Induction period, radical polymerization

Given that the rate of generation of primary radicals, at a given light flux, is a function of the concentration of photoinitiator, the induction period depends also on initiator concentration. In order to eliminate this induction period of polymerization in industrial scale applications, many photocuring processes are carried... 

Inhibitors are useful in determining induction rates, because their reaction with radicals is so rapid that the decomposition of the inhibitor is not dependent on its concendation but gives the rate of generation of radicals directly. As a result, the length of the induction period/before polymerization starts is directly proportional to the number of inhibitor molecules initially present. This number then represents the number of radicals produced during the time of the induction period. 

Kinetic plots for polymerization of vinyl acetate (VAc) initiated by azo radical sources (AIBN and V-70) and mediated by cobalt(ii) tetramesityl porphyrin ((TMP)Co ) in CgDg (Figure 5.16) appear very similar to the corresponding plots for MA polymerization (Figure 5.7). During the induction period radicals that enter solution from the azo radical sources react and are trapped by (TMP)Co as the initially formed organo-cobalt complexes ((TMP)Co-CH(OC(0)CH3)CH3 in a manner that parallel the acrylate... 

Inhibitors and retarders differ in the extent to which they interfere with polymerization, and not in their essential activity. An inhibitor is defined as a substance which blocks polymerization completely until it is either removed or consumed. Thus failure to totally eliminate an inhibitor from purified monomer will result in an induction period in which the inhibitor is first converted to an inert form before polymerization can begin. A retarder is less efficient and merely slows down the polymerization process by competing for radicals. 

At the conclusion of the induction period due to oxygen, polymerization sets in at a rate exceeding that for pure monomer under the same conditions. The polymeric peroxides apparently furnish a source of free radicals. Oxygen therefore combines the roles of inhibitor, comonomer, and (indirectly) of initiator. 

The induction period observed at the very beginning of the irradiation is due to the well known inhibition effect of oxygen on these radical-induced reactions. Once it is over, after the, v10 ms needed to consume essentially all of the oxygen dissolved in the liquid film (19), the polymerization starts rapidly to reach 75 % conversion within 0.08 s. Further UV exposure leads only to a slow increase in the cure, mainly because of mobility restrictions in the rigid matrix, so that there still remains about 15 % of acrylic unsaturation in coatings heavily irradiated for 0.4 s. 

An easy way to evaluate the initiation rate, Y[ = Iq (1-e " 2,3 e l [pl]) Oj, and a possible deviation from its first-order dependence on the light intensity, is by monitoring the induction period (tj). Under air diffusion-free conditions (laminate), a given number of radicals (N) must be generated by photolysis of the initiator to consume the 02 molecules dissolved in the formulation, before polymerization can start ... 

Therefore, the reciprocal of the induction period (1/tj) should vary linearly with the light intensity. Figure 6 shows that this is true, up to a certain value of the light intensity above which 1/tj is levelling off, very much like (Rp)max does. This result suggests that the polymerization rate limitation is due, at least partly, to a decrease in the production of initiating radicals under intense illumination. 

Free radical polymerization processes [41] are carried out in bulk, solution, suspension, emulsion, or by precipitation techniques. In all cases the monomer used should be free of solvent and inhibitor or else a long induction period will result. In some cases this may be overcome by adding excess initiator. 

An ideal inhibitor for a polymerization introduces an induction period during which it is completely converted into substances which have no subsequent effect upon the polymerization. When the inhibitor is completely consumed, polymerization proceeds at the rate to be expected for a similar reaction performed in the absence of inhibitor due allowance must be made for consumption of initiator during the induction period. Even in an ideal case however there must be a transition between complete inhibition and the full rate of reaction because, at very low concentrations of the inhibitor, the monomer may compete successfully for the capture of radicals in the system. An ideal inhibitor should not become incorporated in polymer except for the possibility of it becoming included in the low polymer formed during the transition period. 

In practice, all inhibitors show some deviations from ideal behaviour for example when diphenylpicrylhydrazyl is used as a radical scavenger it is converted to products which are reactive towards radicals as shown by the fact that polymerizations occurring after the induction periods are obviously retarded. The use of 14C-hydrazyl in the radical polymerization of styrene has shown that substantial quantities of the material are combined in the polymer 36). 

Also important is that the induction period, which was about 3 min in the ESR experiment, was found to increase to about 30 min in isothermal DSC scans performed at the same cure temperature (Tollens and Lee, 1993). This is possibly due to the presence of dissolved oxygen (coming from air) in the DSC samples. Oxygen is a known inhibitor of the UP-S free-radical polymerization. This is a very important fact rate equations determined... 

The time to tQ is the time for the wood-monomer mass to reach oven or curing temperature at T5. During the period of constant temperature, the induction period, the inhibitor is being removed by reaction with the free radicals. Once the inhibitor is eliminated from the monomer and wood, the temperature rises to a maximum which corresponds to the peak of the exothermic polymerization reaction. Polymerization continues to completion although at a decreased rate and the temperature returns to that of the curing chamber. The time to the peak temperature depends upon the amount of catalyst present, the type of monomer, the type of crosslinker, and the ratio of the mass of monomer to that of the wood. The wood mass acts as a heat sink. Figure 4 illustrates the effect of increased Vazo catalyst on the decrease in time to the peak temperature, and the increase in the peak temperature(10)... 

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... 

Very rapid initiations are known, manifested by an instantaneous start to the polymerization after which the number of active centres is not further increased. Polymerizations with slow initiation are also quite frequent, starting only after some inhibition and/or induction period. In the course of these polymerizations, the concentration of active centres is not usually constant. A stationary state is not excluded, of course but it occurs much less frequently than with radical polymerizations. 

Stabilize new particles, thereby increasing the total number of particles. Since the nucleation period is lengthened, the polydispersity increases. Figure 14 shows that the dependence of the inhibitor concentration on the number of particles is 0.176 0.010. Conversion time curves indicate that an induction period results from the presence of the inhibitor. Since polymer-stabilized miniemulsion polymerization occurs via droplet nucleation, it should be less sensitive to oil-phase inhibition. Initiator radicals will enter the droplet one after the other until all of the inhibitor is used up, and the monomer polymerizes. This does not affect the number of droplets or particles. As seen in Fig. 15, the number of particles is proportional to the DPPH concentration raised to the power of 0.0031 0.0001. Therefore, the number of particles is essentially independent of the presence of inhibitor. 

As reported in Table 15, the kinetic data clearly indicate that the photoinitiation activity of poly(BMOA-co-MtA) is not substantially affected by the content of BMOA co-units along the polymer chain and is of the same order of magnitude as that found for the model compound BMOAc. The absence of a polymer effect in the above photoinitiators has been interpreted [84] in terms of a photodegradation mechanism of the macromolecules involving the ftee radical species anchored to the main chain, even in the presence of acrylic monomers, analogous to what is reported in Scheme 18. Moreover, the induction period of the HDDA/BA photoinduced polymerization increases, on decreasing the content of... 

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