Chain polymerization heterocyclic monomers

Recent work on the synthesis, structure and some properties of macromolecules bearing furan rings is discussed. Two basic sources of monomers are considered, viz. furfural for monomers apt to undergo chain polymerization and hydroxymethylfurfural for monomers suitable for step polymerization.Within the first context, free radical, catiomc and anionic systems are reviewed and the peculiarities arising from the presence of furan moieties in the monomer and/or the polymer examined in detail. As for the second context, the polymers considered are polyesters, polyethers, polyamides and polyurethanes. Finally, the chemical modification of aU these oligomers, polymers and copolymers is envisaged on the basis of the unique reactivity of the furan heterocycle. 

A wide range of carbon-carbon double bonds undergo chain polymerization. Table 3-1 shows monomers with alkyl, alkenyl, aryl, halogen, alkoxy, ester, amide, nitrile, and heterocyclic substituents on the alkene double bond. 

These and related heterocyclic monomers are usually highly polar and strongly nucleophilic conpounds. During polymerization chains containing heteroatoms are formed and they can, as well as monomers themselves, interact with components of ionic growing species. The interaction of the macroion-pairs with the elements of the chains has well been documented for the polymerization of ethylene oxide [Z]. 

The second major difference stems from the fact that the nucleophilic site of vinyl monomer, i.e., double bond, is consumed in the propagation step, whereas the nucleophilic site of the heterocyclic monomer, i.e., heteroatom, is still present in the formed chain. Thus, the saturated all-carbon chain becomes a neutral component of the polymerizing system, while the heteroatom-containing chain may still participate in the reaction ... 

Trimethylsilyl triflate can initiate the cationic polymerization of different vinyl and heterocyclic monomers (28). The triflated poly silane can react in the same way and can induce growth of a large number of chains. In this way, comblike graft copolymers can be prepared. 

Cationic polymerization of heterocyclic monomers can proceed not only by the SjjZ mechanism involving onium ions located at the chain end, and analysed in this paper, but also by another Sj 2 mechanism involving activated monomer, adding to the neutral chain ends. For the latter no correlation is yet available and can differ from these described for onium ions in this section. 

Chain transfer to polymer is a general phenomenon in ring-opening cationic polymerization of heterocyclic monomers and its basic mechanistic and kinetic features have been discussed in Vol. I Adv. Polymer Sci. 37 (1980), Sect. 5.3. 

In the polymerization of heterocyclic monomers, the polymer backbone possesses heteroatoms able to participate in chain transfer. The result of this reaction is the formation of inactive branched onium ions (termination). In other instances scrambling is observed leading to changes in the microstructure of the chain and interconversion of active centers. Intramolecular chain transfer to polymer is accompanied by pie formation of macrocycles. Low-molecular-weight cyclic oligomers may escape detection, particularly when the polymer is separated by precipitation. Cyclic oligomers are better soluble and may remain in solution which leads to an erroneous... 

Crivello and Lee have described the synthesis and characterization of a series of (4-alkoxyphenyl)phenyliodonium salts 7, which are excellent photo- and thermal-initiators for the cationic polymerization of vinyl and heterocyclic monomers [17]. Iodonium salts 7 are conveniently prepared by the reaction of alkoxyphenols 6 with [hydroxy(tosyloxy)iodo]benzene followed by anion exchange with sodium hexafluoroantimonate (Scheme 7.2). Products 7 have very good solubility and photoresponse characteristics, which make them especially attractive for use in UV curing applications. Compounds 7 with alkoxy chains of eight carbons and longer are essentially nontoxic, compared to diphenyliodonium hexafluoroantimonate, which has an oral LD50 of 40 mg kg (rats) [17]. 

Additional well-defined side-chain liquid crystalline polymers should be synthesized by controlled polymerizations of mesogen-ic acrylates (anionic or free radical polymerizations), styrenes (anionic, cationic or free radical), vinyl pyridines (anionic), various heterocyclic monomers (anionic, cationic and metalloporphyrin-initiated), cyclobutenes (ROMP), and 7-oxanorbornenes and 7-oxanorbornadienes (ROMP). Ideally, the kinetics of these living polymerizations will be determined by measuring the individual rate constants for termination and... 

Cationic and anionic polymerizations of heterocyclic monomers provide many examples in which the concurrent formation of cyclics of various sizes is observed during the ring-opening polymerization. As illustrated in Scheme 1, in these systems active species follow three pathways they can react with a functional group of the monomer, of its own polymer chain, or of other chains. When the function / involved belongs to a linear polymer chain, intramolecular chain saambling or intermolecular macrocycle formation takes place, as observed in the cationic polymerization of cyclic ethers, acetals, esters, amides, siloxanes, and so forth. 

Whereas the monomers shown in Scheme 6.11 are specifically suited for chain polymerization reactions through the moieties external to the ring and thus give rise to polymers or copolymers with the heterocycle pendant to their backbone, the monomers shown in Schemes 6.12 and 6.13 give rise to polycondensate architectures in which the heterocycle is an integral part of the backbone. 

Chain extension reactions based on tbe activation of the cellulose surface aimed at provoking the grafting-from through the ring opening polymerization of heterocyclic monomers have been practiced industrially in the specific instance of the synthesis of hydroxypropyl and hydroxyethyl cellulose. The aim of these processes is, however, to... 

For less reactive anionic chain ends such as those involved in the propagation of heterocyclic monomers, a wider range of solvents can be utilized. For example, dipolar aprotic solvents such as dimethylsulfoxide (DMSO), A,A -dimethylformamide (DMF), and hexamethylphosphoramide (HMPA) can be used for polymerizations of epoxides. 

Where a polymer has functional groups in the main chain, typical of polymerization by step-reaction chemistry or chain poljunerization of heterocyclic monomers, the fimctional groups in the chain are often the weakest and may nndergo reactions which do not produce radicals. Thermal degradation by random scission, without significant yield of volatiles is typical of polymers such as polyesters, polyamides, and polyurethanes, all of which undergo thermolysis by reaction at the functional groups (5). 

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