Emulsion thermal initiation

Fig. 7. Time-conversion curves of thermally initiated emulsion polymerization of 1,4-DVB at 0.1 (I) 0.65 (II) and 0.85 (III) M SDS concentrations. Polymerization temperature = 90 °C water/monomer volume ratio = 12.5. [Reproduced from Ref.84 with permission,Hiithig Wepf Publ., Zug, Switzerland].

For the thermally initiated (potassium persulphate) emulsion polymerisation of styrene, we have observed [73] a twofold increase in the initial polymerisation rate in the presence of ultrasound (20 kHz), the increase being dependent upon the level of surfactant employed. Several workers have suggested that possible explanations for the observed increase in rate are ... 

Recently Biggs [74] has investigated the emulsion polymerisation of styrene using ultrasonic irradiation as the initiation source (i. e. in the absence of a chemical initiator). Similar to Lorimer and Mason using a thermally initiated system, Biggs found both a marked increase in monomer conversion rate as a function of time as the ultrasonic intensity was increased but remarkable constancy in the resultant latex particle... 

The thermal initiator system. This system is made up of water-soluble materials that produce free radicals at a certain temperature to initiate polymerization. The most commonly used i materials for such thermal emulsion polymerizations are potassium persulfate, sodium persulfate, or ammonium persulfate. 

The latexes were prepared using a conventional semi-batch emulsion polymerization system modified for power-feed by the addition of a second monomer tank. Polymerization temperatures ranged from 30-85°C using either redox or thermal initiators. Samples were taken periodically during the polymerization and analyzed to determine residual monomer in order to assure a "starved-feed" condition. As used in this study this is a condition in which monomer feed rate and polymerization rate are identical and residual monomer levels are less than 5%. 

Catalysis of Thermal Initiation of Styrene Emulsion Polymerization by Emulsifiers... 

The mechanism by which emulsifiers could influence the rate of the thermal initiation reaction is obscure. Most probably the emulsifiers increase the efficiency with which one of the radicals produced in the thermal initiation process escapes into the aqueous phase so that emulsion polymerization may begin. If so those emulsifiers for which exchange between the micelle or the adsorbed layer on a latex particle and true solution in the aqueous phase is most rapid should be most effective in promoting the thermal polymerization. Recently the kinetics of micellization has attracted much attention (29) but the data which is available is inadequate to show whether such a trend exists. 

It would be necessary to determine the activation energy for thermal initiation in emulsion with two emulsifiers of similar structure selected to have exchange rates which differed as much as possible. 

If styrene is polymerized at SO C in emulsion, the initiating radicals are essentially produced by the decomposition of the persulfate to radical anions. In case of monomers like 1,4-DVB, however, additional radicals in substantial amounts are formed by thermal initiation. Therefore the radical formation is significantly higher, more micelles are initiated and consequently more but smaller particles are formed. 

The possibility of a thermal initiation respectively polymerization of 1,4-DVB at 50 u in emulsion without K S,0q or other initiators has been established. 2 2 8... 

From the discussion above, it is clear that there is no evidence for catalysis of persulfate initiation in emulsion polymerization systems. However, many ionic reactions have been shown to be subject to large catalytic effects in the presence of emulsifier micelles (Fendler and Fendler, 1975) so that the question arises as to whether there are any radical reactions that are subject to micellar catalysis and whether this phenomenon plays any part in any emulsion polymerization systems, Prima fade evidence that uiicellar catalysis may be important when emulsified monomer is allowed to polymerize thermally is provided by the work of Asahara et al. (1970, 1973) who find that several emulsifiers decrease the energy of activation for thermal initiation of alkyl methacrylate and styrene, [n particular, the energy of activation for thermal initiation of styrene emulsified with sodium tetrapropylene benzene solfonate was reported as S3 kl mol. much lower than any value determined in bulk. Hui and Hamielec s value of ] IS kj tnol (1972) seems to be representative of the data available on thermal initiation in bulk. The ctmclusions of Asahara et al. are based on observations of the temperature dependence of the degree of polymerization and are open to several objections. 

