Polar solvents, polymerization dienes

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

Two-shot techniques for acyclic diene metathesis, 435-445 for polyamides, 149-164 for polyimides, 287-300 for polyurethanes, 241-246 for transition metal coupling, 483-490 Anionic deactivation, 360 Anionic polymerization, 149, 174 of lactam, 177-178 Apolar solvents, 90 Aprotic polar solvents, 185, 338 Aprotic solvents, low-temperature condensation in, 302 Aqueous coating formulations, 235 Aqueous polyoxymethylene glycol, depolymerization of, 377 Aqueous systems, 206 Ardel, 20, 22... 

The anionic polymerization of 1,3-dienes yields different polymer structures depending on whether the propagating center is free or coordinated to a counterion [Morton, 1983 Quirk, 2002 Senyek, 1987 Tate and Bethea, 1985 Van Beylen et al., 1988 Young et al., 1984] Table 8-9 shows typical data for 1,3-butadiene and isoprene polymerizations. Polymerization of 1,3-butadiene in polar solvents, proceeding via the free anion and/or solvent-separated ion pair, favors 1,2-polymerization over 1,4-polymerization. The anionic center at carbon 2 is not extensively delocalized onto carbon 4 since the double bond is not a strong electron acceptor. The same trend is seen for isoprene, except that 3,4-polymerization occurs instead of 1,2-polymerization. The 3,4-double bond is sterically more accessible and has a lower electron density relative to the 1,2-double bond. Polymerization in nonpolar solvents takes place with an increased tendency toward 1,4-polymerization. The effect is most pronounced with... 

There is no mechanism that adequately explains all features of the anionic polymerization of 1,3-dienes. NMR data indicate the presence of it- and cr-bonded propagating chains (L and LI) When reaction occurs in polar solvents, the carbanion center is delocalized as both... 

Kinetics in Non-Polar Media. Polymerization of vinyl monomers in non-polar solvents, i.e., hydrocarbon media, has been almost entirely restricted to the organolithium systems (7), since the latter yield homogeneous solutions. In addition, there has been a particularly strong interest in the polymerization of the 1,3-dienes, e.g., isoprene and butadiene, because these systems lead to high 1,4 chain structures, which yield rubbery polymers. In the case of isoprene, especially, it is possible to actually obtain a polymer with more than 90% of the eis-1,4 chain structure (7, 8, 9), closely resembling the microstructure of the natural rubber molecule. 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, arc efficient difunctionai initiators, However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization. 

Most of these compounds show a limited solubility in non-polar solvents. In addition, the respective alkyl derivatives are rather unstable in solution and decompose easily [297]. The peculiarity about CpNd complexes is their ability to polymerize various alkenes such as a-olefines, styrene, c -dienes as well as polar acrylates [298,299]. 

The photochemistry of 1,3-dienes can be highly dependent on the diene structure and reaction conditions. Important variables include the ground state conformation [22,23], the reaction concentration, the use (or not) and properties of a triplet sensitizer [14] or an electron acceptor [18], and solvent polarity. The simplest dienes also often yield the most complex chemistry. For example, 1,3-butadiene 3 undergoes unimolecular isomerization in dilute solution to give only cyclobutene 4 and bicyclobutane 5 (Sch. 2), and polymerization in concentrated solution [24]. At intermediate... 

Defined in this way, anionic polymerizations can only be expected when the cation is derived from one of the most electropositive metals, the cation is strongly solvated, and the polymer anion is highly stabilized by resonance. These conditions are frequently met with sodium (206—208) or potassium (209, 210) catalysts in basic solvents with polar monomers or dienes. 

In anionic polymerization of vinyl monomers (nondiene), low temperatures and polar solvents favor the preparation of syndiotactic polymers.21 Nonpolar solvents tend to favor isotactic polymerization. In the case of diene monomers such as butadiene and isoprene, the use of lithium based initiators in nonpolar... 

