Free-Radical Polymerization Homogeneous Systems

Cationic polymerizations differ from free-radical and homogeneous anionic syntheses of high polymers in that the cationic systems have not so far been fitted into a generally useful kinetic framework involving fundamental reactions like initiation, propagation, and so on. To explain the reasons for the peculiar problems with cationic polymerizations we will, however, postulate a conventional polymerization reaction scheme and show where its inherent assumptions are questionable in cationic systems. 

In summary, cationic polymerizations are much more variable and complex than homogeneous free-radical or anionic chain-growth polymerizations. No convincing general mechanism has been provided for cationic reactions, and each polymerization system is best considered as a separate case. 

The data in Table 2.4 provide evidence that the slow rates and low molecular weights obtained in homogeneous free radical polymerization of these dienes are not due to a low rate constant for propagation but rather must be caused by a high rate constant for termination (as indicated in Table 2.1) (Matheson et al., 1949,1951 Morton and Gibbs, 1963). Hence, under the special conditions of emulsion polymerizations, where the termination rate is controlled by the rate of entry of radicals into particles, it becomes possible to attain both faster rates and higher molecular weights. It is this phenomenon which led to the rise of the emulsion polymerization system for the production of diene-based synthetic rubbers. 

Table 4.1 lists the range of concentration and rate coefficient values typically encountered in homogeneous free-radical polymerization systems at low conversion. These can be combined with Eqs. (9)-(12) to illustrate the tradeoffs involved be-... 

The rate of termination reaction is slower than that observed in the homogenous bulk or solution polymerization since the limited number of free radicals exists in the polymerization loci having a reasonably small volume (i.e., monomer swollen forming latex particle). Higher degree of polymerizations can be achieved in an emulsion system relative to the homogenous polymerization due to the existence of this limitation. 

Free radical copolymerizations of the alkyl methacrylates were carried out in toluene at 60°C with 0.1 weight percent (based on monomer) AIBN initiator, while the styrenic systems were polymerized in cyclohexane. The solvent choices were primarily based on systems which would be homogeneous but also show low chain transfer constants. Methacrylate polymerizations were carried out at 20 weight percent solids... 

A simple mathematical description of the postgel stage will be presented for stepwise and free-radical chainwise polymerizations (in this case, the description will be limited to the range of low concentrations of the polyfunctional monomer leading to a homogeneous system). Calculations will be restricted to the evolution of sol and gel fractions, the mass fractions of pendant and elastic chains, and the concentration of crosslinks and EANC as a function of conversion. 

Lanthanides in homogeneous systems As organometallics As cerium(IV) salts As coordination complexes As nitrates, chlorides, alkoxides etc. For olefin polymerization For olefin hydrogenation For free radical polymerization For Diels-Alder reactions For olefin polymerization In organic synthesis... 

Claverie et al. [325] have polymerized norbornene via ROMP using a conventional emulsion polymerization route. In this case the catalyst was water-soluble. Particle nucleation was found to be primarily via homogenous nuclea-tion, and each particle in the final latex was made up of an agglomeration of smaller particles. This is probably due to the fact that, unlike in free radical polymerization with water-soluble initiators, the catalyst never entered the polymer particle. Homogeneous nucleation can lead to a less controllable process than droplet nucleation (miniemulsion polymerization). This system would not work for less strained monomers, and so, in order to use a more active (and strongly hydrophobic) catalyst, Claverie employed a modified miniemulsion process. The hydrophobic catalyst was dissolved in toluene, and subsequently, a miniemulsion was created. Monomer was added to swell the toluene droplets. Reaction rates and monomer conversion were low, presumably because of the proximity of the catalyst to the aqueous phase due to the small droplet size. 

Initiators are not used efficiently in free-radical polymerizations. A significant proportion of the primary radicals that are generated are not captured by monomers, and the initiator efficiency / in Eq. (6-10) is normally in the range 0.2-0.7 for most initiators in homogeneous reaction systems. It will be lower yet in polymerizations in which the initiator may not be very well dispersed. 

