Cryptands, anionic polymerization

Table 2. Anionic Polymerization of Masked Disilene 7 initiated by Silyl Anions in the Presence of Cryptand in Benzene at Room Temperature...

Crown ethers and cryptands have also found widespread use as modifiers in the polymerization process (79MI52104.81MI52110). Anionic polymerization is strongly affected by the addition of crown ethers and cryptands which cause increases in the polymerization rate and in the yield and molecular weight of the final polymer. 

Anionic Polymerization of Da. It has been recently shown that addition of cryptands markedly increases the rate of the bulk polymerization of D4 Initiated by KOH at 160°C (35). 

The complex Me3SiCH2SiMe20Li (2) (7Li shift 0.74 ppm) is the only product identified in the reaction of 1 (7Li shift 2.5 ppm) with D3 or D4 at 20 °C in toluene even in the presence of excess siloxane, as shown in equation 1. Addition of the cryptand [211] shifted the Li resonance to —0.93 ppm in agreement with other lithium cryptand [211] complexes. This lithium silanolate was then shown to initiate polymerization of D3, D4, Dg and functional cyclics such as (SiMe(HC=CH2)0)4 and (SiMe(CH2CH2CF3)0)3. Kinetic measurements using this initiator show a reactivity order of D3 D4 >Ds >Dg and the results are in good agreement with those previously reported for anionic polymerization under similar conditions. Co-polymerization reaction involving vinyl dimethyl cyclics... 

By the addition of crown ethers or cryptands, the rate of the anionic polymerization of methylthiiran, /Mactones, and other monomers may be enhanced by up to two orders of magnitude [218-220]. 

A method for preparing homopolymers or copolymers of oxiranes by anionic polymerization using s-butyl lithium and triisobutylaluminum but without crown ethers or cryptands during the polymerization process is described. 

It has been shown that addition of cryptands markedly increases the rate of the bulk polymerization of D4 initiated by KOH at 160 C (19) The anionic polymerization of D4, in toluene, at room temperature, with Li + [211] as counterion appears to follow a different course than in the case of D3. The rate of polymerization is much lower and cyclic oligomers formation is Important as can be seen from the results of Figure 3 for [C] 2.76 x 10 mole.l . ... 

Other areas of interest include stabilization of noncommon oxidation states, solvent extraction of cations, transfer of cations through membranes, isotopic separation, detoxification of harmful and radioactive metals, metal recovery, metal trace analysis, ion chromatography on polymer-supported cryptands, and chromo- and fluoro-ionophores. More organic-chemistry-orientated applications can be mentioned, including enhancement of metal salt solubility in organic solvents, anion activation, phase-transfer catalysis, and anionic polymerization. Many of these applications are covered in other articles in this encyclopedia as well as in the literature. 

Although the reactivity increase caused by crown ethers and cryptands in anionic polymerizations has already found a wide range of application, more details have been reported and a number of questions concerning the type and the behavior of the different species present, both in the initiation and in the propagation steps, have been clarified by, for example, kinetic studies [235], Special polymerization reactions that were effected in the presence of crown compounds are those starting with butadiene, propene, styrene, 2-vinyl pyridine, ethylene oxide, propylene sulfide, isobutylene sulfide, methyl methacrylate, p-propyllactone, or e-caprolactone as monomers and alkali metals as initiators [238-246],... 

Organometallics as well as metals such as sodium or potassium are well Known as initiators for anionic polymerizations. Moreover, in order to increase or modulate the reactivity of these classical initiators, the influence of additives (such as tertiary amines, linear and macrocyclic polyethers, cryptands, alcohols and alKoxides) has been the subject of many worKs l l. 

Complexation of alkali metal counterions was shown to significantly increase the reactivity of propagating species in anionic polymerization. Besides the use of crown ethers or cryptands, the use of large organic molecules as metal-complexing agents or directly as organic counterions has been studied. 

The polyaminophosphazene base t-BuP4 was used in combination with alkyllithium as initiator for the anionic polymerization of EO (Scheme 15(a)). The space inside the molecule is sufficient to host the compact lithium cation and the base works as a cryptand for Li ions with the polar amino and imino groups located inside the globular molecule and the outer shell formed by alkyl substituents. The equilibrium between complexed lithium alkoxide ion pairs and reactive free anions is thus shifted allowing polymerization. 

This effect has been shown to be more effective in media of low permittivity, which are more sensitive to a gain in dissociation. Indeed, addition of cryptand has only doubled the rate of polymerization for 2-p)T rolidone, while it has enhanced the rate by two orders of magnitudes for 2-piper-idone. The same influence has been found in the anionic polymerization of CL conducted in THF at low temperature (25 C). ... 

Figure 4. GPC Profiles of Polysilane in the Propagation Process of the Anionic Polymerization in Benzene Initiated by llc-Cryptand[2.1.1].

In order to avoid the multiple deprotonation observed during P-lactone polymerizations, in 1976 both BoUeau and Penczek showed, independently, that the introduction of macrocyclic ligand such as a crown ether [31] or a cryptand [32] in the anionic polymerization of PL (initiated with potassium acetate) would lead to a living polymerization (Scheme 9.7). As shown later by same authors, the M of the growing polyester became a linear function of monomer conversion, while the semilogarithmic monomer conversion kinetic plot was a Unear function of time [33]. 

In order to avoid the SET process, we chose diphenylmethylsilyl anions (PI MeSiM 8a, M = K 8b, M = Na 8c, M = Li) as initiators for 7 instead of alkyllithium and benzene as a solvent. The polymerization did not take place in benzene with silyl anions alone. However, in the presence of an equimolar amount of suitable cryptands, the silyl anions initiated the polymerization. The results are summarized in Table 2. The molecular weights of polysilylenes thus obtained were in good agreement with the calculated values within experimental error. 

The redox chemistry of [4]radialenes shows similarities as well as differences with respect to [3]radialenes (see elsewhere1 for a more detailed comparison). The simplest [4]radialene for which a redox chemistry in solution is known appears to be octa-methyl[4]radialene (94). It has been converted into the radical anion 94 (with potassium, [2.2.2]cryptand, THF, 200 K) and into the radical cation 94 + (with AICI3/CH2CI2, 180 K)82. Both species are kinetically unstable, but the radical cation is less stable than the radical anion and disappears even at 180 K within 2 hours, probably by polymerization. For the success of the oxidation of 94 with the one-electron transfer system... 

In this method, attack by an anionic initiator ( -BuLi, potassium alkoxides/cryptand[2.2.2],62 or silyl anions in benzene)63 occurs regioselectively on the less hindered silicon of 9, resulting in an anionically terminated disilanyl-lithium which then attacks another monomer at the less hindered silicon atom. The process continues rapidly (the reaction is usually complete within a few minutes) in a living polymerization fashion to yield 10 on alcohol workup. 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

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