Emulsion polymerization random copolymers

Poly(butadiene- (9-acrylonitrile) [9008-18-3] NBR (64), is another commercially significant random copolymer. This mbber is manufactured by free-radical emulsion polymerization. Important producers include Copolymer Rubber and Chemical (Nysyn), B. F. Goodrich (Hycar), Goodyear (Chemigum), and Uninoyal (Paracdl). The total U.S. production of nitrile mbber (NBR) in 1990 was 95.6 t (65). The most important property of NBR mbber is its oil resistance. It is used in oil well parts, fuels, oil, and solvents (64) (see Elastomers, synthetic— nitrile rubber). 

Random copolymers of vinyl chloride and other monomers are important commercially. Most of these materials are produced by suspension or emulsion polymerization using free-radical initiators. Important producers for vinyl chloride—vinyUdene chloride copolymers include Borden, Inc. and Dow. These copolymers are used in specialized coatings appHcations because of their enhanced solubiUty and as extender resins in plastisols where rapid fusion is required (72). Another important class of materials are the vinyl chloride—vinyl acetate copolymers. Principal producers include Borden Chemicals Plastics, B. F. Goodrich Chemical, and Union Carbide. The copolymerization of vinyl chloride with vinyl acetate yields a material with improved processabihty compared with vinyl chloride homopolymer. However, the physical and chemical properties of the copolymers are different from those of the homopolymer PVC. Generally, as the vinyl acetate content increases, the resin solubiUty in ketone and ester solvents and its susceptibiUty to chemical attack increase, the resin viscosity and heat distortion temperature decrease, and the tensile strength and flexibiUty increase slightly. 

The copolymerization occurs in an aqueous emulsion. When free radicals are used, a random copolymer is obtained. Alternating copolymers are produced when a Zieglar-Natta catalyst is employed. Molecular weight can be controlled by adding modifiers and inhibitors. When the polymerization reaches approximately 65%, the reaction mixture is vacuum distilled in presence of steam to recover the monomer. 

Polystyrene is unusual among commodity polymers in that we can prepare it in a variety of forms by a diversity of polymerization methods in several types of reaction vessel. j Polystyrene may be atactic, isotactic, or syndiotactic. Polymerization methods include free radical, cationic, anionic, and coordination catalysis. Manufacturing processes include bulk, solution, suspension, and emulsion polymerization. We manufacture random copolymers ... 

The unsulfonated random copolymers are reportedly synthesized at 50 °C over a period of 48 h using emulsion polymerization with dodecylamine hydrochloride surfactant in water as the reaction system and potassium persulfate as the initiator. The copolymer is then dissolved in an appropriate solvent such as dichloroethane or chloroform and sulfonated using reagents such as chlorosulfonic acid or a sulfur trioxide complex. It has been reported that this generation of BAM membranes exhibited some su-... 

The solution random copolymer generally contains about 32 percent cis-, 41 percent trans- and 27 percent vinyl-unsaturation compared to 8 per cent cis-, 74 percent trans- and 18 percent vinyl-unsaturation in emulsion copolymer of the same monomer composition. The principal effect of slightly higher vinyl unsatura-tion in solution copolymer is a small increase in the glass transition temperature (-58 C versus -62 C for the emulsion copolymer). However, both solution and emulsion polymerized copolymers exhibit satisfactory low temperature performance for general uses. 

Figure 15. Molecular weight distribution of anionically polymerized styrene-butadiene random copolymer and emulsion polymerized SBR.

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... 

Core-shell polymers were commercially introduced as impact modifiers for poly(vinyl chloride) PVC, in the 1960s. They are produced by a two-stage latex emulsion polymerization technique (Cruz-Ramos, 2000). The core is a graftable elastomeric material, usually crosslinked, that is insoluble in the thermoset precursors. Typical elastomers used for these purposes are crosslinked poly(butadiene), random copolymers of styrene and butadiene,... 

