Living polymerization, initiated

Thus, Lewis acids do not appear to be prominent initiators for living polymerizations. Initiation is usually slow, efficiency is low and side products (e.g. POP3, reactive end groups) could induce termination or undesirable transfer reactions. 

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. 

Adachi, T. Sugimoto, H. Aida, T. Inoue, S. Controlled synthesis of high molecular weight poly(methyl methacrylate) based on Lewis acid-assisted high-speed living polymerization initiated with aluminum porphyrin. Macromolecules 1992, 25, 2280-2281. 

Inoue and coworkers (100,101) have examined these compounds for the polymerization of alkylene oxides, lactones, and lactides and termed them as initiators for immortal polymerization, in contrast to the living polymerization initiators (42). 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

The formation of polymer can be considered as a quasi-living polymerization. After the polymerization is complete, it can be reinitiated with the addition of more monomer to the unquenched polymer. However, the degree of polymerization cannot be predicted by the monomer/initiator molar ratio, the polydispersity is 1.5-2.0, and water, or even carboxylic acids, act as inhibitors and do not terminate the polymerization [10]. 

Formation of block polymers is not limited to hydrocarbon monomers only. For example, living polystyrene initiates polymerization of methyl methacrylate and a block polymer of polystyrene and of polymethyl methacrylate results.34 However, methyl methacrylate represents a class of monomers which may be named a suicide monomer. Its polymerization can be initiated by carbanions or by an electron transfer process, the propagation reaction is rapid but eventually termination takes place. Presumably, the reactive carbanion interacts with the methyl group of the ester according to the following reaction... 

The reaction of radicals with nitroxides is reversible. 09 This means that the highest temperature that the technique can reasonably be employed at is ca 80 °C for tertiary propagating species and ca 120 °C for secondary propagating species.22 These maximum temperatures are only guidelines. The stability of alkoxyamines is also dependent on solvent (polar solvents favor decomposition) and the structure of the trapped species. This chemistry has led to certain alkoxyamines being useful as initiators of living polymerization (Section 9.3.6). At elevated temperatures nitroxides are observed to add to monomer albeit slowly. 3IS 5" 523... 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

Figure 9.1 Predicted evolution of molecular weight (arbitrary units) with monomer conversion for a conventional radical polymerization with a constant rate of initiation (---------------) and a living polymerization (--).

In order for the characteristics of living polymerization to be displayed, initiators should possess the following attributes ... 

Since the dithiocarbatnyl end groups 8 are thermally stable but pholochemically labile at usual polymerization temperatures, only photo-initiated polymerizations have the potential to show living characteristics. However, various disulfides, for example, 9 and 10, have been used to prepare end-functional polymers37 and block copolymers38 by irreversible chain transfer in non-living thermally-initiated polymerization (Section 7.5.1). 

In comparing observed reactivity ratios between various polymerization systems, it is important to take into account the possible effect of molecular weight on copolymer composition.3475 19 In conventional radical eopolymeri/.ation, the specificity shown in the initiation and termination steps can have a significant effect on the composition of low molecular weight copolymers (usually <10 units). These effects are discussed in Section 7.4.5. In a living polymerization molecular weights are low at low conversion and increase with conversion. In these... 

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