Radical chain polymerization carbonyl monomer

Isoprene can be polymerized in the laboratory by a radical chain mechanism. As shown in the following equations, the odd electron of the initially produced radical is delocalized onto both C-2 and C-4 by resonance. Either of these carbons may add to another isoprene monomer to continue the chain reaction. If C-2 adds, the process is called 1,2-addition if C-4 adds, the process is called 1,4-addition. (This is similar to the addition of electrophiles to conjugated dienes discussed in Section 11.13 and the addition of nucleophiles to a,/8-unsaturated carbonyl compounds described in Section 18.10.)... 

In the emulsion-polymerization of methyl methacrylate by carbonyl initiators, intramicellar processes have been found to reduce the conversion rate. On the other hand, the application of weak magnetic fields has been found to increase the conversion of emulsion-polymerized vinyl monomers using triplet ketone sensitizers. Here external magnetic fields will decrease the efficiency of triplet-to-singlet radical-pair intersystem crossing within the micelles and thus increase the function of radical pairs that escape without terminating the polymer chains. 

Polymerization by ionic initiation is much more limited than that by free-radical initiation with vinyl monomers, but there are monomers such as carbonyl compounds that may be polymerized ionically but not through free radicals because of the high polarity. The polymerization is much more sensitive to trace impurities, especially water, and proceeds rapidly at low temperature to give polymers of narrow molar-mass distribution. The chain grows in a living way and, unlike in the case of free-radical polymerization, is generally terminated not by recombination but rather by trace impurities, solvent or, rarely, the initiator s counter-ion (Fontanille, 1989). 

Major commercial synthetic specialty polymers are made by chain-growth polymerization of functionalized vinyl monomers, carbonyl monomers, or strained ring compounds. Depending on monomer structure, the polymerization may be initiated free radically, anionically, or cationically. Copolymers or terpolymers with random, alternating, block, or graft sequences can be prepared under appropriate reaction conditions. There are numerous mediods used to prepare specialty polymers in the research laboratory. However, only a few are of commercial interest. Of particular commercial interest is synthesis of specialty polymers in solutions, dispersions, suspensions, or emulsions. 

Photolysis of aromatic ketones, such as benzophenone, in the presence of hydrogen donors, such as alcohols, amines, or thiols, leads to the formation of a radical stemming from the carbonyl compound (ketyl-type radical in the case of benzophenone) and another radical derived from the hydrogen donor [see reaction (14)]. Provided vinyl monomer is present, the latter may initiate a chain polymerization. The radicals stemming from the carbonyl compound are usually not reactive toward vinyl monomers due to bulkiness and/or the delocalization of the unpaired electron. 

Copolymers containing carbonyl groups in the main chain have been used in early works as photoinitiators. These copolymers undergo main chain scission upon irradiation. In the presence of the vinyl monomer, the terminal radicals react with the monomer, initiating its polymerization. Following this method, various block copolymers (second comonomer in the block copolymer methyl methacrylate) have been obtained. See Table 18. 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

Bulk Polymerization. The bulk polymerization of acryUc monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to form by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acryUc monomers is of limited importance. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

The above examples of free-radical ring-opening polymerization, which have been explored by Bailey and Endo, produce polymers containing ketonic carbonyl and/or ester groups in the main chain. In addition, these cyclic monomers can be copolymerized with vinyl monomers by free-radical mechanism. Thus, the variety of the polymers produced by radical polymerization has been enlarged. 

The use of peptide-derived lipids (Glu-1, Phe-1, and Phe-2) as organic stationary phases also has been described in this entry. These lipids not only have high potential ability as a self-assembling system but also are attractive as a carbonyl Tr-electron source. As a result, double-alkylated L-glutamide-derived stationary phase has been developed and extremely high selectivity is detected in HPLC as predicted. Polymeric types of peptide-derived hpids were also considered and developed, but their selectivity could not exceed that of the monomeric type hpids. This is probably due to the fact that radical polymerization of peptide-derived monomers disturb the stereoregularity of the resultant polymer main chain, and thus, sufficient molecular ordering is not obtained to increase the selectivity (multiple TT-TT interaction). Finally, it is concluded that subsidiary weak interactions, such as tt-tt interaction, can... 

Monomers which have been successfully polymerized using ATRP include styrenes, acrylates, methacrylates, and several other relatively reactive monomers such as acrylamides, vinylpyridine, and acrylonitrile, which contain groups (e.g., phenyl, carbonyl, nitrile) adjacent to the carbon radicals that stabilize the propagating chains and produce a suf cientiy large atom transfer equilibrium constant. The range of monomers polymerizable by ATRP is thus greater than that accessible by nitroxide-mediated polymerization, since it includes the entire family of methacrylates. However, acidic monomers (e.g., methacrylic acid) have not been successfully polymerized by ATRP and so also halogenated alkenes, alkyl-substimted ole ns, and vinyl esters because of then-very low intrinsic reactivity in radical polymerization and radical addition reactions (and hence, presumably, a very low ATRP equilibrium constant). 

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