The copolymerization with alkyllithium to produce uniformly random copolymers is more complex for the solution process than for emulsion because of the tendency for the styrene to form blocks. Because of the extremely high rate of reaction of the styryl-lithium anion with butadiene, the polymerization very heavily favors the incorporation of butadiene units as long as reasonable concentrations of butadiene are present. This observation initially was somewhat confusing because the homopolymerization rate of styrene is seven times that for butadiene. However, the cross-propagation rate is orders of magnitude faster than either, and it therefore dominates the system. For a 30 mole percent styrene charge the initial polymer will be almost pure butadiene until most of the butadiene is polymerized. Typically two-thirds of the styrene charged will be found as a block of polystyrene at the tail end of the polymer chain ... 

Some rather interesting, if complex, materials have been prepared by Vollmert (18). Previously we considered some of his materials composed of two polymers now we will consider mixtures of three or more polymers. (Notations relating to mode of polymerization and state of the polymer (14, 15) are omitted for clarity.) Because the individual polymers, in most cases, are multi-mer random copolymers in their own right, one monomer will be considered per polymer for simplicity in the following. In his example 5, the Vollmert emulsion polymerized 7i-butyl acrylate, styrene, 1,4-butanediol monoacrylate, and 1,4-butanediol... 

FIGURE 3-49 BUNA rubber, a styrene/butadiene random copolymer made by emulsion polymerization manufacture, circa 1940s (Courtesy Bayer). 

In emulsions, amine hydrochloride constitutes the aqueous phase and acrylic ester the organic phase. Cetyltrimethylanunonium bromide (CTAB) or span/twin (S/T)-type surfactants are used for emulsion polymerization. Solid dispersants such as talc and colloidal silica are often used to stabilize emulsions which are difficult to stabilize with usual surfactants. HydrophiUc colloidal silica (Aerosil 200) drastically increases the stability of some emulsions provided high amounts (up to 10%) of Aerosil are used. Random copolymers containing 10% hydroxyl groups can be used as polymeric dispersants for preparing w/o emulsions. 

Lee DY, Kim JH. Emulsion polymerization using alkali-soluble random copolymer as a polymeric emulsifier. Am Chem Soc Polym Prepr 1997 38(2) 418-419. 

Lee DY, Kim JH. Preparation of small-sized carboxylated latexes by emulsion polymerization using alkali-soluble random copolymers. J Appl Polym Sci 1998 69 543-550. 

Lee DY, Kim JH, Min TI. Role of alkali-soluble random copolymer in emulsion polymerization. Colloids Surf 1999 A153 89-97. 

When a mixture of styrene and acrylonitrile is polymerized in the presence of a polybutadiene latex by an emulsion radical process, an acrylonitrile-butadiene-styrene (ABS) copolymer is obtained. This ABS copolymer is actually a mixture of (a) a graft copolymer which contains some of the styrene/acrylonitrile (ST/AN) copolymer chemically bound to the polybutadiene backbone, and (b) a random copolymer, conventionally designated as a linear copolymer, which is not bound to the polybutadiene backbone but which consists of the portion of the styrene/acrylonitrile monomer that has polymerized separately. 

Polymeric micelles are called the new generation of colloid drug delivery systems [28]. In an aqueous medium, block and random copolymers associate into micelles with a structure similar to that of surfactants. Active substances which are insoluble or poorly soluble in water can be built into the lipophilic part of the micelle in two ways i) with dialysis and ii) with the emulsion method (see Figure 18.4). 

EMULSION POLYMERIZATION Used for standard SBR. Monomer is emulsified in water with emulsifying agents. Polymerization is initiated by either decomposition of a peroxide or a peroxydisulfate. Hot SBR is initiated by free radicals generated by thermal decomposition of initiators at 50°C or higher. Cold SBR is initiated by oxidation-reduction reactions (redox) at temperatures as low as —40°C. Stjrrene content normally is 23%. Copolymer is randomly distributed. Structure of butadiene contents is about 18% ds-1,4, 65% frans-1,4, and 15-20% vinyl. 

---