Nucleophilic agents, effect polymerization

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is broken. Water has an especially negative effect on polymerization, since it is an active chain-transfer agent. For example, C s is approximately 10 in the polymerization of styrene at 25°C with sodium naphthalene [Szwarc, 1960], and the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. The adventitious presence of other proton donors may not be as much of a problem. Ethanol has a transfer constant of about 10-3. Its presence in small amounts would not prevent the formation of high polymer because transfer would be slow, although the polymer would not be living. 

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is thus broken. Water is an especially effective chain terminating agent. For example, Ctr,s is approximately 10 in the polymerization of styrene at 25° C with sodium naphthalene. Thus the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. 

A decisive influence on the effectiveness of the anionic polymerization has the chemical structure of used monomer. In the extreme case, very reactive monomers can be polymerized using very weak bases as nucleophilic agents. 

The polymerizations require the use of dipolar aprotic solvents such as N-methylpyrrolidone (NMP), dimethyl acetamide (DMAC), dimethyl sulfoxide (DMSO) or N,N -dimethylpropylene urea (DMPU). Nucleophilic aromatic substitution polymerizations are t q>ically performed in a high boiling aprotic polar solvent with the monomer(s) reacted in the presence of a base, potassium carbonate, at elevated temperatures (ca. 180 C). Potassium carbonate is used to convert the phenol into the potassium phenolate and since K2CO3 is a weak base, no hydrolytic side reactions are observed. Dipolar aprotic solvents are used in these poly(aryl ether) syntheses, since they effectively dissolve the monomers and solvate the polar intermediates and the final polymer. DMPU has been shown to be an excellent solvent for poly(ether) syntheses, particularly for those polymers which are only marginally soluble in other dipolar aprotic solvents (22). Furthermore, DMPU allows higher reaction temperatures (260 C). We have observed that DMPU, when used in conjunction with toluene as a dehydrating agent, accelerates many nucleophilic substitution reactions. 

Chain Transfer. A number of materials act as tme transfer agents in THF polymerization notable examples are dialkyl ethers and orthoformates. In low concentrations, water behaves as a transfer agent, and hydroxyl end groups are produced. The oxygen of dialkyl ethers are rather poor nucleophiles compared to THF and are therefore not very effective as transfer agents. On the other hand, orthoformates are effective transfer agents and can be used to produce alkoxy-ended PTHFs of any desired molecular weight (169). 

Mackay and coworkers [33,34] and Moss and coworkers [35] showed that reactions of nucleophiles with phosphorus(V) esters, which are models for nerve agents, can be carried out effectively in microemulsions. The active agents often contain polymeric thickeners, and the solubilizing power of microemulsions is of crucial importance. Typically cationic surfactants are used with alcohols or alMpyrollidinones as cosurfactants. lodosobenzoate ions (2) are nucleophilic turnover catalysts toward fluorophosphonate nerve agents and phosphate esters, and Moss and coworkers used amphiphilic iodosobenzoate ions extensively in this work [36] (see Scheme 2). 

Thiols may be used as transfer agents in a wide variety of free radical polymerization processes. Scheme 1.12 shows the general reaction mechanism for this class of transfer agents. Nucleophilic radicals react more readily with thiols than electrophilic radicals, so transfer coefficients are higher for vinyl esters and styrene than for acrylates and methacrylates. Aromatic thiols react more readily than aliphatic ones, i.e., the chain transfer constant is higher, but they also show a stronger retardation effect as the resulting S-centered radicals are less prone for monomer addition due to their increased stability. The product of the transfer reaction is a thiyl radical, which is electrophilic and will react preferably with the more electron rich monomer in copolymerizations. 

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