Polymerization, anionic radical

C. Formation of MAIs By Anionic Chain Polymerization-Anion Radical Transfer... 

Theoretical discussion of A/-parameters, spin density calculations. ) Some measurements on similar compounds for instance the polymerization anion radical of 1,1-Diphcnylethylene. 

Although the anionic polymerization mechanism is the predominant one for the cyanoacryhc esters, the monomer will polymerize free-radically under prolonged exposure to heat or light. To extend the usable shelf life, free-radical stabilizers such as quinones or hindered phenols are a necessary part of the adhesive formulation. 

Regarding anion radical transfer, low-molecular weight azo compounds were used as terminating agents in anionic polymerizations. An interesting example is the addition of a living polystyrene chain to one nitrile group of AIBN [71]. The terminal styryl anion is likely to form... 

The reactions of polymeric anions with appropriate azo-compounds or peroxides to form polymeric initiators provide other examples of anion-radical transformation (e.g. Scheme 7. 6). ""7i However, the polymeric azo and peroxy compounds have limited utility in block copolymer synthesis because of the poor efficiency of radical generation from the polymeric initiators (7.5.1). 

One of the major advantages of radical polymerization over most other forms of polymerization, (anionic, cationic, coordination) is that statistical copolymers can be prepared from a very wide range of monomer types that can contain various unprotected functionalities. Radical copolymerization and the factors that influence copolymer structure have been discussed in Chapter 7. Copolymerization of macromonomers by NMP, ATRP and RAFT is discussed in Section 9.10.1. 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

First, we examined the efficiency of the initiation process. A solution of buthyllithium was added to a THF solution of 7 at -70°C. The color of the solution turned to red immediately and a strong ESR signal was observed with a well separated hyperfme structure. The observed radical species was identified as the anion radical of 2-butyl-l,l,2,2-tetramethyldisilanyl-substituted biphenyl by computational simulation as well as by comparison with the spectra of a model compound. The anion radical should be a product of a single electron transfer (SET) process from buthyllithium to the monomer. Since no polymeric product was obtained under the above-mentioned conditions, the SET process is an undesired side reaction of the initiation and one of the reasons why more higher molecular weight polymer was observed than expected. ... 

Ionic Polymerization. Ionic polymerizations, especially cationic polymerizations, are not as well understood as radical polymerizations because of experimental difficulties involved in their study. The nature of the reaction media is not always clear since heterogeneous initiators are often involved. Further, it is much more difficult to obtain reproducible data because ionic polymerizations proceed at very fast rates and are highly sensitive to small concentrations of impurities and adventitious materials. Butyl rubber, a polymer of isobutene and isoprene, is produced commercially by cationic polymerization. Anionic polymerization is used for various polymerizations of 1,3-butadiene and isoprene. 

The currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

Polymers can be made by vibromilling of some monomers with steel balls. No initiators are needed. Kramer effect, that is, the action of the electron stream developed by mechanoemission during vibratory milling initiates the polymerization. On vibratory milling, acryl and methacrylamides give anion-radicals, which are key species in the reaction (Simonescn et al. 1983) ... 

In many cases, homopolymerization can be initiated by the anion-radicals of the monomers themselves. Of course, such monomers must have pronounced electron affinity (EA) and be stabilized by delocalization of an unpaired electron. Typical examples are represented by the anion-radicals of 1,1-dicyanoethylene (EA = 1.36 eV) and methyl or ethyl 2-cyanoacrylates (EA = 1.08 eV). In all of these anion-radicals, an unpaired electron is primarily localized on C atom of the CH2 segment and characterized by appreciable resonance stabilization (Brinkmann et al. 2002). These anion-radicals are nucleophilic and attack the neutral monomers to initiate polymerization. 

These examples demonstrate the well-known process of polymerization initiated by anion-radicals. Our next consideration is devoted to an unusual case of initiation. Intercalation of fullerenes by metals results in the formation of fullerene-metal derivatives. Paramagnetic metallofullerenes (anion-radicals) are the fullerenes doped with endohedral metal. According to calculations and structural studies, LaCs2, for example, contains La in the center of one hexagonal ring of the fuller-ene cage (Akasaka et al. 2000, Nishibori et al. 2000, Nomura et al. 1995). Intrafullerene electron transfer in metallofullerenes is possible (Okazaki et al. 2001). 

A possible explanation of this conductivity assumes that polymerization of fullerene anion-radicals results in the formation of a long conjugated chain. This is why the conduction electrons can move along the chain. 

Interestingly, if the C50 fullerene doped by alkali metals is rapidly cooled down to the liquid nitrogen temperature, polymerization does not occur. Only monomeric anion-radical salts are obtained. Warming up these monomers to 80-160 K results in dimerization polymerization does not take place. The dimer (KCgo)2 is dielectric (Pekker et al. 1995). It has been shown that the tris(anion)-radical Cgo can polymerize too. Particularly, Na2CsCgo forms a polymer that maintains superconducting properties (Mizuki et al. 1994). 

The work function of the rubbing surfaces and the electron affinity of additives are interconnected on the molecular level. This mechanism has been discussed in terms of tribopolymerization models as a general approach to boundary lubrication (Kajdas 1994, 2001). To evaluate the validity of the anion-radical mechanism, two metal systems were investigated, a hard steel ball on a softer steel plate and a hard ball on an aluminum plate. Both metal plates emit electrons under friction, but aluminum produced more exoelectrons than steel. With aluminum, the addition of 1% styrene to the hexadecane lubricating fluid reduced the wear volume of the plate by over 65%. This effect considerably predominates that of steel on steel. Friction initiates polymerization of styrene, and this polymer formation was proven. It was also found that lauryl methacrylate, diallyl phthalate, and vinyl acetate reduced wear in an aluminum pin-on-disc test by 60-80% (Kajdas 1994). 

Whether a vinyl monomer polymerizes by radical, anionic, or cationic initiators depends on the inductive and resonance characteristics of the substituent(s) present. The effect of the... 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

---