Diene, polymerization

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

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). 

A number of different synthetic rubbers are produced commercially by diene polymerization. Both cis- and frrms-polyisoprene can be made, and the synthetic rubber thus produced is similar to the natural material. Chloroprene (2-chloro-l,3-butadiene) is polymerized to yield neoprene, an excellent, although expensive, synthetic rubber with good weather resistance. Neoprene is used in the production of industrial hoses and gloves, among other things. 

When the cnolate of an enone is brought into reaction with an enone, usually a carbocyclic system is prepared by two consecutive Michael additions (M1MIRC reactions). Due to the lower temperatures employed and the absence of diene polymerization these reactions are useful alternatives for Diels-Alder reactions and proceed in general with high diastereoselectivities. When neither enolate nor enone is cyclic a monocyclic system is formed 338 which can be converted into a bicyclic system when the Michael addition is followed by an aldol reaction339. When, however, the enolate is cyclic a bicyclic or a tricyclic system is formed340 341. 

Measurements of polymerization rate and parallel measurements on the resultant polymer microstructure in the butadiene/DIPIP system cannot be reconciled with the supposition that only one of the above diamine solvated complexes (eg. Pi S) is active in polymerization 162). This is probably true of other diene polymerizations and other diamines. The observations suggest a more complex system than described above for styrene polymerization in presence of TMEDA, This result is clearly connected with the increased association number of uncomplexed diene living ends which permits a greater variety of complexes to be formed. 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

Figure 9. Catalyst activity of lanthanides in diene polymerization. Data from Ref. 22.

Several articles on these topics have appeared in earlier volumes. For the convenience of readers they are listed here. In addition, articles on Ziegler-Natta catalysis and on organolithium compounds in diene polymerization are planned for the next volume of this serial publication. 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

Porri, L. Giarrusso G. Conjugated Diene Polymerization. In Comprehensive Polymer Science, Vol. 4. Eastmond, G. C. Ledwith, A. Russo, S. Sigwalt, P. (Eds.). Pergamon, Oxford, 1989, Chapter 5. 

Catalytic Applications of SOLnC and SOAnC 491 Table 12.8 1,3-Diene polymerization by silica-supported neodymium complexes. 

The stereochemistry of diene polymerizations is also affected by solvent polarity. For instance, the proportion of cA-1,4 units is increased by using organolithium or lithium itself as the initiator in the polymerization of isoprene or 1,3-butadiene in nonpolar solvents. 

Purified diene polymerizes within 24 hr If stored neat. Its lifetime can be indefinitely prolonged by storage as a 10% by weight solution In hexane under an argon atmosphere. 

Acyclic diene metathesis polymerization (ADMET) is a related polymerization in which an unconjugated diene polymerizes with loss of ethene [Lehman and Wagener, 2002, 2003 Schwendeman et al., 2002], ADMET is carried out using the Schrock and Gmbbs initiators at about 40-80°C. The process is a step polymerization, not a ROP chain reaction. The reaction is reversible, and high polymer MW is achieved by removal of ethene (usually by reduced... 

The relative extents of the different structural units in 1,3-diene polymerization are not strongly dependent on polymerization temperature in the range —20 to 50°C [Morton, 1983], Table 8-10 shows other features of 1,3-diene polymerization in nonpolar solvent... 

Mechanisms have been proposed to explain stereoselectivity in 1,3-diene polymerizations, but these processes are less understood than the polymerization of ethylene and 1-alkenes [Peluso et al., 1997]. The ability to obtain cis 1,4-, trans 1,4-, and st 1,2-polymers from 1,3-butadiene, each in very high stereoregularity, hy using different initiators has great practical utility for polymer synthesis even if it is not well understood why a particular initiator gives a particular stereoregular polymer. 

Porri,L. and A. Giarruso, Conjugated Diene Polymerization, Chap. 5 in Comprehensive Polymer Science, Vol. 4, G. C. Eastmond, A. Ledwith, S. Russo, and P. Sigwalt, eds., Pergamon Press, Oxford, 1989. Prabhu, P, A. Shindler, M. H. Theil, and R. D. Gilbert, J. Polym. Sci. Polym. Lett. Ed., 18, 389 (1980). Pregaglia, G. F. andM. Binaghi, Ionic Polymerization of Aldehydes, Ketones, andKetenes, Chap. 3 in The Stereochemistry of Macromolecules, Vol. 2, Marcel Dekker, New York, 1967. 

Currently, topochemical polymerization is opened not only to the diacetylene library but also to the diene library, which is more popular and a larger collection. Diene polymerization has the potential of being applied to the construction of advanced organic materials in the solid state, because the topochemical polymerization of diacetylene and diene monomers provides different types of polymers, that is, conjugate and nonconjugate polymers, respectively (Scheme 8) [16,61]. 

The diene polymerizations catalyzed by organometallic compounds are probably further examples of diene insertion reactions of metal alkyls (25). But there is little evidence on the detailed mechanisms of these polymerizations. 

However, for the purpose of synthesis, they are almost equivalent. Only for diene polymerizations, when high 1,4 unit contents are usually desired, it is of importance to choose nonpolar solvents (and Li as a counter ion) as the polymerization media. 

NMR spectra have been taken on diene polymeric systems by Morton et al. Ji j and of models formed by the addition of t-butyl1ithium to dienes by Glaze et al. L5/ and Bywater et al. i / Compounds with other counter ions have been made from these. 

These efforts coupled with the much earlier work on sodium and lithium initiated polymerizations led to an appreciation of the stereospecificity of the alkyllithium initiators for diene polymerization both industrially and academically. Polymerization of isoprene to a high cis polyisoprene with butyllithium is well known and the details have been well documented 2 Control over polybutadiene structure has also been demonstrated. This report attempts to survey the unique features of anionic polymerization with an emphasis on the chemistry and its commercial applications and is not intended as a comprehensive review. 

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