Mechanism of Emulsion Polymerisation

According to the theory of Smith and Ewart [4] of the kinetics of emulsion polymerisation, the rate of propagation Rp is related to the number of particles N formed in a reaction by the equation. 

According to Equation (17.1), the rate of polymerisation and the number of particles are directly related to each other that is, an increase in the number of particles will increase the rate. This has been found for many polymerisations. 

Most reports on emulsion polymerisation have been limited to commercially available surfactants which, in many cases, are relatively simple molecules such as sodium dodecyl sulphate and simple nonionic surfactants. However, studies on the effects of surfactant structure on latex formation have revealed the importance of the structure of the molecule. Block and graft copolymers (polymeric surfactants) are expected to be better stabilisers when compared to simple surfactants. The use of these polymeric surfactants in emulsion polymerisation and the stabilisation of the resulting polymer particles is discussed below. 

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Gardon, J.L., "Mechanism of Emulsion Polymerisation", AIChE Symp., May 1969... 

The mechanism of emulsion polymerisation is complex. The basic theory is that originally proposed by Harkins21. Monomer is distributed throughout the emulsion system (a) as stabilised emulsion droplets, (b) dissolved to a small extent in the aqueous phase and (c) solubilised in soap micelles (see page 89). The micellar environment appears to be the most favourable for the initiation of polymerisation. The emulsion droplets of monomer appear to act mainly as reservoirs to supply material to the polymerisation sites by diffusion through the aqueous phase. As the micelles grow, they adsorb free emulsifier from solution, and eventually from the surface of the emulsion droplets. The emulsifier thus serves to stabilise the polymer particles. This theory accounts for the observation that the rate of polymerisation and the number of polymer particles finally produced depend largely on the emulsifier concentration, and that the number of polymer particles may far exceed the number of monomer droplets initially present. 

Several mechanisms have been proposed to explain the mechanism of emulsion polymerisation, but no single mechanism can explain aU of the happenings. Barrett and Thomas [11] suggested that particles are formed in emulsion polymerisation by two main steps ... 

The core of a micelle has been shown to have properties akin to a liquid hydrocarbon. One piece of evidence for this is that a surfactant solution above the c.m.c. is capable of taking up substantial quantities of non-polar organic (lipophilic) substances. These enter the core of the micelle, which can now swell because the added material has no hydrophilic moiety which needs to he on the surface (Figure 11.8). Solubilisation plays an important role in detergency and in the detailed mechanism of emulsion polymerisation. 

Clearly, an objective of commercial production is to obtain a stable latex which has as high a solids content as remains consistent with producing the required balance of pol3naer properties after drying. The colloidal stability of latices is enhanced by the addition of surfactants. Latices prepared by emulsion pol3mierisation techniques usually have solids content of 40-45%. The kinetics and mechanism of emulsion polymerisation of VCM have been extensively reviewed by Ugelstad et al (9). 

The precise mechanism of emulsion polymerisation remains a matter for debate. Whilst theoretical studies have given rise to the classical theories of Harkins and Smith-Ewait, commercial polymerisations do not always behave as those theories predict. There are a number of reasons for this. For example the water solubility of various monomers has a profound effect on the mechanism by which the monomer undergoes polymerisation, any one of several options being possible. 

Emulsion Polymerization. Emulsion SBR was commercialised and produced in quantity while the theory of the mechanism was being debated. Harkins was among the earliest researchers to describe the mechanism (16) others were Mark (17) and Elory (18). The theory of emulsion polymerisation kinetics by Smith and Ewart is still vaUd, for the most part, within the framework of monomers of limited solubiUty (19). There is general agreement in the modem theory of emulsion polymerisation that the process proceeds in three distinct phases, as elucidated by Harkins (20) nucleation (initiation), growth (propagation), and completion (termination). 

The mechanism of dispersion polymerisation has been discussed in detail in the book edited by Barrett [11]. A distinct difference between emulsion and dispersion polymerisation may be considered in terms of the rate of reaction. As mentioned above, with emulsion polymerisation the rate of reaction depends on the number of particles formed. However, with dispersion polymerisation, the rate is independent of the number of particles formed. This is to be expected, since in the latter case polymerisation initially occurs in the continuous phase, whereby both monomer and initiator are soluble, and the continuation of polymerisation after precipitation is questionable. Although in emulsion polymerisation the initial monomer initiation reaction also occurs in the continuous medium, the particles formed become swollen with the monomer and polymerisation may continue in these particles. A comparison of the rate of reaction for dispersion and solution polymerisation showed a much faster rate for the former process [11]. 

Like the theory of emulsion polymerisation, there are many differing theories about the mechanism of film formation. The reader is advised to consult specialist works for more details. 