Although this was one of the first examples of a living anionic polymerization, there are a number of drawbacks. A high level of polar solvent must be present, so that a diene block formed by this process will have a fairly high vinyl (low 1,4) content and the polymerization must generally be executed at a low temperature. It is also difficult to determine quantitatively the initiator concentration, so control of the molecular weight is difficult. A second approach involves addition of 2 mol of a lithium alkyl to a nonpolymerizable diolefin... 

Polymerization of styrene and dienes in polar solvents 4.1 THE INITIATION STEP... 

The diene monomers give predominantly 1,4-polymers in hydrocarbon solvents if polymerized using lithium-based initiation. Isoprene, under these conditions, gives a predominantly cis-1,4 polymer but with butadiene the proportions of cis- and frans-1,4 are fairly evenly distributed. Once ain this phenomenon is characteristic of lithium compounds sodium- and potassium-based initiation gives mixed structures even in hydrocarbon solvents. Polymerization in polar solvents such as tetrahydrofuran leads to largely 3,4-polyisoprene or 1,2-butadiene with... 

The reactive intermediates used in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes. Of the three common metal catalyzed polymerizations - coordination-insertion, ring-opening metathesis and diene polymerization - the last appears to possess the greatest tolerance toward protic solvents. The polymerization of butadiene in polar solvents was first reported in 1961 using Rh salts [18]. It was discovered that these polymerizations could be performed in aqueous solution with an added emulsifier (sodium dodecyl sulfate, for example). 

Alkyllithium initiators yield stereoregular polymers of conjugated dienes if the polymerization is carried out in hydrocarbon solvents. Addition of tetrahydrofuran or other more polar solvents changes the microstructure of the polymers that are produced... 

Hydrocarbon solubility is important for diene polymerization, where a high 1,4 polymer microstructure is often desired. Polar solvents have a tendency to decrease the 1,4 content and elevate the 1,2 addition product. Hydrocarbon solubility is less of an issue for styrene or epoxide polymerization. Thus, the hydrocarbon soluble t-butyldimethylsilyl (TBDMS) protected initiator (5) (Table I) is recommended for high 1,4 polydienes. Most of the other conpounds in Table I can be used with other anionic monomers or where high 1,4 microstructure is not needed. 

Cumyl potassium (pTf 43 based on toluene) [2] is a useful initiator for anionic polymerization of a variety of monomers, including styrenes, dienes, methacrylates, and epoxides. This carbanion is readily prepared from cumyl methyl ether as shown in Equation 7.9, and is generally used at low temperatures in polar solvents such as THE [63]. 

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes, because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cw-1,4 in structure. The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-lA in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. 

Anionic polymerization requires unsaturated monomers having substituents that stabilize the negative charge on the active centre (electrophilic substituents) and initiators based on the most electropositive elements. Among initiators for diene polymerizations, or-ganolithiums are nearly ideal compounds. This stems from some of their main characteristics listed in Table 1. Solubility in non-polar solvents results from their covalent character and their ability to form electron-deficient bonding giving rise to associated forms, usually tetramers or hexamers. 

Dienes, such as cyclopentadiene, are often used to trap the more reactive ketenes, which otherwise would undergo oligomerization or polymerization reactions. In the reactions of monoketenes with cyclopentadiene endo cycloaddition products are usually obtained, with the exception of t-butylketene, where the exo adduct is predominantly formed . In the reaction of MeC(X)=C=0 (X = Cl, Br) with cyclopentadiene, a strong solvent effect on the exo/endo ratio is observed . The more polar solvents favor the exo isomer, indicating the presence of a polar intermediate. 

It is possible to polymerize conjugated dienes into polymers with more cu-1,4 segments using ionic initiators. In particular it is found anionic initiators such as lithium or organolithium compounds in non-polar solvents will polymerize isoprene to produce a product which is very similar in structure to natural rubber (c -l,4-polyisoprene). It is thought that the Li ion holds the isoprene molecule in a cisoid conformation through the formation of a rr-complex type of structure. 

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