation mechanism, and the possibility of many modes of reactor operation. 

In general the mechanism of polymerisation for thiophene appears to be similar to that of pyrrole (Section 4.11.2), occurring via a radical coupling mechanism [423] giving mainly a-a linkages [293,400,405], and involves oligomer as well as monomer radicals, with evidence to suggest that the polymerisation reaction occurs at a lower... 

Kruus also conducted experiments in the presence of the radical scavenger diphe-nylpicryhydracyl (DPPH) and observed induction periods which were roughly proportional to concentration of DPPH employed. This clearly demonstrates the free radical nature of the polymerisation. By assuming that each of the monomer radicals produced by the cavitation process (Eq. 5.30) reacted with one DPPH molecule, he was able to deduce the following kinetic relationship ... 

In solution the colorless 2,4,6-triphenylphenoxyl dimer attains a rapid equilibrium with its red monomer radical (dissociation constant in benzene 4 X 10 at 20°). The radical is surprisingly stable toward oxygen and can be stored in solution for a long time when it is protected from light. The stability of the 2,4,6-triphenylphenoxyl radical is ascribed to steric and mesomeric effects. The e.s.r. spectrum and an ENDOR-spectrum of the radical are described. 

The product obtained is analytically pure Calcd. for ( aillirO C, 89.69 H, 5.33 O, 4.98. Found C, 89.94 11,5.27 O, 5.00. The product is stable for several months when stored in the dark. The 2,4,6-triphenylphenoxyl dimer is piezo-chromic rubbing in a mortar produces a red color. Solutions of the colorless dimer in organic solvents are red owing to dissociation to the monomer radical. 

In the photochemical one-electron oxidation of aromatic sulfides, dimer radical cations were formed in rapid equilibrium with monomeric radical cation (59). The complex formation of a- and tt-types has been shown to be sensitive to the steric and electronic influence of substituent. For the case of jo-(methylthio)anisole the formation of TT-type dimer was shown to be reduced due to steric hindrance of two methyl groups. No formation of dimer radical cation was observed for jo-(methoxy)thioanisole and diphenyl disulfide where the corresponding monomer radical cations are stabilized by the delocalization of positive charge on the sulfur atom. Density-functional calculations supported the experimental results. The intramolecular formation of similar radical... 

Some electroinitiated polymerizations proceed via monomer radical-cations (VII) formed by electron transfer... 

Electron-transfer initiation also occurs in heterogeneous polymerizations involving dispersions of an alkali metal in monomer. Initiation involves electron transfer from the metal to monomer followed by dimerization of the monomer radical-anion to form the propagating... 

Figure 6-11 shows that the order of reaction rate constants for the various monomer-radical reactions is... 

As in the case of hexafluorobenzene solvent anion, EPR and ODMR spectroscopies suggests that no dimerization of monomer radical anions of benzene and toluene occur in liquid benzene and/or in alkane solutions of benzene (whereas the radical cation of benzene is known to dimerize rapidly). The conductivity studies also indicate that there is no volume change associated with the dimerization [45]. 

The results are surprising since they apparently infer that monomer radical propagation starts at one end of the template and continues to the other end of the template when termination occurs. According to the authors this needs not necessarily be the case. The explanation is illustrated in Figure 4.4. 

Once the monomer radical has been initiated, high polymer is formed through addition of monomer units to the radical in the propagation phase of the polymerization. Each time a monomer unit is added, the radical transfers to the end of the chain to allow the polymerization to continue ... 

If the reaction takes place in the presence of monomer, grafting occurs in the usual manner. The fact that some homopolymerization also occurs in the redox-catalyzed grafting system can be explained by a chain-transfer mechanism (Reaction 10). The growing polymer radicals can abstract hydrogen atoms from the monomer, forming monomer radicals and thereby initiating homopolymerization. 

One measure of the resonance component of radical stability in polymerizations is the so-called Q value of the monomer, which quantifies the resonance stabilization of the radical (Stevens 1990). However, the experimentally determined value of Q can be influenced by other factors unrelated to resonance. To evaluate the extent to which their measured Q values were consistent with resonance stabilization of the monomer radical, the authors compared isodesmic energies from Eq. (6.14) to measured Q values for R = Me, rBu, PhO, CN, Ph, vinyl, and phenylethynyl. The largest stabilization energy was computed for the R = phenylethynyl case, about 101 kJ mol-1, although at the HF/3-21G level the expected linear correlation between log(2 and stabilization energy was only fair (R2 = 0.86 a better correlation for the non-phenylethynyl substituents had been obtained previously at a higher level of theory). 

Generation of free-radicals by Kolbe s reaction is well-known [Eq. (10)]. Formation of a radical-cation of monomer [Eq. (11)] has never been been proved and is only a possible conjecture from the right reverse consideration of the radical-anion formation at the cathode [Eq. (6)], although the perchlorate anion has actually been found to yield an unstable perchlorate free-radical by discharge at the anode. Nor is it certain that the monomer radical-cation is formed by direct discharge from the anode [Eq. (12)]. The ring-opening polymerization of oxides, caprolactam and isocyanides is also initiated on the electrode. A few examples of condensation polymerization have developed recently, like Eq. (7) and (12). Details of this work are described in the appropriate section. 

The copolymerization experiments indicate that polymerization does not proceed through an anionic but a free-radical mechanism. Free radical has been found to remain after polymerization. The authors explain that the dimerization of semiquinones is inhibited with its steric hindrance, while the formation of monomer radical is not disturbed, which initiates the polymerization. An anionic end may be protonized with di-methylformamide. 

It has been known for some time that UV photopolymerization of multifunctional monomers does not obey the classical rate expression, Rp proportional to I0 5, but follows an approximately first-order relationship [196,197]. These results have been explained by postulating that, in these viscous monomers, radical occlusion competes with bimolecular termination. 

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