Emulsion polymerisation represents the next stage in development from the suspension technique and is a versatile and widely used method of polymerisation. In this technique droplets of monomer are dispersed in water with the aid of an emulsifying agent, usually a synthetic detergent. The detergent forms small micelles 10-100 /im in size, which is much smaller than the droplets that can be formed by mechanical agitation in suspension polymerisation. These micelles contain a small quantity of monomer, the rest of the monomer being suspended in the water without the aid of any surfactant. 

A few studies have reported the embedding of an MIP film between two membranes as a strategy for the construction of composite membranes. For example, a metal ion-selective membrane composed of a Zn(II)-imprinted film between two layers of a porous support material was reported [253]. The imprinted membrane was prepared by surface water-in-oil emulsion polymerisation of divinylbenzene as polymer matrix with 1,12-dodecanediol-0,0 -diphenylphosphonic acid as functional host molecule for Zn(II) binding in the presence of acrylonitrile-butadiene rubber as reinforcing material and L-glutamic acid dioleylester ribitol as emulsion stabiliser. By using the acrylonitrile-butadiene rubber in the polymer matrix and the porous support PTFE, an improvement of the flexibility and the mechanical strength has been obtained for this membrane. 

The particles are heterogeneous by definition. As with singlepolymer particles, heterogeneities in density can arise as a result of the mechanism of particle formation. Usually a polymer has a different density to the liquid monomer from which it is derived, and in most emulsion polymerisation processes to produce reasonably concentrated dispersions, propagation is dominated by arrival at the particle surface of oligomeric radicals which can lead to non-homogeneous shrinking within the particle. 

One of the main drawbacks to the commercial development of multiple emulsions is their inherent instability. The intention of this paper is to review studies on the stability and mechanism of breakdown of multiple systems and attempts to minimise such instability, for example, by appropriate choice of surfactant, polymerisable surfactants or gelation of the aqueous or oily phases. 

There is an enormous range of industrial applications. For the main part, latexes are prepared by emulsion polymerisation. The process was developed industrially during the last World War as a way of replacing natural rubbers. Much fundamental and applied research has gone into this area. The relevant reaction mechanisms and physical processes have been quite well understood, although a few minor points of controversy are still discussed in the literature [6.4],... 

Before describing the polymerisation mechanism in microemulsions, it will be useful to recall the main features for the emulsion case, and also the principle of radical polymerisation [6.8]. In general, radical polymerisation has four steps initiation, propagation, transfer and termination. These are represented in relations (6.2-6.5). 

Most emulsion polymerisations are free radical processes (318). There are several steps in the free radical polymerisation mechanism initiation (324), propagation and termination (324, 377, 399). In the first step, an initiator compound generates free radicals by thermal decomposition. The initiator decomposition rate is described by an Arrhenius-type equation containing a decomposition constant ( j) that is the reciprocal of the initiator half-life (Ph). The free radicals initiate polymerisation by reaction with a proximate monomer molecule. This event is the start of a new polymer chain. Because initiator molecules constantly decompose to form radicals, new polymer chains are also constantly formed. The initiated monomeric molecules contain an active free radical end group. 

Surfactant keeps emulsion droplets and latex particles colloidally stable against coalescence/aggregation. The surfactant plays another important role in emulsion polymerisation besides stabilisation. Surfactant is critically involved in the nucleation mechanism (i.e., how the particles are formed) of the polymer latex particles (418,419). The amount of surfactant used is critical in controlling the latex particle size distribution. As surfactant is added to an emulsion, some remains dissolved in the aqueous phase, and some adsorbs onto the surface of the emulsion droplets according to an adsorption isotherm (e.g., Langmuir, Freundhch, or Frumkin adsorption isotherms) (173). 

The primary distinction between miniemulsion and conventional emulsion polymerisation is the nucleation mechanism. In miniemulsion polymerisation (104,132, 193, 361) radicals from the water phase enter the dispersed monomer droplets directly to initiate polymerisation (i.e., the droplets act as individual reactors). This nucleation mechanism is referred to as droplet nucleation. Because of the small size and large surface area of the miniemulsion droplets, they are competitive for radicals relative to the homogeneous and micellar nucleation mechanisms. The monomer droplets polymerise to become polymer particles (275). Miniemulsion latex particles are typically prepared in the size range of 50 to 500 nm in diameter. 

Surfmers , i.e. surfactants which also acted as copolymerisable monomers, were synthesised from the hemi-ester of a fatty alcohol and maleic anhydride and were then used in the preparation of self-crosslinking dispersions by seeded semi-continuous emulsion polymerisation of acrylate monomers. Water-borne exterior wood stains were prepared from the dispersions and their properties were studied. The use of surfmers as sole emulsifiers in emulsion polymerisation was considered and data were obtained on the effects of surfmers on film formation, water barrier properties, gloss retention and mechanical properties. Environmental aspects of the use of products involving surfmers were examined. 6 refs. 

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