Isoprene, 1,4-polymerization

Another group of isoprene polymerization catalysts is based on alanes and TiCl. In place of alkyl aluminum, derivatives of AlH (alanes) are used and react with TiCl to produce an active catalyst for the polymerization of isoprene. These systems are unique because no organometaHic compound is involved in producing the active species from TiCl. The substituted alanes are generally complexed with donor molecules of the Lewis base type, and they are Hquids or soHds that are soluble in aromatic solvents. The performance of catalysts prepared from AlHCl20(C2H )2 with TiCl has been reported (101). 

The association phenomena occurring with alkyllithium initiators in nonpolar solvents results in very low polymerization rates. A typical styrene or isoprene polymerization by... 

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... 

Evidence for the absence of termination or transfer reactions in the organolithium-initiated polymerization of styrene and iso-prene is shown in Table I for representative examples of these polymers. It can be seen that these polymers exhibit the expected low kfo/Mn values, except in the case of the isoprene polymerized in the H4furan, where a slow side reaction seems to occur between the solvent, on the one hand, and both the initiator (Ifo vs Mg) and the growing chains (1% vs Mn)>on the other hand. 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

A more detailed description of the mechanism of isoprene polymerization by lithium compounds has been given (99, 104). The poly-isoprenyllithium first complexes with isoprene in the cis-form. The complex subsequently rearranges to form a transition state in the form of a six-membered ring. H CH... 

Polymerization. Isoprene polymerization can proceed by either 1.4- ttr 1.2- tv ins 11 addition. 

Contrarily, the rate of isoprene polymerization has been found to decrease when TMEDA was added 92)188>. if isoprene is analogous to styrene92), the direction of change in rate on adding TMEDA will depend upon the concentration of chain end. 

In other words, high temperatures would shift the equilibrium in favor of the dissociated species 10 which would, in turn, favor the 1,4-addition product. H-NMR and UV data for isoprene polymerization by n-BuLi-TMEDA also support this notion (28a). 

Activation of such homoleptic trimethylaluminum adduct complexes with 1 -3 equivalents of the chloride source Et2AlCl produced binary catalyst mixtures revealing the expected activity and cis- 1,4-stereospecificity in isoprene polymerization (Table 9) [142]. High-yield polymeric materials with relatively narrow molecular weight distributions were obtained. No activity was observed for a n(Cl) n(Ln) ratio of 3 1. 

Preformation of homoleptic lanthanide tris(tetramethylaluminate) complexes Ln(AlMe4)3 (Ln = Y, La, Ce, Pr, Nd, Sm, Gd) and Et2AlCl generated extremely active and highly cis-1,4-slcrcosclective binary diene polymerization initiators (Fischbach et al., 2006, personal communication) [150]. Optimal performance in isoprene polymerization was observed for most of the lanthanide metal centers for (Cl) (Ln) ratios of 2 1 (Table 16). Only the samarium... 

Metathesis of the borohydride ligand for cyclopentadienyl in a reaction with KCp or diorganomagnesium compounds gives half sandwich borohydride complexes [Mg(THF)6][Cp Ln(BH4)3] (Cp =Cp, Cp C5H2Ph3, Ln — La, Nd) that are stable to redistribution. Combined with dia-lkylmagnesium, the Nd complexes provide useful catalysts for stereospecific isoprene polymerization.62... 

The rotation barrier about the fi-y bond is not negligible, so that the centres exist in either the cis or trans forms (Z, E) mutual transitions between these two forms only occur under certain conditions and at a certain rate. In diene polymerizations in hydrocarbon medium with Li+ as counter-ion, monomer addition transforms a cis centre into a cis segment of the chain, and a trans centre into a trans polymer (the former case is typical for isoprene polymerization in THF the equilibrium is shifted towards the cis form over the whole of the available temperature range). A cis tram transition is, of course, not excluded (for example with butadienyllithium), and when it is more rapid than addition, chain stereospecificity is reduced. 

Zimmermann, M., Tornroos, K.W., and Anwander, R. (2008) Cationic rare-earth-metal half-sandwich complexes for the living trans-l,4-isoprene polymerization. Angewandte Chemie IntemationalEdition, 47, 775. 

Zhang, L.X., Nishiura, M., Yuki, M. etal. (2008) Isoprene polymerization with yttrium amidinate catalysts switching the regio- and stereoselectivity by addition of AlMes. Angewandte Chemie International Edition, 47, 2642. 

Table IV. Isoprene Polymerization with Ziegler-Natta Catalysts...

Initiation with amides has also been investigated. Tait and his co-workers showed that in hydrocarbons, diethyl ether and tetrahydrofuran isoprene polymerization can be readily initiated with LiN(C2Hs)2, while polymerization of styrene could be achieved only in tetrahydrofuran and dimethoxyethane. °° More detailed evidence in the isoprene system suggests a mechanism in hydrocarbon solvents where the essentially insoluble amide is slowly dissolved by complexation with isoprene, a similar interaction with the active centre occurring during propagation (Scheme 16). The characterization of... 

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