Polymerization, degree emulsion

It can be shown that the degree of polymerization in emulsion polymerization can be derived from general polymerization kinetics [Eq. (3.79)] by neglecting chain transfer ... 

Polymerization of emulsion SBR is started by free radicals generated by the redox system in cold SBR and by persulfate or other initiator in hot SBR. The initiators are not involved in the molecular structure of the polymers. Almost all molecules are terminated by fragments of the chain transfer agent (a mercaptan). Schematically, the molecules are RSM H. where RS is the C H S pan of a dodccyl mercaptan molecule M is the monomer involved n is the degree of polymerization, and H is a hydrogen atom formerly attached to the sulfur of a mercaptan. In the case of free-radical-initiated polymerization of butadiene, by itself to form homopolymers or with other monomers for fonn copolymers, the butadiene will be about 18% 16% fix-1.4 and 66% trms-1,4-... 

In general, for most investigations, y radiation has been used because of its high degree of penetration and the ccanparative ense of estimating dose-depth characteristics and because radical fluxes comparable to those used with chemical initiation can easily be achieved- There have also been a few, comparatively brief, studies using electron accelerators to initiate emulsion polymerization in emulsion. These have mainly been conducted in Japanese laboratories. 

These relations for rate and degree of polymerization, although based on a relatively simple chemical picture, agree well with experimental data obtained in solution and bulk polymerizations. In emulsion polymerization, however, a complication arises from the fact that the volume of the reaction mixture is subdivided into a very large number of very small volume elements, the particles, which are suspended in water. These particles are so small that they can accommodate only a limited number of polymer radicals at any one time. The peculiarities of emulsion polymerization stem from this limitation. Polymer radicals, being insoluble in water, are confined to the particle in which they are generated. Thus, a radical in one particle cannot be terminated by a radical in another particle. Therefore, the rate of termination in emulsion is much lower than that given by Equation 1. 

Polyaddition and polycondensation reactions usually lead to functional polymers, since the polymers produced are terminated with reactive functional groups. A higher degree of functionality is easily affordable if monomers with additional reactive groups are used that do not participate in the step-growth polymerization. In emulsion polymerizations, neither polyaddition nor polycondensation reactions can be carried out consequently, the miniemulsion technique is of special interest as no diffusion of the monomers takes place. The first polyaddition in miniemulsion were performed in 2000, with the reaction of polyepoxides and hydrophobic diamines, bisphenols, and dimercaptanes [105]. Stable latexes of epoxy resins could be obtained, and apparent molecular weights of up to 20 000 g mol were measured. 

By contrast to other polymerization methods, emulsion polymerization offers a series of technological advantages. The temperature can easily be maintained constant by the water. The use of redox initiators means that polymerizations at fast rates are possible even at relatively low temperatures. The degrees of polymerization can be made quite high also, the unpolymerized monomer can be removed relatively easily by steam distillation. 

The molecular structure of this polymer corresponds to the maximal steric hindrance for a polymer. The rigidity of the chains is increased compared to that of PTFE and, in spite of the atacticity of the polymer obtained by radical polymerization in emulsion or suspension, PCTFE is highly crystalline. Its melting zone is about 211-216°C. Its degree of crystallinity is approximately 50%. Its glass transition temperature is 85°C due to the high rigidity of its chains. 

Latex-modified concrete is conventional Portland cement concrete with the addition of a polymeric latex emulsion. The water of suspension in the emulsion hydrates the cement and the polymer provides supplementary binding properties to produce a concrete with a low water-cement ratio, good durability, good bonding characteristics and a high degree of resistance to penetration by chloride ions, all of which are desirable properties in a concrete overlay. 

The aqueous phase into which the monomer mix is dispersed is also prepared in a separate tank before transferring to the copolymerization ketde. It contains a catalyst, such as benzoyl peroxide [94-36-0], to initiate and sustain the polymerization reaction, and chemicals that aid in stabilizing the emulsion after the desired degree of dispersion is achieved. Careful adherence to predeterrnined reaction time and temperature profiles for each copolymer formulation is necessary to assure good physical durabiHty of the final ion-exchange product. 

Emulsion polymerizations of vinyl acetate in the presence of ethylene oxide- or propylene oxide-based surfactants and protective coUoids also are characterized by the formation of graft copolymers of vinyl acetate on these materials. This was also observed in mixed systems of hydroxyethyl cellulose and nonylphenol ethoxylates. The oxyethylene chain groups supply the specific site of transfer (111). The concentration of insoluble (grafted) polymer decreases with increase in surfactant ratio, and (max) is observed at an ethoxylation degree of 8 (112). 

Distribution of the monomer units in the polymer is dictated by the reactivity ratios of the two monomers. In emulsion polymerization, which is the only commercially significant process, reactivity ratios have been reported (4). IfMj = butadiene andM2 = acrylonitrile, then = 0.28, and r2 =0.02 at 5°C. At 50°C, Tj = 0.42 and = 0.04. As would be expected for a combination where = near zero, this monomer pair has a strong tendency toward alternation. The degree of alternation of the two monomers increases as the composition of the polymer approaches the 50/50 molar ratio that alternation dictates (5,6). Another complicating factor in defining chemical stmcture is the fact that butadiene can enter the polymer chains in the cis (1), trans (2), or vinyl(l,2) (3) configuration ... 

During emulsion polymerization, a high conversion of monomer to polymer produces cross-linked rubber which is insoluble. To obtain a high conversion in the polymerization reaction and a processable polymer, suitable polymer modification should be made. The use of sulphur moieties allows this goal to be reached [2]. Sulphur-modified polychloroprenes contain di- and polysulphide sequences in the polymer chains. After the polymerization reaches the desired degree, reaction is stopped by adding thiuram disulphide ... 

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. 

All these effects increase the overall polymerization rate and decrease the degree of polymerization. The effect of polymerization temperature on the variation of monomer conversion with the polymerization time is exemplified in Fig. 8 for the emulsion polymerization of styrene. 

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

The degree of dissociation is very small but the diphenylcyanomethyl radical is sufficiently reactive to induce polymerization in styrene. Methyl radicals or hydrogen atoms bring about polymerization of vinyl monomers in the gas phase.Hydrogen peroxide in the presence of ferrous ions initiates polymerization in the aqueous phase or in aqueous emulsions through generation of hydroxyl radicals according to the Haber-Weiss mechanism... 

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