Polymer emulsion free radical polymerization

The preparation of polymers through free radical polymerization can be carried out in several ways, both in laboratory and industrial scales. Among the techniques, bulk, suspension, emulsion and solution polymerization methods are of the most practical importance. 

Overall, it is clear that significant hydrolysis of silanes in waterborne formulations occurs when the hindered silanes are used as monomers in emulsion polymerization. Si NMR spectroscopy on latex solids also reveals that a significant amount of hindered vinyl silane monomer forms condensates with itself, so that not all of the vinyl groups are incorporated into the backbone of the growing polymer through free-radical polymerization. 

The production of polyolefins by means of coordination polymerization, which is the highest tonnage polymerization process, is discussed first. The following chapters present the production of polymers by free-radical polymerization in homogeneous, heterogeneous and dispersed (suspension and emulsion) media. Afterwards, the reaction engineering of step-growth polymerization is discussed. The last chapter is devoted to the control of polymerization reactors. 

The polymers produced through emulsion free radical polymerization are synthesized from modified alkenes (eg, styrene, methyl methacrylate, methyl acrylate, butyl acrylate, vinyl acetate) and are characterized by the presence of C—C bonds that bind the monomers used. As a result, the obtained materials can be biocompatible [as for poly(methyl methacrylate)], but they are not biodegradable [9]. 

The intrinsic nature of the emulsion free radical polymerization, starting from a monomer functionalized with a C—C double bond, leads to a final product in the form of a polymer latex. This polymer is more or less a stable dispersion of polymer... 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

Water is extensively used to produce emulsion polymers with a sodium stearate emulsifrer. The emulsion concentration should allow micelles of large surface areas to form. The micelles absorb the monomer molecules activated by an initiator (such as a sulfate ion radical 80 4 ). X-ray and light scattering techniques show that the micelles start to increase in size by absorbing the macromolecules. For example, in the free radical polymerization of styrene, the micelles increased to 250 times their original size. 

Free-radical polymerization of alkenes has been carried out in aqueous conditions.115 Aqueous emulsion and suspension polymerization is carried out today on a large scale by free-radical routes. Polymer latexes can be obtained as products (i.e., stable aqueous dispersions... 

Finally, addition polymerization of suitably substituted furans allows incorporation of the furan nucleus into heterocyclic polymers (77MH1102). 2-Vinylfuran apparently exhibits free radical polymerizability comparable with that of styrene, although rates, yields and degrees of polymerization are low under all conditions except for emulsion polymerization. Cationic polymerization is quite facile and leads not only to the poly(vinylfuran) structure (59), as found in free radically produced polymers, but also to structures such as (60) and (61) in which the furan nucleus has become involved. Furfuryl acrylate and methacrylate undergo free radical polymerization in the manner characteristic of other acrylic esters. 

Free-radical initiation of emulsion copolymers produces a random polymerization m which the trans/ds ratio cannot be controlled. The nature of ESBR free-radical polymerization results in the polymer being heterogeneous, with a broad molecular weight distribution and random copolymer composition. The microstructurc is not amenable to manipulation, although the temperature of the polymerization affects the ratio of trans to cis somewhat... 

Acrylic Monomers. The physical properties of a polymer are dependent upon the monomers used in the polymerization. Impurities contained in the monomers also affect the physical properties. Acrylosilane resins and crosslinkable emulsions can be prepared using 3-(Diethoxymethylsilyl) propyl methacrylate (I) (H)). Free-radical polymerization using (I) will yield a resin... 

Polymer colloids involve dispersions containing polymer particles having sizes greater than about 1 nm. If dispersed in aqueous solution, such a polymer dispersion is called a latex. These are usually synthetic polymer particles formed by free radical polymerization [784], Many kinds of polymerization systems exist, involving almost all of the possible kinds of colloidal dispersion, including emulsion polymerization, hence the more general term heterophase polymerization is sometimes used. Several reviews are available [785-789]. Emulsion polymerization provides a convenient means of controlling the polymerization of monomers and is used to make, for example, synthetic rubber which is mostly a co-polymer of butadiene and styrene. 

