Free radical initiators half-life time

A series of simulations were performed to study the effect of variables such as initiator concentration, initiator half-life and activation energy on the optimum temperature and optimum time. It was assumed that initially the polymerization mixture contained S volume percent monomer, the rest of the mixture being solvent and polymer formed earlier. It was required to reduce the monomer concentration from S volume percent to 0.S volume percent in the minimum possible time. The kinetic and tbeimodyamnic parameters used are similar to those of free radical polymerization of MMA. The parameter values are given in Appendix B. 

One of the successful radical homopolymerizations of VPA was performed by Levin et al in DMF in the presence of AIBN as initiator, in a yield of 95%. PVPA was obtained in protic solvents both from pure VPA, crude VPA , and ester-containing crude VPA in the presence of initiator. Suitable protic solvents were water and aliphatic alcohols such as isopropanol, which keep the mixtures stirrable and workable. The free radical initiators that can be used are peroxides such as dibenzoyl peroxide, di-tert-butyl peroxide, tert-butyl peroxybenzoate, tert-butyl pero3gr-2-ethylhexanoate, and ammonium or potassium persulfate. The amount of initiator necessary is 1.5-4% versus monomer and depends directly on the amount of diluent. The authors recommend that the calculated amount of initiator be added in equal portions during the reaction, after the reaction temperature has been reached, because the polymerization is highly exothermic at the start when the monomer concentration is high and relatively higher residual monomer content could be obtained. The reaction temperature was maintained between 80 °C and 110 °C and depends on the dissociation half-life of the initiator. A reaction time of between 5 and 12 h is approximately inversely proportional to the concentration of VPA monomer in solvent. The yield of PVPA varied from 32% to 60% when peroxide initiators were used and from 3% to 6% when ammonium or potassium persulfate were used. Pure PVPA can be obtained by precipitation. The homopolymerization of VPA in methanol did not occur. ... 

One of the nice features of free-radical polymerization is that values of the preexponential coefficients and activation energies (or alternately half-life values at various temperatures) can be obtained in the literature (such as in Odian (1991)) or from their manufacturers (such as Wako Chemical Corp.) for a variety of initiators, and these numbers do not normally change no matter what the fluid environment the initiator molecules are in. Thus, if we want to decompose more than 99% of the starting initiator material in the reactor, we just have to wait for the reaction to proceed up to five times the initiator half-life. The other attractive feature of free-radical polymerization is that free-radical reactions are well known and radical concentrations can be directly measured. Thus, we know, for example, that if we want to preserve radicals in solution, we should not allow oxygen gas (O2) in our system, because reactive radicals will combine with oxygen gas to form a stable peroxy radical. That is why reaction fluids were bubbled with N2, CO2, Ar, or any inert gas, in order to displace O2 gas that comes from the air. Finally, Iree-radical polymerization is not sensitive to atmospheric or process water, compared to other polymerization kinetic mechanisms. 

TFE and monomer (I) were also copolymerized in supercritical carbon dioxide using a free radical initiator such as bis(perfluoro-2-propoxypropionyl)peroxide (III) at 35°C (the half-life time of the initiator is 40 minutes at 35°C) [9], The decomposition of the initiator proceeds through a single-bond homolysis mechanism [10], resulting in the formation of perfluorinated end group that yields thermally stable polymers [9] (Scheme 16.1). The reaction conditions and properties of the copolymers of (I) and TFE obtained in supercritical carbon dioxide are shown in Table 16.1. PTFE is crystalline, so that when the amount of TFE increases in copolymers, the polymer has some microcrystalline regions. The polymers obtained in carbon dioxide have similar properties with the commercial polymers. 

These temperature properties determine the polymerization initiation temperature and the length of time the free-radical initiation process is active. For example, low-temperature peroxides possess a relatively short free-radical half-life and offer a low-temperature initiation but also a lower peak temperature, while a high-temperature peroxide with a relatively long... 

Under appropriate conditions and in the presence of free-radical initiators, cyclopentadiene and MA will also react to form a saturated 1 2 copolymer.Copolymerizations are most effectively carried out at 80-205°C. In general, the highest yield of 1 2 copolymer is obtained when the initiator is used at a temperature where it has a short half-life. At a given temperature, the higher the initiator concentration or more rapidly the initiator decomposes, the higher the yield, independent of total reaction time. Copolymerizations may be run in bulk or solvents such as dioxane or chlorobenzene. The amount of solvent is critical, i.e., the yield of copolymer decreases with increasing solvent concentration unless the initiator concentration is increased. 

In general, free radicals initiators are used under conditions of half-life times of about 10 hours. The decomposition of peroxides can be single-step or multistep for example, dicumyl peroxide (DICUP) decomposes as follows ... 

Polymerisation can only proceed efficiently and economically if sufficient free radicals are present throughout the polymerisation. However, the presence of too many free radicals can have a deleterious effect upon the polymer, resulting in too low a molecular weight, or excessive chain grafting etc. Thus, it is desirable to know how the number of free radicals relates to the initiator concentration, initiator type, and reaction conditions, including reaction temperature. This relationship is expressed as the initiator half-life, and is defined as the time taken for half of a given mass of initiator to decompose. 

Initially, R M radicals are generated by some initiation process [11]. A large number of compounds are known to decompose thermally or photolytically to generate free radicals (see Volume 1, Chapter 1.1). Generally, 5-10 mol% of initiator is added either all in one portion or by slow addition over a period of time. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azo compounds are used from time to time depending on the reaction conditions. Peroxides are used when the reaction requires a more reactive... 

A list of some peroxo compounds that generate free radicals is given in Table 3.3, extensive information can be found in the literature. The initiators are selected according to their thermal half-lives to ensure that at the polymerization temperature they provide a source of free radicals. The rate equation for the thermal half-life is given by tj/2 = 0.693-kd, where kd is the rate constant for the thermal decomposition. In technical applications one often uses the temperature at which within a certain time interval one half of the initiator is decomposed (e.g., quoted as 10 h half-life temperature). 

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