Early work indicated that the nature of the reaction medium had no effect on the course of free radical copolymerizations in homogeneous reaction systems. More recent studies have not always supported this conclusion and it has been suggested that a bootstrap effect may be operating whereby there is a partitioning of the comonomers between the bulk of the reaction medium and the polymerization locus (i.e., the macroradical end) [29]. 

The discussion of free-radical polymerizations in Chapters 6 and 7 focused primarily on homogeneous reaction systems, in which monomer, polymer, and any solvent were all miscible. This conventional presentation makes it much easier to grasp the fundamentals of free-radical polymerizations. In fact, however, many large-scale processes are carried out in heterogeneous systems, because these offer advantages over alternative procedures. Their overall importance is such as to justify this chapter describing the effects of process conditions on polymer properties. 

The free-radical kinetics described in Chapter 6 hold for homogeneous systems. They will prevail in well-stirred bulk or solution polymerizations or in suspension polymerizations if the polymer is soluble in its monomer. Polystyrene suspension polymerization is an important commercial example of this reaction type. Suspension polymerizations of vinyl ehloride and of acrylonitrile are described by somewhat different kinetic schemes because the polymers precipitate in these cases. Emulsion polymerizations aie controlled by still different reaetion parameters because the growing macroradicals are isolated in small volume elements and because the free radieals which initiate the polymerization process are generated in the aqueous phase. The emulsion process is now used to make large tonnages of styrene-butadiene rubber (SBR), latex paints and adhesives, PVC paste polymers, and other produets. 

The model for the reaction system will be considered in detail in Section II. However, it is convenient to note here that, in principle, the free radicals that initiate the polymerization may be generated cither within the external phase (external initiation) or within the reaction loci themselves (internal initiation). Whereas very brief reference will be made at the conclusion of this chapter to reaction systems of the latter type, the concern here will be almost exclusively with reaction systems of the former type. Insofar as the initiating radicals are generated exclusively within the external phase (and therefore have to be by some means acquired by the loci by absorption from the external phase), we have a farther important distinction between homogeneous and compartmentalized reactions. In the latter case, the processes that lead to the generation of the initiatit radicals are physically isolated from the propagation, termination, and transfer reactions. One minor consequence of this is that transfer-to-initiator reactions may be virtually eliminated in the latter case. 

The theory also has relevance to the so-called seeded " emulsion polymerization reactioas- In these reactions, polymerization is initial in the presence of a seed latex under conditions such that new particles are unlikely to form. The loci for the compartmentalized free-radical polymerization that occurs are therefore provided principally by the particles of the initial seed latex. Such reactions are of interest for the preparation of latices whose particles have, for instance, a core-shell" structure. They are also of great interest for investigating the fondamentals of compartmentalized free-radical polymerization processes. In this latter connection it is important to note that, in principle, measurements of conversion as a function of time during nonsteady-state polymerizations in seeded systems offer the possibility of access to certain fundamental properties of reaction systems not otherwise available. As in the case of free-radical polymerization reactions that occur in homogeneous media, investigation of the reaction during the nonsteady state can provide information of a fundamental nature not available through measurements made on the same reaction system in the steady state. 

In a few instances, poly(methylmethacrylate) has been prepared exceeding the syndiotactic content attainable through a free ion pair or free radical mechanism at the same temperature [20]. A possible mechanism for homogeneous syndiotactic propagation has been proposed. However, none of these highly syndiotactic systems has been reproducible [10], and it appears to be no real need for such a mechanism. Coordination-directed stereospecific polymerization of methyl methacrylate seems to be limited to isotactic propagation. 

In aqueous solution polymerization a water-soluble monomer is polymerized to a water-soluble polymer. For example, polyacryUc acid is produced on a large scale by free radical techniques in this manner [10]. Again, fhe non-flammabiUty and high heat capacity of water are advantageous. Such polymerizations in homogeneous solution can, in some cases, offer better molecular weight control by comparison to polymerization in a multiphase system. 

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