Oxygen, living polymerization

Termination. Anionic polymerization has no termination associated with it in the time scale of the polymerization reaction. For this reason, anionic polymerization is sometimes called living polymerization. As a result, if the starting reagents are pure and if the polymerization is moisture- and oxygen-free, propagation can proceed until all monomer is consumed. In this case, termination occurs only by the deliberate introduction of oxygen, carbon dioxide, methanol or water as follows ... 

Solution-polymerized SBR is made by termination-free, anionic/live polymerization initiated by alkyl lithium compounds. Other lithium compounds are suitable (such as aryl, alkaryl, aralkyl, tolyl, xylyl lithium, and ot/p-naphtyl lithium as well as their blends), but alkyl lithium compounds are the most commonly used in industry. The absence of a spontaneous termination step enables the synthesis of polymers possessing a very narrow molecular weight distribution and less branching. Carbon dioxide, water, oxygen, ethanol, mercaptans, and primary/secondary amines interfere with the activity of alkyl lithium catalysts, so the polymerization must be carried out in clean, near-anhydrous conditions. Stirred bed or agitated stainless steel reactors are widely used commercially. 

Water and compounds with active hydrogen must be excluded from the reaction medium. Oxygen, on the other hand, does not interfere with the reaction. Tetrahydrofuran, acetonitrile, and aromatic solvents are commonly used in polymerizations catalyzed by nucleophiles. Chlorinated solvents and dimethylformamide are utilized in many reactions catalyzed by electrophiles. Living polymerizations of methacrylate esters can be carried out at 0 to 50 C. The acrylate esters, however, require temperatures below 0 °C for living, group-transfer polymerizations, because they are more reactive and can undergo side reactions. 

Increased rates of polymerization have classically been explained by a decrease in the rate of termination. We can take advantage of the resulting longer-lived polymeric radicals by proper selection of conditions and careful experimental techniques to avoid contamination with possible termination agents such as oxygen. Consider the copolymerization of SAN in tert.-butyl alcohol. The resulting copolymer precipitates, and is capable of further polymerization even after 96 hrs. The latter interval was selected to allow near quantitative decomposition of the primary initiator. Representative of the extended reactivity, when MMA was added to precipitated PSAN, a copolymer with block... 

A chain reaction without termination produces so-called living polymers. Even if on polymerization the initial monomer is used up, a new monomer can be added in a second step and the polymerization restarted as long as the active sites are not destroyed. The reaction became possible when initiation of vinyl polymerization with anionic mechanism was discovered by Szwarc in 1956 [19]. The process is easy to understand. A fixed number of initiator molecules, N, is added to the monomer under conditions that eliminate termination (i.e., in the absence of water and oxygen). Figure 3.31 illustrates living polymerization with 10 initiator and 42 monomer molecules. Without termination, the reaction stops when all monomers are used up. 

It was not until the invention of iodonium and sulfonium salts as photo-initiators by Crivello (1975) that cationic photo-polymerization became practical (see Crivello et al., 1977,1990,2000). Upon irradiation of these Crivello salts, acids are generated. Another significant difference between free radical and cationic polymerizations is the latter process is a living polymerization— once the acid species is formed, it remains active even after the irradiation is stopped. In contrast to this behavior free radicals die soon after irradiation is stopped. Also, unhke free radical polymerizations cationic reactions are not inhibited by oxygen. Quite often the dark reaction following irradiation can play an important role in enabhng a cationic system to develop its full properties and this leads manufacturers of commercial cationic photopolymers to often recommend a thermal (dark) postcure after carrying out the photo-irradiation process. 

With methyl oxirane, propylene oxide, ring opening is by cleavage of the less-substituted carbon-to-oxygen bond. Since the terminal oxyalkanol groups are still active, the polymerizations, as represented, are "living" polymerization reactions. However, there are termination side reactions that become particularly important at elevated reaction temperatures. 

When a counteranion is not so nucleophilic as the iodide anion, the propagating carbocation may be stabilized instead by adding an base (Z) so that living polymerization proceeds (Eq. 9 see also section 1.1) (4). This method is particularly effective for the polymerization initiated with ethylaluminum dichloride (EtAlCl2) (13) and typically, the bases may be 1,4-dioxane and related ethers that form a "base-stabilized" carbocationic species like where Z is an ether oxygen. We have recently synthesized end-functionalized polymers via these base-stabilized living species (14). 

Unfortunately, non-radical, in particular ionic, polymerization processes are incompatible with many functional groups and require highly pure monomers and solvents as well as the exclusion of oxygen and water. In contrast, the controlled and living radical polymerization combines the robustness of a classical RP with the power of living polymerization and thus allows the preparation of complex polymeric structures. 

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