Thermal decomposition of initiators

The thermal scission of a compound is the most common means of generating radicals to initiate polymerization. The number of different types of compounds which can be used as thermal initiators is rather limited. Compounds with bond dissociation energies in the range 100-170 kJ/mol are usually suitable. (Others with higher or lower dissociation energies will dissociate too slowly or too rapidly to be useful.) The major class of compounds with bond dissociation in this range contain the 0-0 peroxide linkage. There are numerous varieties of compounds of this type and some are listed in Table 6.4. 

Dialkyi peroxy-dicarbonates CH3 0 0 CH3 1 II II 1 H-C-O-C-O-O-C-O-C-H 1 1 C2H5 C2H5 Di-sec-butylperoxydicarbonate 45 

Thermal decomposition is ideally a unimolecular reaction with a first-order rate constant, kj, which is related to the half-life of the initiator, ti/2, by Eq. (6.29). For academic studies it is convenient to select an initiator whose concentration will not change significantly during the course of an experiment so that instantaneous kinetic expressions, such as Eq. (6.26), may be applicable. From experience it seems that an initiator with a ti/2 of about 10 h at the particular reaction temperature is a good choice in this regard. This corresponds to a kj of 2xl0 s from Eq. (6.29). For the peroxide initiators listed in Table 6.4 the required reaction temperatures for 10 h half-life (ti/2) are also shown. It should be noted, however, that the temperature-half-life relations given in Table 6.4 may vary with reaction conditions, because some peroxides are subject to accelerated decompositions by specific promoters and are also affected by solvents or monomers in the system. 

Aside from peroxides, the main other class of compounds used extensively as catalysts are the azo compounds. By far the most important member of this class of initiators is 2,2 -azobisisobutyro—nitrile (AIBN) which generate radicals by the decomposition reaction  

Initiators are not used efficiently in free-radical polymerizations. This becomes apparent when a material balance is performed on the amount of initiator that is decomposed during a polymerization and compared with the amount that initiates polymerization and thus becomes a part of the polymer formed. 

It may be noted that Eq. (6.23) for the rate of polymerization Rp contains a general term Rt representing the rate of initiation. The expression for Rt will vary depending on the method used for the generation of primary radicals. For unimolecular thermal decomposition of initiator compounds [Eq. (6.3)], Ri is given by Eq. (6.11). Inserting it in Eq. (6.23) yielded the corresponding expression, Eq. (6.24), for the rate of polymerization. 

A variety of other means, besides thermal decomposition of initiator, can be used to produce radicals for chain initiation, such as redox reactions, ultraviolet irradiation, high-energy irradiation, and thermal activation of monomers. The expression for Rt will be different in each case and inserting it into the same equation (6.23) will yield the corresponding expression for the rate of polymerization. 

In spite of the high dissociation energy ( 290 kJ/mol) of the C-N bond, AIBN undergoes facile dissociation because it leads to the formation of very stable nitrogen gas. This initiator has a 10-h ti/2 at 64°C. 

The temperature of an initiator depends on the rate of decomposition as re ected in its half-life. A good rule-of-thumb in this regard is a tyz of about 10 h at the particular reaction temperature (Table 6.2). The practical use temperature ranges of some common initiators are diacetyl peroxide 70-90°C, dibenzoyl peroxide 75-95°C, dicnmyl peroxide 120-140°C, and AIBN 50-70°C (Odian, 1991). 

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Free radical polymerization is the most widely used process in PVDF synthesis. It involves the reaction of VDF and other comonomers with active center followed by successive addition of monomer(s) under the condition in which monomers cannot react with each other without intervention of the active center. Active centers are generated by thermal decomposition of initiator and in some cases by photoinitiation of the catalyst. The average lifetime of each active center (free radical) is approximately few seconds depending on the degree of polymerization and the initiator concentration. For successful polymerization, the sequence of reaction must take place. 

For polymerization initiated through radicals, generated by thermal decomposition of initiator, Eq. (6.26) may be combined with Eq. (6.124) to give an alternative expression for v ... 

In some cases, non-covalent adducts between functional monomer and template are too unstable to be used at higher temperatures, and the polymerization must be carried out at lower temperatures. Under these conditions, the thermal decomposition of initiator cannot be used to initiate the polymerization, and the initiators are decomposed with UV-light irradiation (photo-initiation never requires high temperatures). If the monomers themselves absorb UV light sufficiently, the polymerization is initiated even in the absence of any radical initiators. 

Thermal decomposition of initiator molecules produces pairs of radicals which are very likely to recombine when produced within the small volume of a latex particle or of monomer solubilized within a micelle. But if one radical escapes to the aqueous phase, a single radical is left in an isolated locus which is the prerequisite for emulsion polymerization. This still seems the most probable reason... 

EMULSION POLYMERIZATION Used for standard SBR. Monomer is emulsified in water with emulsifying agents. Polymerization is initiated by either decomposition of a peroxide or a peroxydisulfate. Hot SBR is initiated by free radicals generated by thermal decomposition of initiators at 50°C or higher. Cold SBR is initiated by oxidation-reduction reactions (redox) at temperatures as low as —40°C. Stjrrene content normally is 23%. Copolymer is randomly distributed. Structure of butadiene contents is about 18% ds-1,4, 65% frans-1,4, and 15-20% vinyl. 

During radical pol3nnerization, the initiation rate depends on the way the initiator bonds with the surface. Chemisorption of initiator on the solid surface decreases the initiator decomposition rate, probably due to diminishing of the degree of freedom of initiator molecules which hinders diffusion separation of the radical pair. The restriction of mobility and recombination of radicals bound to the surface lowers the rate and effectiveness of pol3nnerization. The rate of thermal decomposition of initiators and their efficiency of initiation also depend... 

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