Solubility monomers

In a suspension polymerisation monomer is suspended in water as 0.1—5-mm droplets, stabilised by protective coUoids or suspending agents. Polymerisation is initiated by a monomer-soluble initiator and takes place within the monomer droplets. The water serves as both the dispersion medium and a heat-transfer agent. Particle sise is controlled primarily by the rate of agitation and the concentration and type of suspending aids. The polymer is obtained as small beads about 0.1—5 mm in diameter, which are isolated by filtration or centrifugation. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

Suspension Polymerization. This method (10) might be considered as a number of bulk polymerizations carried out simultaneously in the monomer droplets with water acting as a heat-transfer medium. A monomer-soluble initiator, eg, a peroxide or azo compound, and a protective coUoid like poly(vinyl alcohol) or bentonite, are requited. After completion of the polymerization, the excess of monomer(s) is steam stripped, and the beads of polymer are collected and washed on a centrifiige or filter and dried on a vibrating screen or by means of an expeUer—extmder. 

Emulsions stabilized with a nonionic surfactant and catalyzed with a monomer soluble initiator were found to foUow kinetics dependent on initiator concentration (17). 

Suspension polymerisation of styrene is widely practised commercially. In this process the monomer is suspended in droplets 5 -Min. in diameter in a fluid, usually water. The heat transfer distances for the dissipation of the exotherm are thus reduced to values in the range s-fisin. Removal of heat from the low-viscosity fluid medium presents little problem. The reaction is initiated by monomer-soluble initiators such as benzoyl peroxide. 

Styrene-based polymer supports are produced by o/w suspension polymerization of styrene and divinylbenzene. Suspension polymerization is usually carried out by using a monomer-soluble initiator such as benzoperoxide (BPO) or 2,2-azo-bis-isobutylnitrile (AIBN) at a temperature of 55-85°C (19). A relatively high initiator concentration of 1-5% (w/w) based on the monomer is used. The time required for complete monomer conversion must be determined by preliminary experiments and is usually between 5 and 20 h, depending on the initiator concentration, the temperature, and the exact composition of the monomer mixture (11-18). 

Monomer-soluble initiators are used in this polymerization technique. The monomer phase containing an initiator is dissolved in an inert solvent or solvent mixture including a steric stabilizer. The polymers or oligomer... 

Ueno, M., Tanford, C., and Reynolds, J. A. (1984). Phospholipid vesicle formation using nonionic detergents with low monomer solubility. Kinetic factors determine vesicle size and polydis-persity. Biochemistry, 3070-3076. 

Polymerization of vinyl chloride occurs through a radical chain addition mechanism, which can be achieved through bulk, suspension, or emulsion polymerization processes. Radical initiators used in vinyl chloride polymerization fall into two classes water-soluble or monomer-soluble. The water-soluble initiators, such as hydrogen peroxide and alkali metal persulfates, are used in emulsion polymerization processes where polymerization begins in the aqueous phase. Monomer-soluble initiators include peroxides, such as dilauryl and benzoyl peroxide, and azo species, such as 1,1 -azobisisobutyrate, which are shown in Fig. 22.2. These initiators are used in emulsion and bulk polymerization processes. 

Figure 22.2 Chemical structure of monomer soluble initiators for polyvinyl chloride a) dilauryl peroxide, b) benzoyl peroxide and c) azobisisobutyronitrile...

Why are some initiators water soluble and others monomer soluble Which type is used for suspension polymerization Emulsion polymerization ... 

The first term on the RHS of (III-8) represents mutual termination of radicals in the polymer particles (i.e. second order termination). The second term represents a first order termination of radicals in the polymer particles by monomer soluble impurities (MSI), which are present in the polymer particles due to their transfer in there with monomer during the monomer diffusion phase from monomer droplets. 

By using this technique only water insoluble monomers can be polymerised. In this process, the monomer is suspended as discrete droplets (0.1 to 1.0 mm diameter) in dilute aqueous solution containing protective colloids like polyvinyl alcohol and surfactants, etc. The droplets have large surface area and can readily transfer heat to water. Suspension is brought about by agitating the suspension. Protective colloids prevent coalescence of the droplets. A monomer soluble initiator is used. The product is obtained by filtration or spray drying. This process cannot be carried out yet in a continuous process hence batch processing has to be used. 

Simplest of the techniques requiring only monomer and monomer-soluble initiator, and perhaps a chain-transfer agent for molecular weight control. Characterized, on the positive side, by high polymer yield per volume of reaction, easy polymer recovery. Difficulty of removing unreacted monomer and heat control are negative features. Examples of polymers produced by bulk polymerization include poly(methyl methacrylate), polystyrene, and low-density (high pressure) polyethylene. 

The term suspension polymerization refers to the polymerization of macroscopic droplets in an aqueous medium. The kinetics is essentially that of a bulk polymerization with the expected adjustments associated with carrying out a number of bulk polymerizations in small particles more or less simultaneously and in reasonably good contact with a heat exchanger (i.e., the reaction medium) to control the exothermic nature of the process. Usually, suspension polymerizations are characterized by the use of monomer-soluble initiators and the use of suspending agents. 

Polymerization in suspension (bead or pearl polymerization) under normal pressure in the range from 60 to 80°C operates with a suspension of globules of an oil-soluble monomer in water and uses a monomer soluble catalyst. Substantial quantities of polystyrene and poly vinyl acetate are made by this method. 

Emulsions may be polymerized by use of a water-soluble catalyst (initiator), such as potassium persulfate, or a monomer-soluble catalyst, such as benzoyl peroxide, lauroyl peroxide or azobisisobutyronitrilc. Suspension and solution polymerizations employ the monomer soluble catalysts only. In addition to the above-mentioned initiators, diisopropyl pcroxydi-carbonatc may also be employed, where lower-temperature polymerization may be desired, e.g., to reduce branching and minimize degradation. 

Microsuspension Polymerization. Microsnspension polymerization uses a monomer-soluble initiator. The monomer is homogenized in water... 

The homopolymerization of DADMAC is possible in several organic solvents such as acetone, l-methyl-2-pyrrolidone, tetramethylurea, or dimethylform-amide. Various initiation methods including radical, ionic, or x-ray induced polymerization have been employed [19]. Since the monomer solubility is limited in these solvents, and the resulting homopolymer is soluble only in water, methanol and acidic acid, the polymerization in aqueous solutions are preferred. Polymerization in both homogeneous and heterogeneous systems have been studied and the kinetics and mechanisms were investigated in aqueous solution and in inverse-emulsion [6-16,52,53]. 

To see the effects of monomer solubility and other variables on nucleation rate, calculations using Equation 16 were made for a series of monomers with reactivity similar to that for methyl methacrylate. These are shown in Figure 5. Curve 1 represents... 

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