Pressure effects radical polymerization

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

Sonication, the application of high-intensity ultrasound at frequencies beyond the range of human hearing (16 kHz), of a monomer results in radical polymerization. Initiation results from the effects of cavitation—the formation and collapse of cavities in the liquid. The collapse (implosion) of the cavities generates very high local temperatures and pressures. This results in the formation of excited states that leads to bond breakage and the formation of... 

A significant fraction, more than 25%, of the low-density polyethylene (LDPE) (Sec. 3-14a) produced by radical polymerization consists of various copolymers of ethylene. LDPE has come under increasing economic pressure in recent years because of a combination of factors [Doak, 1986]. High-density polyethylene (HDPE) has displaced LDPE in applications such as blow-molded bottles and thin films where the increased strength of HDPE is preferred over the clarity of LDPE. Linear low-density polyethylene (LLDPE) (Sec. 8-1 lc) competes effectively with LDPE in terms of both cost and properties. New producers of ethylene have entered the LDPE market because of a lack of alternatives for their feedstocks. Many LDPE producers use copolymerization as a strategy to obtain products more resistant to displacement by HDPE and LLDPE. 

Organic peroxides, which readily decompose into free radicals under the effect of thermal energy, are used under high pressures as initiators for radical polymerizations. The measurement of the influence of pressure on the rate of decomposition gives rise to the determination of the activation volume, which, in turn, allows conclusions to be drawn on the decomposition mechanism and the transition state. 

The authors [20] regard eqn. (28) as a semiempirical relation which is generally useful for predicting the effect of pressure on fct in all radical polymerizations. 

Derive the expression for the rate of a free radical polymerization. Using this expression, account for the Trommsdorff effect and the inability of ethylene to polymerize free radically at ordinary temperatures and pressures. 

If surfactant is added to a suspension polymerization system, a number of phenomena may occur. If the surfactant is added in small amounts (below the critical micelle concentration or CMC), the reduction in interfacial tension between the organic and aqueous phases will result in smaller monomer droplets, but it has hardly any other effect. If surfactant is added above the CMC, and an oil-soluble initiator is used, the process is commonly termed a microsuspension polymerization. Due to the reduced interfacial tension, the droplet diameter (and hence bead diameter) is reduced to approximately 10-40 pm. Little polymerization takes place in the aqueous phase or in particles generated from surfactant micelles because of the hydrophobic nature of the initiator. However, some smaller particles initiated from surfactant micelles may be found. The kinetics are still essentially those of a bulk free radical polymerization. Microsuspension polymerization is used to produce pressure-sensitive adhesives for repositionable notes. 

Another aspect of free radical polymerizations under pressure which has been recently studied is the effect of pressure on comonomer reactivity ratios (5). In two copolymerization systems—styrene-acrylonitrile and methyl methacrylate-acrylonitrile—it was found that the product of the reactivity ratios, rif2, approaches unity as the pressure is increased. The monomer-polymer composition curves for these two copolymerizations at 1 and 1000 atm. are illustrated in Figures 1 and 2. The effect of pressure on the individual reactivity ratios and on the fit2 product is given in Table II. 

In this article we describe the phase behavior of a microemulsion system chosen for the free radical polymerization of acrylamide within near-critical and supercritical alkane continuous phases. The effects of pressure, temperature, and composition on the phase behavior all influence the choice of operating parameters for the polymerization. These results not only provide a basis for subsequent polymerization studies, but also provide data on the properties of reverse micelles formed in supercritical fluids from nonionic surfactants. 

In139) theoretical and experimental data are examined for radical polymerization under reaction front propagation conditions, and in140 the impact of pressure and the gel effect on polymerization kinetics is considered also. In this case, a heat conductivity equation is added to the kinetic equations. 

The rapidity of the reaction can be seen by the large effect low pressures ( 1 torr) of oxygen can have on the free radical polymerization of a reactive olefin such as styrene [22]. The reaction rate coefficients are expected to be typical for exothermic radical—radical reactions with essentially no activation energy. Thus, if R is alkyl, log(feQ/l mole-1 s-1) would be 9.0 0.5, and be independent of temperature. For simple resonance-stabilized radicals, log(feD/l mole-1 s-1) would be 8.5 0.5. 

We have also studied the kinetics of free radical initiation in CO2 using azobis(isobutyronitrile) (AIBN) as an initiator [35]. These experiments were accomplished using high pressure UV spectroscopy, and illustrated that AIBN decomposes more slowly in CO2 than in traditional hydrocarbon solvents, yet the initiator efficiency is much greater in CO2 due to the reduced solvent cage effect in the low viscosity supercritical medium. The main conclusion drawn from this work was that CO2 can therefore be employed effectively as a solvent for free radical polymerizations and remains an inert solvent even in the presence of highly electrophilic hydrocarbon radicals. 

Impurities can also act as chain transfer agents. In some instances, as in the production of low density polyethylene via high pressure radical polymerization, impurities and/or the so-called inerts (methane, ethane, and propane), which come as impurities in the ethylene, are used as effective chain transfer agents to lower the MW of the polymer. 

The same author prepared alternating copolymers of a-olefins with sulfur dioxide since here the possibility of a hydrogen shift is evidently eliminated. These processes usually were initiated spontaneously when the components were stirred in sealed pressure bottles at room temperature. Sluggishly initiating systems were accelerated by the addition of a few drops of cumene hydroperoxide. These processes were free-radical polymerizations. The effect of cumene hydroperoxide on the initiation is interesting in view of the very low half life of this reagent at room temperature. The products from these polymerizations were optically active. 

Only a few measurements of the effect of pressure on the rates of radical polymerizations have been reported, but the general principles are well enough established that some information about mechanism can be obtained. The main experimental facts of many radical polymerizations can be accounted for in terms of the following reaction scheme (for a recent account see Bamford et al., 1958). 

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