Initiator systems

Two simulation methods—Monte Carlo and molecular dynamics—allow calculation of the density profile and pressure difference of Eq. III-44 across the vapor-liquid interface [64, 65]. In the former method, the initial system consists of N molecules in assumed positions. An intermolecule potential function is chosen, such as the Lennard-Jones potential, and the positions are randomly varied until the energy of the system is at a minimum. The resulting configuration is taken to be the equilibrium one. In the molecular dynamics approach, the N molecules are given initial positions and velocities and the equations of motion are solved to follow the ensuing collisions until the set shows constant time-average thermodynamic properties. Both methods are computer intensive yet widely used. 

For constant energy simulations without temperature regulation, use heating steps of about 0.5 ps and a healing time of 20-30 ps. In gen eral, short h eating tim es and large temperature steps perturb th e initial system m ore than Ion gcr heating times and small tern -perature steps. 

To appreciate the action of a drying agent of class (a), let us imagine some anhydrous copper sulphate in an evacuated vessel provided with a pressure gauge, and water is allowed to enter slowly the temperature is assumed constant at 25°. The results may be best expressed by means of a vapour pressure - composition diagram (Fig. 7, 20, 1). The initial system is represented by the point A the pressure will rise along AB until the monohydrate CuS04,H20 commences to form at B. 

Total dissociation of the initiator system is rare, so the simplifications we explored for this condition in the anionic case cannot be used. 

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

Residual monomers in the latex are avoided either by effectively reacting the monomers to polymer or by physical or chemical removal. The use of tert-huty peroxypivalate as a second initiator toward the end of the polymeri2ation or the use of mixed initiator systems of K2S20g and tert-huty peroxyben2oate (56) effectively increases final conversion and decreases residual monomer levels. Spray devolatili2ation of hot latex under reduced pressure has been claimed to be effective (56). Residual acrylonitrile also can be reduced by postreaction with a number of agents such as monoamines (57) and dialkylamines (58), ammonium—alkali metal sulfites (59), unsaturated fatty acids or their glycerides (60,61), their aldehydes, esters of olefinic alcohols, cyanuric acid (62,63), andmyrcene (64). 

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

Wheieas the BPO—DMA ledox system works well for curing of unsaturated polyester blends, it is not a very effective system for initiating vinyl monomer polymerizations, and therefore it generally is not used in such appHcations (34). However, combinations of amines (eg, DMA) and acyl sulfonyl peroxides (eg, ACSP) are very effective initiator systems at 0°C for high conversion suspension polymerizations of vinyl chloride (35). BPO has also been used in combination with ferrous ammonium sulfate to initiate emulsion polymerizations of vinyl monomers via a redox reaction (36). 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymeri2ation of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkaU metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Usehil initiating systems for acrylate polymeri2ation have been prepared from complexes of ester enolates with alkak metal alkoxides (104,105). 

In place of a proton source, ie, a Briimsted acid, a cation source such as an alkyl haUde, ester, or ether can be used in conjunction with a Friedel-Crafts acid. Initiation with the ether-based initiating systems in most cases involves the haUde derivative which arises upon fast haUdation by the Friedel-Crafts acid, MX (2). 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

The radicals are destroyed and are not available to take part in the desired radical reactions, eg, polymerizations. Thus, transition-metal ion concentrations of metal—hydroperoxide initiating systems are optimized to maximize radical generation. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Neutral or alkaline salts, eg, KCl, K SO, K CO, or Na PO, are often present in synthetic latices in quantities of - <1%, based on the weight of the mbber. During emulsion polymerization the salts help control viscosity of the latex and, in the case of alkaline salts, the pH of the system. Many polymerizations are carried out at high pH, requiring the use of fixed alkaH, eg, KOH or NaOH. Very small amounts of ferrous salts can be employed as a component of the initiator system, in which case a sequesteriag agent, eg, ethyldiaminotetraacetic acid (EDTA) may be iacluded to complex the iron. Water-soluble shortstops, eg, potassium dithiocarbamate, may also be iacluded ia very small amounts (ca 0.1 parts). 

Redox initiator systems are normally used in the emulsion polymerization of VDC to develop high rates at low temperatures. Reactions must be carried out below - 80° C to prevent degradation of the polymer. Poly(vinyHdene chloride) in emulsion is also attacked by aqueous base. Therefore, reactions should be carried out at low pH. 

Emulsion Polymerization. Poly(vinyl acetate)-based emulsion polymers are produced by the polymerization of an emulsified monomer through free-radicals generated by an initiator system. Descriptions of the technology may be found in several references (35—39). 

Ammonia H2O2 Initiation. The lower molecular weight grades (K-15 and K-30) ofPVP ate prepared industrially with an ammonia /H O initiation system. Such products ate the standards for the pharmaceutical industry and conform to the various national pharmacopeias. Several papers have appeared concerning the mechanism of this polymerization (15). 

The polysulfide impression materials can be formulated to have a wide range of physical and chemical characteristics by modifying the base (polysulfide portion), and/or the initiator system. Further changes may be obtained by varying the proportion of the base to the catalyst in the final mix. Characteristics varied by these mechanisms include viscosity control from thin fluid mixes to heavy thixotropic mixes, setting-time control, and control of the set-mbber hardness from a Shore A Durometer scale of 20 to 60. Variations in strength, toughness, and elasticity can also be achieved. 

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