Isoprene, stereospecific polymerization

In the mid-1950s, the Nobel Prize-winning work of K. Ziegler and G. Natta introduced anionic initiators which allowed the stereospecific polymerization of isoprene to yield high cis-1,4 stmcture (3,4). At almost the same time, another route to stereospecific polymer architecture by organometaHic compounds was aimounced (5). 

Stereospecific polymerization has particular significance for the preparation of stereoregular polymeric dienes. In the radical polymerization of butadiene or isoprene the molecular chains always consist of varying proportions of adjacent cis- and trans-1,4-units as well as 1,2- and 3,4- linked units, depending on the polymerization conditions but it is now possible, using particular ionic initiation systems to make a synthetic natural rubber that contains more than 90% cfs-l,4-isoprene repeating units (see Example 3-21). 

Stereospecific Polymerization of Isoprene with Butyllithium in Solution... 

From such data it is then possible to derive the absolute values of the propagation rate constants representing the interaction of the growing chain with the incoming monomer unit. These are shown for isoprene and styrene in Table II. Obviously these propagation rate constants are all characterized by a low activation energy as well as an extremely low frequency factor. This is true both for the non-stereospecific polymerization of isoprene in THF as well as for the stereospecific case of isoprene in hexane (although it seems somewhat more extreme in the latter and for the styrene in benzene). 

Annex 2 Stereospecific Polymerization of Conjugated Diolefins Butadiene and Isoprene... 

The discovery of. stereospecific polymerization methods, which led to the production of practically pure cis-1,4 polyisoprene (natural rubber contains 85 per cent of this isomer), raises the problem of the economic production of isoprene monomer (bPi.013 = 340c, df =0.681 >). 

The catalysis of the stereospecific polymerization of conjugated dienes is of considerable interest from both the scientific and the industrial points of view [1,2]. From butadiene and isoprene, as the industrially most important 1,3-dienes, in comparison with the polymerization of olefins many more structurally different stereoregular polymers can be derived cf the structures of the stereoregular polybutadienes and polyisoprenes given in Scheme 1 [106]. 

Stereospecific Polymerization of Butadiene or Isoprene 309 Cobalt-Polybutadiene... 

This review article is concerned with chemical behavior of organo-lithium, -aluminum and -zinc compounds in initiation reactions of diolefins, polar vinyls and oxirane compounds. Discussions are given with respect to the following five topics 1) lithium alkylamide as initiator for polymerizations of isoprene and 1,4-divinylbenzene 2) initiation of N-carboxy-a-aminoacid anhydride(NCA) by a primary amino group 3) activated aluminum alkyl and zinc alkyl 4) initiation of stereospecific polymerization of methyloxirane and 5) comparison of stereospecific polymerization of methyloxirane with Ziegler-Natta polymerization. A comprehensive interpretation is proposed for chemistry of reactivity and/or stereospecificity of organometallic compounds in ionic polymerizations. 

Stereospecific polymerization of 1,3-dienes (10-18) (to butadiene) and isoprene homo- and copolymers), dimerization of propene (19) and recently stereospecific polymerization of acetylene (20) to high cis-content polyacetylene have all been reported using lanthanide catalysts. Sen (21) has reported the preparation of cationic europium systems (which perhaps function as cationic initiators) for polymerization of norbornadiene and 1,3-cyclohexa-diene. 

How many different structures are possible, in principle, from isoprene polymerization Why does no 1,2-polymerization occur with this monomer Mention some catalysts for stereospecific polymerization of isoprene. 

Organolanthanide(III) based binary initiator systems were used by Yu et al. [65] for stereospecific polymerization of butadiene and isoprene. Typically, the po-... 

The attractive feature of this mechanism is its ability to account for either A, A, or any in between order of propagation of the presumably dimeric living polymers. Hence, it is applicable to the styrene as well as to the isoprene systems. Nevertheless, this writer has reservations about its validity because the stereospecific polymerization of lithium isoprene seems to demand the presence of Li+ cation in the transition state of propagation. 

While earlier attempts to produce satisfactory synthetic rubber from iso-prene were unsuccessful, in 1955 American chemist Samuel Emmett Horne Jr. (b. 1924) prepared 98 percent czr-l,4-polyisoprene via the stereospecific polymerization of isoprene. Home s product differs from natural mbber only in that it contains a small amount of rfr-l,2-polyisoprene, but it is indistinguishable from natural mbber in physical properties. First produced in 1961, BR (for butadiene mbber), a mbberlike polymer that is almost ex-clnsively czr-1,4-polybutadiene, when blended with natural or SBR mbber, has been nsed for tire treads. 

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