Emulsion chain termination

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Emulsion polymerisation is a special case of heterogeneous addition polymerisation in which the reaction kinetics are modified because the A are compartmentalised in small polymer particles [48, 49]. These particles are usually dispersed in water and reaction (78) occurs in the aqueous phase. Initiating radicals diffuse to the particles which are stabilised by surfactant material. Chain termination becomes retarded physically and a relatively high polymerisation rate is obtained. If chain transfer is not prominent, a high molecular weight polymer is produced. The polymerisation rate is given by the expression... 

The polyazophenylene units are formed from the polyrecombination of the decomposition products from bis(nitrosoacetyl)benzidine. Chain termination can occur by disproportionation of the polymer radicals and by recombination with acetoxy radicals. Despite the rate constant for the recombination of the phenyl and azophenyl radicals being much larger than that of the initiation reaction for isoprene, it is possible to synthesise copolymers from these materials by a careful choice of the various reaction parameters. However, block copolymers could only be obtained using emulsion techniques (see Table 4.11) and not in bulk or in solution. 

The acrylic acid and aciylonitrile are fed into the reactor containing the bal i diene emulsion and the chain termination is achieved by adding a mercaptan when the molecular weight reaches 2000-4000. Al lei that an aniioxidant is added, the polymer puriried by washing with water and vacuum drying. 

The thermal process is perhaps the most universally applicable of all the phase inversion processes because it can be utilized over the widest range of both polar and nonpolar polymers. However, its commercial use for membrane applications will probably be restricted to polyolefins, particularly polypropylene. A large number of the substances can function as latent solvents (Table X). They usually consist of one or two hydrocarbon chains terminated by a polar hydrophilic end group. Therefore, they exhibit surface activity which may explain their ability to form the emulsion-like Sol 2 micelles at elevated temperatures. One latent solvent which is worthy of special mention because of its broad applicability is N-Tallowdiethanolamlne (TDEA). 

In treatments of polymerization reactions that concentrated on a single feature, the effect of molecular weight upon the termination rate constant has been deduced, the relative rates of initiation of two monomers in a copolymerization have been assessed, constants for chain transfer to monomer have been obtained in an emulsion copolymerization, the relative amounts of chain termination by combination and disproportionation have been discovered from a molecular weight distribution, and the rate constant for long-chain branch formation in the free-radical polymerization of ethylene has been found by fitting a probalistic model. ... 

Kinetic studies on the radiation-induced polymerization and post-polymerization of TFE were carried out using chlorofluorohydrocarbons as solvents. The remarkable postpolymerization is again explained by the unusually slow rate of the bimolecular chain termination. A chain transfer reaction was also discussed by Hisasue et al. [721]. Suwa et al. [679] discussed the emulsion polymerization of TFE by radiation with ammonium perfluorooctanoate (FC-143) as the emulsifier. The polymerization rate is proportional to the 0.8 power of the dose rate and is almost independent of the emulsifier concentration (up to 2wt% in water). Molecular weights between 10 and 10 were observed, which increases with reaction time but decreases with the emulsifier concentration. In general, the molecular weight of PTFE prepared by radiation-induced polymerization in solution and in emulsion is relatively low compared with commercial PTFE. However, it is also possible to produce molecular weight of up to 3 x 10 if an emulsifier-free polymerization are carried out [677,678,723]. 

Tertiary butanol is preferred because of its low chain-terminating properties. Three different methods for the production of ethylene vinyl acetate copolymers are known, these being high-pressure process (0%-45% vinyl acetate) low-pressure emulsion process (55%-100% vinyl acetate) and medium-pressure process in solution (30%-100% vinyl acetate). 

The completion stage is identified by the fact that all the monomer has diffused into the growing polymer particles (disappearance of the monomer droplet) and reaction rate drops off precipitously. Because the free radicals that now initiate polymerization in the monomer-swollen latex particle can more readily attack unsaturation of polymer chains, the onset of gel is also characteristic of this third stage. To maintain desirable physical properties of the polymer formed, emulsion SBR is usually terminated just before or at the onset of this stage. 

Copolymers with butadiene, ie, those containing at least 60 wt % butadiene, are an important family of mbbers. In addition to synthetic mbber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers are made in an emulsion polymerization system, which favors chain propagation but not termination (199). The result is economically acceptable rates with desirable chain lengths. Usually such processes are mn batchwise in order to achieve satisfactory particle size distribution. 

It is appropriate to mention here an alternative method for synthesizing monodispersed polymers which was developed by Zimm.67 Emulsion polymerization is initiated by flash photolysis. The second flash terminates the polymers initiated by the first one, starting on a new chain to be terminated by the third flash, and... 

Many emulsion polymerizations can be described by so-called zero-one kinetics. These systems are characterized by particle sizes that are sufficiently small dial entry of a radical into a particle already containing a propagating radical always causes instantaneous termination. Thus, a particle may contain either zero or one propagating radical. The value of n will usually be less than 0.4. In these systems, radical-radical termination is by definition not rate determining. Rates of polymerization are determined by the rates or particle entry and exit rather than by rates of initiation and termination. The main mechanism for exit is thought to be chain transfer to monomer. It follows that radical-radical termination, when it occurs in the particle phase, will usually be between a short species (one that lias just entered) and a long species. 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

Some of the issues associated with RAFT emulsion polymerization have been attributed to an effect of chain length-dependent termination.528 In conventional emulsion polymerization, most termination is between a long radical and a short radical. For RAFT polymerization at low conversion most chains are short thus the rate of termination is enhanced. Conversely, at high conversion most chains are long and the rate of termination is reduced. 

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