Block copolymers ionic mechanisms

Miranda, D.F., Versek, C., Tuominen, M.T., Russell, T.P., Watkins, J.J. 2014. Cross-linked block copolymer/ionic liquid self-assembled blends for polymer gel electrolytes with high ionic conductivity and mechanical strength. Macromolecules 46 9313-9323. 

Non-ionic polymers have also been blended with ionic block copolymers. Poly(vinyl phosphanate)-l7-polystyrene and PS-l -SPS have been blended with PPO. In both cases, improvements were seen in MeOH permeability over that of fhe unmodified block copolymers and conductivity values dropped as a function of increasing PPO confenf. PVDF has been blended wifh SEES in order fo improve its mechanical and chemical stability, but aggregation was found fo be a problem due fo incompafibility between components. However, it was found that a small amount (2 wt%) of a methyl methacrylate-butyl acrylate-methyl methacrylate block copolymer as com-patibilizer not only led to greater homogeneity but also improved mechanical resistance, water management, and conductivity. ... 

In the present paper we will review principally the different methods of synthesis which proceed through a radical or ionic mechanism. Condensation reactions will only be considered when they unite preformed polymer molecules into a graft or a block copolymer. 

Transformation of epoxy resins, which are viscous liquids or thermoplastic solids, into network polymers is a result of interaction with alkali or acid substances by means of to polyaddition and ionic polymerization mechanisms.10 A resin solidified by to the polyaddition mechanism, is a block copolymer consisting of alternating blocks of resin and a hardener or curing agent. A resin solidified by the ionic mechanism is a homopolymer. Molecules of both resin and hardener contain more than one active group. That is why block copolymer formation is a result of multiple reactions between an epoxy resin and a curing agent.11... 

Nitroxides have been used to prepare numerous block copolymers. Initially, using the TEMPO moiety, only styrene-based monomers could be incorporated into copolymers, but with the use of new nitroxides like DEPN and BPPN, the list has expanded to include acrylate-type monomers, as well as dienes, something that previously could only be accomplished through ionic mechanisms. Unfortunately, chain extension of either St or diene-based macroinitiator with an acry-... 

Addition polymers can also be produced by mechanisms that involve ionic intermediates, reacting in much the same way as illustrated for free radical intermediates. These t)rpes of polymerization are used much less frequently than the free radical types, but are very important in some specialty reactions, such as preparation of block copolymers. 

Several methods can be used to synthesize block copolymers. Using living polymerization, monomer A is homopolymerized to form a block of A then monomer B is added and reacts with the active chain end of segment A to form a block of B. With careful control of the reaction conditions, this technique can produce a variety of well-defined block copolymers. This ionic technique is discussed in more detail in a later section. Mechanicochemical degradation provides a very useful and simple way to produce polymeric free radicals. When a rubber is mechanically sheared (Ceresa, 1965), as during mastication, a reduction in molecular weight occurs as a result of the physical pulling apart of macromolecules. This chain rupture forms radicals of A and B, which then recombine to form a block copolymer. This is not a preferred method because it usually leads to a mixture of poorly defined block copolymers. 

Ji, J., Keen, J., Zhong, W.-H., 2011. Simultaneous improvement in ionic conductivity and mechanical properties of multi-functional block-copolymer modified solid polymer electrolytes for lithium ion... 

In a well-controlled radical system, the monomer conversion is first order, molar mass increases linearly with monomer conversion, and the molar mass distribution MJM is below 1.5. In addition, chain end functionalization and subsequent monomer addition allow the preparation of well-controlled polymer architectures, for example, block copolymers and star polymers by a radical mechanism, which had been up to now reserved for ionic chain growth polymerization techniques. 

Many copolymers of styrene are ntanufactured on a large commercial scale. Because styrene copolymerizes readily with many other monomers, it is possible to obtain a wide distribution of properties. Random copolymers form quite readily by a free-radical mechanism. Some can also be formed by ionic mechanism. In addition, graft and block copolymers of styrene are also among commercially important materials. 

These techniques rely upon high shear to cause bond scissions. Ruptured bonds result in formations of free-radical and ionic species. When this application of shear is carried out in the presence of monomers, block copolymers can form. This approach is exploited fairly extensively. Such cleavages of macromolecules can take place during cold mastication, milling, and extrusion of the polymers in the viscoelastic state. Both homolytic and heterolytic scissions are possible. The first yields free-radical and the second ionic species. Heterolytic scissions require more energy but should not be written off as completely unlikely." Early work was done with natural rubber. It swells when exposed to many monomers and forms a viscoelastic mass. When this swollen mass is subjected to shear and mechanical scission, the resultant radicals initiate polymerizations. The mastication reaction was shown to be accompanied by formation of homopolymers. Later, the technique was applied to many different polymers with many different monomers. ... 

Introduction. We have, so far, considered ionic propagation, coordination catalysis, and the step reactions of a pol37mer terminus as techniques for the preparation of block copol3nners. Free radical polymerization may also be utilized by application of one of several chemical manipulations. For example, block copolymers may be prepared by coupling macroradicals, or by generating new radicals in the presence of a second monomer by photolytic or mechanical degradation. As an alternate, difunctional initiators may be employed. 

Ionic mechanisms for the preparation of block copolymers are a very important tool of the synthetic polymer chemist. A feature of many homogeneous anionic polymerizations in solution is that termination can be avoided by careful control of experimental conditions. In fact, an infinite life of the active chain end is theoretically possible, and this has led to the term living polymers. Polymer carbanions can resume growth after the further addition of monomer. By changing the monomer composition, block copolymerization is readily initiated, and this process can be repeated. A major advantage of this... 

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