Microemulsion polymerization, as the name implies, involves free-radical polymerization in extremely small size, microemulsified monomer droplets of about 1-10 nm diameter [792], The produced polymer particles tend to be small (10 nm) and have higher molar mass (106-107 g/mol) than can be obtained from conventional emulsion polymerization [792]. 

Emulsion polymerization—Free-radical polymerization carried out in micelles suspended in water. Ingredients include a surfactant (detergent) to form the micelles, a monomer that is not very soluble in water, and an initiator. The product consists of small particles of polymer suspended in water called a latex. ... 

Free-Radical Polymerization in Emulsion. In suspension polymerization, the particle size is fixed by the size of the monomer droplet which contains the initiator. Emulsion polymerization differs from suspension polymerization in that the initiator is dissolved in the aqueous phase and the polymer particle grows during polymerization. Free radicals are generated in the water and diffuse to the monomer-water interface. The length of the polymer chain formed, or equivalently the molecular weight, depends on the rate of free radical arrival and termination. S. Katz,... 

Nonlinear polymer formation in emulsion polymerization is a challenging topic. Reaction mechanisms that form long-chain branching in free-radical polymerizations include chain transfer to the polymer and terminal double bond polymerization. Polymerization reactions that involve multifunctional monomers such as vinyl/divinyl copolymerization reactions are discussed separately in Sect. 4.2.2. For simplicity, in this section we assume that both the radicals and the polymer molecules that formed are distributed homogeneously inside the polymer particle. 

Tobita H (2004) Scale-free power-law distribution of emulsion-polymerized nonlinear polymers Free-radical polymerization with chain transfer to polymer. Macromolecules... 

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. 

Free-radical polymerization is the most widely used process for polymer synthesis. It is much less sensitive to the effects of adventitious impurities than ionic chain-growth reactions. Free-radical polymerizations are usually much faster than those in step-growth syntheses, which use diFFereiit monomers in any case. Chapter 7 covers emulsion polymerization, which is a special technique of free-radical chain-growth polymerizations. Copolymerizalions are considered separately in Chapter 8. This chapter focuses on the polymerization reactions in which only one monomer is involved. 

Macroradicals are isolated I rom each other in emulsion polymerizations because they grow in particles, which can accommodate either one or zero radicals at any instant. The distinguishing feature of the kinetics of such reactions is that the polymerization rate and polymer molecular weight are proportional to the number of particles, as distinct from free-radical polymerizations in bulk, solution or suspension. An interesting consequence is that the rate of polymerization will be inversely proportional to the particle size. This holds affixed final polymer content, which is the way such reactions are usually performed. Polymer molecular weight may also be affected by particle size under the same conditions. 

Polymers can be prepared by many different processes. Free radical polymerization can be accomplished in bulk, suspension, solution, or emulsion. Ionic and other nonradical polymerizations are usually produced in solution polymerizations. Each technique has characteristic advantages and disadvantages. 

Apart from intrinsic interest, the theoiy of compartmentalized free-radical polymerization reactions is of importance primarily because it is believed that most of the polymer which is form in the course of an emulsion polymerization reaction is formed via reactions of this type. The general sl pe of the conversion-time curve for many emulsion polymerization reactions suggests (see Fig. I) that the reaction occurs in three more-or-less distinct stages or intervals. The first of these, the so-called Interval I, is interpreted as the stage of polymerization in which the discrete reaction loci are formed. In the second and third stages—Intervals II and III—the polymerization is believed to occur essentially by compartmentalized free-radical polymerization within the loci which were formed during Interval I. 

In homogeneous free radical polymerization, water is often employed as solvent for water-soluble monomers and polymers with more polar functional substituents such as hydroxyl, amino, oxyethylene, ammonium, and carboxylate groups, along with emulsion, suspension, and dispersion processes. This is also the case for metal-catalyzed living radical polymerization. 

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