Concentration, of trapped radicals

The actual value of the limiting concentration of trapped radicals depends strongly on the physical state of the polymer since this determines the mobility of the radicals in the medium. Thus, no significant amount of trapped radicals is found in elastomers irradiated at room temperature whereas large concentrations of the order of 1019 spins per cc. can be observed in glassy or crystalline polymers. 

The concentration of trapped radicals and the degree of occlusion (how deeply they are buried) have been studied extensively by Bamford and Jenkins (9). They determined the approximate radical concentration by swelling or dissolving polymer in solutions of diphenylpicryl hydrazyl (DPPH), a violet substance that becomes colorless on reaction with radicals. Nitrobenzene was generally used as the medium to dissolve the hydrazyl and to swell the polymer /3-propiolactone, a solvent for the polymer, was used also. 

The concentration of trapped radicals in a typical experiment was estimated as about 3 X 10-5 mol/1 of reaction mixture, or about 2 X 1018 radicals/ml. This concentration increased with the rate of photopolymerization, but the number of radicals that became trapped was a constant fraction of the total number generated. In these experiments that fraction was estimated at about 1% at 25°. 

The work described here was undertaken on the premise that measurements of the number of particles, total particle surface, and concentration of trapped radicals are needed in conjunction with rate measurements over a wide range of initiation rate and monomer concentration to understand more thoroughly the important factors in this type of heterogeneous polymerization. However, as will become apparent from the results reported here, variations in particle number and total surface are small and have little effect on the polymerization rate. Under our conditions trapped radicals were present in too low a concentration to be detected and cannot account for the peculiar features of the reaction kinetics. 

Recently, even higher concentrations of trapped radicals have been observed by electron spin resonance. Bamford et al. (5) found up to 1.3 X 1017 radicals per ml., and Ingram et al. (18) found 3.5 X 1017 per ml. in the photopolymerization of acrylonitrile at 20 °C. At the same time, it was found that the concentration was greatly reduced when polymerization was carried out at higher temperatures, and Bamford et al. were unable to detect any radicals at 60 °C. 

The concentration of trapped radicals in the cellulose lattice depended on the radiation dosage and the amount of moisture present in the cellulose during irradiation. The trapped radical yield in irradiated cellulose, containing about 7% moisture, is shown in Table I. If cellulose was irradiated dry (about 0.5% moisture) and in vacuum, an initial yield of 2.5-3 times that shown was obtained however, the concentration of the radicals decreased rapidly on exposure of the irradiated, dry cellulose to moisture (26). 

Three distinct parts can often be observed in the curves relating the concentration of trapped radicals in low and high molecular weight solids and the irradiation time, (a) an initial linear part, (b) a plateau at high doses, and (c) an intermediate region corresponding to a decrease in the net rate of formation and trapped radicals. 

Finally, mention must be made of a study on PS-PP blends in which PP was a minority component (20wt%). When the blends were yirradiated at low doses (<70kGy) [95], EPR data relating to the concentration of trapped radicals indicated that in these systems the PP was protected against a strong oxidative degradation (chain scission). 

It is unfortunate that typical concentrations of free-radical species present in biological systems are only at the limit of e.s.r. detection sensitivity and, of course, there are major technical difficulties in studying whole animals in this manner. Therefore, the most successful e.s.r. experiments have adopted the approach of spin trapping in which very reactive and thus transient radical species are converted to long-lived adducts via reaction with a trap such as a nitrone, e.g. Equation 1.1 ... 

Several possible complications in the kinetic analysis were pondered and ruled out by the authors. For instance, they demonstrated that the direct reaction of the peroxide with P did not occur because a change of concentration of this radical trap did not affect the reaction rate. 

Table 15.1 Rate constants (k- ) of reaction 15.5 in toluene at several temperatures. The initial concentration of the radical trap was 5.83 x 10-5 M. Data from [323],...

