Living polymerizations, advantages

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... 

Multiblock copolymers are synthesized by step polymerization using prepolymers containing specific end-groups (Eq. 14). Polyester- and polyether-polyurethanes and polyether-polyesters are multiblock copolymers of commercial interest. Step polymerizations has advantages over living polymerization. There is a... 

The advantages provided by the continuous reactor prompted us to explore CCTP in a reactor with two CSTRs connected in series [11]. This reaction scheme depicted in Scheme 6 provides a highly flexible process for production of a wide range of diblock OBC compositions. The block composition can easily be varied by changing the production rate in either reactor. The comonomer content of either block can also be independently tailored by varying the feed compositions because the process operates in two independent reactors. This CCTP scheme also produces multiple chains per catalyst, an advantage over stoichiometric living polymerization systems, but is necessarily stoichiometric in CSA. The reaction produces... 

The stability of alkoxyamine initiators is not only a major synthetic advantage when compared to more traditional living polymerization procedures but it also permits opportunities in the area of surface modification. In analogy with the work that has... 

While possessing many of the key advantages of controlled/ living polymerization methods, nitroxide-mediated free-radical polymerizations do exhibit several limitations. The range of monomers that have been polymerized using nitroxide-mediated techniques include styrenics. acrylamides and (meth)acrylates but these have predominantly been reported in bulk polymerizations (i.e. without solvent) and are conducted at elevated temperature for long time periods. In addition, synthesis of the unimolecular initiator can prove troublesome (dependent upon the type required) and often requires extensive purification in order to attain sufficient purity levels to allow molecular weight control. 

The concept of flash chemistry can be applied to polymer synthesis. Cationic polymerization can be conducted in a highly controlled manner by virtue of the inherent advantage of extremely fast micromixing and fast heat transfer. An excellent level of molecular weight control and molecular-weight distribution control can be attained without deceleration caused by equilibrium between active species and dormant species. The polymerization is complete within a second or so. The microflow system-controlled cationic polymerization seems to be close to ideal living polymerization within a short residence time. 

The ROP of lactide affords high molecular weight PLA polymers with better control of the polymerization process relative to polycondensation. These advantages can be directly attributed to the fact that ROP can be a living polymerization process. Living polymerization is a chain-growth polymerization where chain termination is absent and is characterized by a linear relationship between the monomer to initiator ratio and the experimental molecular weight, and narrow dispersity indicates the... 

Alkyl acrylates were for the first time polymerized in a living fashion with the aid of the unique catalytic action of rare earth metal complexes [4]. Since these monomers have an acidic a-H, termination and chain transfer reactions occur so frequently that their polymerizations generally do not proceed in a living manner. By taking advantages of the living polymerization ability of both MMA and alkyl acrylate, ABA or ABC type tri-block copolymerization was performed to obtain thermoplastic elastomers. 

Potentially important advantages of controlled living polymerization reactions of acetylene derivatives are greater tolerance of functionalities, control over the nature of the capping groups, and the ability to prepare block or random copolymers that contain other monomers that can be polymerized by the well-defined alkylidene complexes. ... 

All previously discussed examples of living cationic polymerization of vinyl ethers were based on homogeneous polymerization media. In 2007, Oashima and coworkers demonstrated the living polymerization of isobutyl vinyl ether in the presence of iron(III) oxide as heterogeneous catalyst and ethyl acetate or dioxane as base [58]. The major advantage of this heterogeneous catalytic system is the easy removal of the metal oxide catalyst. In addition, it was demonstrated that the iron(III) oxide could be reused for at least five times without a decrease in activity. 

As with all living polymerizations, the formation of bloek eopolymers is possible if the active chain end of one polymer block can initiate the polymerization of a second monomer. This may mean that when block copolymers are prepared, the sequence of monomer addition may be critical. In this respect, the CRP techniques are no different from the other ionic reactions, but they do have one specific advantage. Because many of the CRP processes do not reach 100% conversion, it is best to separate and purify the polymer adduct formed in the first step, which can then be stored and used as a macroirfitiator for the growth of the second block, with no loss of activity. The use of such a macroinitiator helps to control the subsequent reaction more effectively and produces products with very low polydispersities, because for the macroinitiator, diffusion and reactivity are decreased, thereby mininuzing radical-radical coupling. 

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