S6). It depended on the variation of the number of latex particles formed iV with temperature. Unfortunately, they have overlooked the fact that the particle growth rate fi which appears to the power —f in the Smith-Ewart expression for the number of latex particles formed coitains the propa gation rate constant which is temperature dependent. It has also recently been realized that another factor on which JV depends, the area occupied by a surfactant molecule at the polymer-water interface Og, is also temperature dependent- Dunn et al. (1981) observed that the temperature dependence of N in the thermal polymerization of styrene differed from different emulsifiers. It seems unlikely that the differences ran be wholly explained by differing enthalpies of adsorption of the emulsifiers and, if not, this observation implies that the energy of activation for thermal initiation of styrene in emulsion depends on the emulsifier used. Participation of emulsifiers in thermal initiation (and probsbly also in initiation by oil-soluble initiators) is most probably attributable to transfer to emulsifier and desorption of the emulsifier radical frcan the micelle x>r latex particle into the aqueous phase the rates of these processes are likely to differ with the emulsifier. 

Most emulsion polymerizations are, of course, carried out in conventional stirred reactors. There are procedures in which all of the monomer is charged at once to the reactor procedures wherein the monomer is gradually added, possibly with gradual addition of surfactant solutions, initiator solutions, and other modifying agents procedures using redox initiation or thermal initiation and procedures using preemulsified monomers. 

The technique of polymerizing emulsions of monomers to produce synthetic latexes which could be processed to produce rubbers similarly to natural rubber latex was developed in industrial laboratories in Germany, USA and the former Soviet Union fiom 1927 onwards. Thermally initiated polymerization of monomer dispersions had beat tried earlier, protective colloids were used to stabilize the monomer droplets these suspension polymerizations were very slow. 

Inverse emulsion polymerization can be thermally initiated with an oil-soluble or wato -soluble initiator. Most of the studies have dealt with oO-soluble initiators such as the dinitrile of azobisisobutyric acid or benzoyl peroxide or with a water-soluble initiator such as potassium posulfate. 

In the case of thermal initiation of styrene [79,80], the polymerization rate was found to be proportional to [AIBN] and [KPS] , in good agreement with other data for three- or four-component microemulsions [66,81]. The dependence on AIBN concentration is consistent with the prediction of 0.40 based on the micellar nucleation theory in emulsion polymerization (Smith-Ewart case 2) (see, e.g.. Ref 129). The dependence on KPS concentration lies between this case and the value of 0.5 for solution or bulk polymerization. 

It is therefore not surprising that the early investigators saw no promise in this mechanism of polymerization of butadiene, isoprene, etc., either by pure thermal initiation or by the use of free radical initiators, such as the peroxides. Instead they turned to sodium polymerization, which, although also rather slow and difficult to reproduce, at least yielded high-molecular-weight rubbery polymers from the dienes. Later, in the 1930s, when emulsion polymerization was introduced, it was found that this system, even though it involves the free... 

The most common method used in emulsion pol)nnerizations is thermal initiation in which the initiator (7) dissociates homolyticaUy to generate a pair of free-radicals (R ) as shown below ... 

Aqueous miniemulsion polymerization of styrene was performed in the presence of CeFis-I as CTA, yielding particles with a good control of the molecular weights, in contrast to emulsion polymerization where the transfer agent efficiency was low due to a slow diffusion of the hydrophobic perfluor-ohexyl iodide from the monomer droplets to the active particles during polymerization.The chains were capped with iodine as evidenced by the successful chain extension upon addition of butyl acrylate. The miniemulsion process was also successfully applied to the preparation of triblock copolymers PS-b-PDMS-b-PS starting from a telechelic diiodo-poly(dimethylsiloxane) macrotransfer agent. A somewhat similar procedure was used to prepare PVAc-b-PDMS- -PVAc triblock copolymers, but the polymerization was performed under UV irradiation (instead of thermal initiation) and in the absence of radical initiator. In this case, the aqueous dispersion medium was a key parameter to achieve a controlled polymerization (Scheme 19). ... 

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