Other researchers have experimentally observed heterogeneity in crosslinked polymers by studying radical concentrations and environment with ESR [101-106], Knowledge of the structure and reactivity of trapped radicals is especially important when considering the long term physical and mechanical properties of dental polymers. Kloosterboer et al. [106] has studied the structure of trapped acrylate radicals while Hamielec and coworkers [102-105] have... 

In some systems the polymer may precipitate in the course of the reaction and this again greatly affects the kinetics of polymerisation, e. g., in a radical polymerisation the precipitation may lead to the formation of "trapped radicals. Moreover, separation into two phases affects the concentration of monomer around the growing centres and this may... 

Fig. 6. The time dependence [12] of the concentration of anion radicals of (a) biphenyl and (b) of triphenylethylene in vitreous ethanol containing 0.15 M of biphenyl + various amounts of triphenylethylene. T — 77 K. The solid lines are theoretical curves calculated using eqn. (7) of Chap. 5 and the values of ve = I0132s 1, ae = 1.90A, a = 13.1 M-1. n(oo) is the concentration of triphenylethylene anion radical in a sample containing 0.15 M of PhaEt and no Ph2 in which, by the time t = 10 6s, all the electrons have been trapped by the molecules of Ph3Et.

The inhomogeneity of the micellar aggregate also affords assisted spin trapping and the exploitation of magnetic field effects on the charge separated ion pairs [48]. Optical modulation spectroscopy can be used, for example, to follow the decay of radicals formed in homogeneous solution and in SDS micelles. Enhancements of a factor of about 50 in the lifetimes and the steady state concentrations of the radical were observed in the micelle, and a kinetic analysis led to a value of 2 x 103 s 1 for the exit rate constant from the micelle [49]. 

Tt is well known that the presence of precipitated polymer can influence the course of polymerization. In bulk acrylonitrile polymerization the effects are most dramatic and have been the subject of many studies. The literature on this subject has been reviewed by Bamford et al. (4) by Thomas (29), and by Peebles (23). Under conditions where the system becomes heterogeneous owing to precipitation of small particles of polymer, a protracted acceleration period is observed at the start of polymerization, and the final rate is found to depend on the 0.8 power of the concentration of free radical initiator. Unusual post-polymerization effects are observed in photoinitiated polymerization of acrylonitrile, owing to the presence of trapped radicals which can be detected by electron spin resonance. None of the detailed mechanisms proposed to... 

The addition of small amounts of radical scavenger (such as benzoquinone and diphenylpicrylhydrazyl) led to the appearance of induction periods in the kinetic curves. The duration of the induction periods are proportional to the concentration of the radical scavenger. The presence of atmospheric oxygen slightly slowed the polymerization. These observations indicate that the polymerization proceeds by a radical mechanism. The radicals are formed from the y-radiolysis of the monomers. By comparison to the ESR spectrum of the radicals formed by thermal initiation with azobisisobutyroni-trile in the presence of a spin trap, the radical formed is... 

The requirements that the radicals are formed and trapped in isolated sites results in low radical concentrations and hence the total concentration of non-radical products is also small. Most samples contain 100-200 ppm of products but in some instances product concentrations as low as 10 ppm have to be determined. Such low concentrations necessitate the use of sensitive detection systems and the meticulous elimination of contaminants from all the gases used in the g.l.c. analysis. The compounds used as radical somrces and as matrices must also be extremely pure and to this end preparative g.l.c. has been used to piudfy the compounds. Mixtures of materials immiscible at room temperatme... 

Figure 7.14 Change in concentration of free radicals trapped in plasma polymers by copolymerization of acetylene with N2 or H2O.

Nitrogen and carbon monoxide have similar electron structures and evidently participate in the chemical reactions of cycle 2. Consequently, the copolymerization of these gases does not decrease the concentration of free radicals trapped in the plasma polymers. 

Some monomer show a more or less anticipated decrease in polymer deposition rates based on the concept that a pulsed discharge decreases the initiation rate, but some monomers show dramatically increased deposition rates. The most significant effect of pulsed discharge, however, can be seen in the concentration of free radicals trapped in plasma polymers (dangling bonds), which reflects the unique mechanisms of polymer formation in plasmas. 

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