Double bonds, living polymerization

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

The role of reactive centers is performed here by free radicals or ions whose reaction with double bonds in monomer molecules leads to the growth of a polymer chain. The time of its formation may be either essentially less than that of monomer consumption or comparable with it. The first case takes place in the processes of free-radical polymerization whereas the second one is peculiar to the processes of living anionic polymerization. The distinction between these two cases is the most greatly pronounced under copolymerization of two and more monomers when the change in their concentrations over the course of the synthesis induces chemical inhomogeneity of the products formed not only for size but for composition as well. 

Anionic polymerization is a powerful method for the synthesis of polymers with a well defined structure [222]. By careful exclusion of oxygen, water and other impurities, Szwarc and coworkers were able to demonstrate the living nature of anionic polymerization [223,224]. This discovery has found a wide range of applications in the synthesis of model macromolecules over the last 40 years [225-227]. Anionic polymerization is known to be limited to monomers with electron-withdrawing substituents, such as nitrile, carboxyl, phenyl, vinyl etc. These substituents facilitate the attack of anionic species by decreasing the electron density at the double bond and stabilizing the propagating anionic chains by resonance. 

The range of monomers that can be incorporated into block copolymers by the living anionic route includes not only the carbon-carbon double-bond monomers susceptible to anionic polymerization but also certain cyclic monomers, such as ethylene oxide, propylene sulfide, lactams, lactones, and cyclic siloxanes (Chap. 7). Thus one can synthesize block copolymers involving each of the two types of monomers. Some of these combinations require an appropriate adjustment of the propagating center prior to the addition of the cyclic monomer. For example, carbanions from monomers such as styrene or methyl methacrylate are not sufficiently nucleophilic to polymerize lactones. The block copolymer with a lactone can be synthesized if one adds a small amount of ethylene oxide to the living polystyryl system to convert propagating centers to alkoxide ions prior to adding the lactone monomer. 

On sonication, surfactants (4, 5) form vesicles which are polymerized by an initiator or by UV irradiation across either their bilayers or their head groups depending on the position of the double bond (Fig. 8). The polymeric vesicles are stable for extended periods even in 25 % C2H5OH. Efficient charge separation has been realized in such chemically disymmetrical polymerized vesicles. Photoexcitation of Ru(bpy)2 + placed on the outside of the vesicle resulted in the formation of long-lived reduced viologens on the inside. 

Organolithum compounds (lithium alkyls) are the most valuable initiators in anionic polymerization.120168 169172-175 Since living anionic polymerization requires the fastest possible initiation, sec- or ferf-butyllithium is usually used. Lithium alkyls add readily to the double bond of styrene [Eq. (13.32)] or conjugated dienes and form free ions or an ion pair depending on the solvent ... 

Thus, it appears that there is no unambiguous mechanistic interpretation for RLi diene polymerization in nonpolar media and more work is needed. Nevertheless, some 13C-NMR data have been obtained that strongly favor some type of interaction between the positive lithium center and the 7r-electrons of the double bond, e.g., the stronger shift centered on the methine (CH2=C) carbon atom. It also appears that the live-end 1,4-structures in hydrocarbon media are in equilibrium with live ends that give rise to 1,2-structures, as alluded to in the earlier discussion. 

With different allenylcarbinols 41 a nickel-catalyzed living polymerization was possible, but no regioselectivity of the C-C bond formation was observed (40% reaction of the a-double bond, 60% reaction at the /1-double bond in 42) [30],... 

Individual methods have also been devised for the preparation of miktoarm stars. One of these approaches involves the preparation of macromonomers possessing either central or end vinyl groups which can be used to produce miktoarm stars either by copolymerization of the double bonds or by reacting the double bonds with living polymer chains, thus creating active centers able to initiate the polymerization of another monomer. All these methods are limited to specific synthetic problems and cannot be used for the preparation of a wide range of different structures. 

The macromonomer method was used by Fujimoto et al. for the preparation of (PS)(PDMS)(PtBuMA) stars [53], as described in Scheme 20. The lithium salt of the p-(dimethylhydroxy)silyl-a-phenyl styrene was synthesized and used as initiator for the polymerization of hexamethylcyclotrisiloxane (D3). Living PS chains were reacted with the end double bond of the macromonomer, followed by the anionic polymerization of the f-BuMA. 

A similar synthetic route was adopted by Stadler et al. for the synthesis of (PS)(PB)(PMMA) stars [54] as shown in Scheme 21. Living PS chains were end-capped with l-(4-bromomethylphenyl)-l-phenyl ethylene to produce the macromonomer. The capping reaction with DPE was employed in order to reduce the reactivity of the PSLi chain ends thus avoiding several side reactions (trans-metallation, addition to the double bond of the DPE derivative). The next step involved the linking of living PB chains, prepared in THF at -10 °C to the end double bond of the macromonomer. This produces a new active center which was used to initiate the polymerization of MMA leading to the formation of the desired product. 

The reaction of an unsaturated compound with an antagonist function located at the end of a polymer chain is still the most commonly used method to synthesize macromonomers. We have already mentioned some processes that can be used to introduce into the chain end of a macromolecule a functional group, e.g. by deactivation of living carbanionic sites and transfer reactions of various kinds in cationic polymerization. We have also described some methods used to link an active terminal double bond to the chain end originally bearing hydroxy groups. 

These are the most important. The two double bonds mutually activate each other conjugation is essentially not destroyed by addition to the growing chain end. Therefore the conjugated dienes are difunctional monomers. They are polymerized by a relatively simple mechanism. Of all the polymers generated in living tissues, we have so far been able to imitate most closely natural rubber, poIy-cis-l,4-isoprene. Butadiene, isoprene and chloroprene are the dienes most often employed in macro-molecular chemistry. 

Busson and van Beylen [205] studied the role of the cation and of the carbanionic part of the active centre during anionic polymerization in non polar media. They were interested in the problem of complex formation between the cation and the monomer double bond [206] and they therefore measured the reaction of various 1,1-diphenylethylenes with Li+, K+ and Cs+ salts of living polystyrene in benzene and cyclohexane at 297 K. Diphenylethy-lene derivatives were selected for two reasons. 

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