Isoprene polymerization mechanism

Monomers for manufacture of butyl mbber are 2-methylpropene [115-11-7] (isobutylene) and 2-methyl-l.3-butadiene [78-79-5] (isoprene) (see Olefins). Polybutenes are copolymers of isobutylene and / -butenes from mixed-C olefin-containing streams. For the production of high mol wt butyl mbber, isobutylene must be of >99.5 wt % purity, and isoprene of >98 wt % purity is used. Water and oxygenated organic compounds iaterfere with the cationic polymerization mechanism, and are minimized by feed purification systems. 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

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... 

Although heterobimetallic complexes with alkylated rare-earth metal centers were proposed to promote 1,3-diene polymerization via an allyl insertion mechanism, details of the polymerization mechanism and of the structure of the catalytically active center(s) are rare [58,83,118-125]. Moreover, until now, the interaction of the cationizing chloride-donating reagent with alkylated rare-earth metal centers is not well-understood. Lanthanide carboxylate complexes, which are used in the industrial-scale polymerization of butadiene and isoprene, are generally derived from octanoic, versatic, and... 

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... 

The first Soviet investigation on the modeling of MWD in the ionic polymerization of butadiene and isoprene in solution on butyllithium catalyst was published in 1958m).In this study one can already find all the elements in the scheme of utilizing MWD to specify the polymerization mechanism. 

The alkyl-lithium initiated, living anionic polymerization of elastomers was described in 1928 by Ziegler. To polymerize styrene-isoprene block copolymers Szwarc et al., [1956] used sodium naphthalene as an anion-radical di-initiator, while Shell used an organolithium initiator. The polymerization mechanism was described by By water [1965]. 

O Driscoll, Yonezawa, and Higashimura proposed a mechanism for steric control. In isoprene polymerization the terminal charges are complexed with the metal cations. These cations are close to the active centers through the occupied zr-orbitals of the chain ends and the unoccupied p-orbitals of the lithium ions, hi the transition state the monomers are conqilexed with the cations in die same way. The lithium cations are assumed to be in hybridized tetrahedral sp configurations with four vacant orbitals. The chain ends are presumed to be allylic and the diene monomers are bidentate. During the propagation steps both the monomers and chain ends complex with the same counterions ... 

Isobutylene Polymerization Mechanism. The carbocationic polymerization of isobutylene and its copolymerization with viable comonomers like isoprene and p-methylstyrene is mechanistically complex (9-11). The initiating system is typically composed of two components an initiator and a Lewis... 

Hydrocarbon Monomers.—The flash polymerization on mixing of isobutylene and EtAlClj was studied by Russian workers. A series of publications by a group of Italian workers describes detailed examination of the structure of homo and copolymers of several alkenes and dienes. This enabled polymerization mechanisms to be explained in terms of relative reactivities of the intermediate carbenium ions towards different monomers and their rearrangements involving hydride and methide shifts. Monomers studied were propylene, but-l-ene, isobutylene/ butadiene, isobutylene/rrans-l,3-pentadiene, isobutylene/isoprene, and propylene/but- 1-ene. ... 

Scheme 27.12 Proposed mechanism to account for the increase in 3,4-stereoselectivity of isoprene polymerization under CCTP conditions. PI represents polyisoprene.

Mechanisms depending on carbanionic propagating centers for these polymerizations are indicated by various pieces of evidence (1) the nature of the catalysts which are effective, (2) the intense colors that often develop during polymerization, (3) the prompt cessation of sodium-catalyzed polymerization upon the introduction of carbon dioxide and the failure of -butylcatechol to cause inhibition, (4) the conversion of triphenylmethane to triphenylmethylsodium in the zone of polymerization of isoprene under the influence of metallic sodium, (5) the structures of the diene polymers obtained (see Chap. VI), which differ. both from the radical and the cationic polymers, and (6)... 

The most industrially significant polymerizations involving the cationic chain growth mechanism are the various polymerizations and copolymerizations of isobutylene. In fact, about 500 million pounds of butyl rubber, a copolymer of isobutylene with small amounts of isoprene, are produced annually in the United States via cationic polymerization [126]. The necessity of using toxic chlorinated hydrocarbon solvents such as dichloromethane or methyl chloride as well as the need to conduct these polymerizations at very low temperatures constitute two major drawbacks to the current industrial method for polymerizing isobutylene which may be solved through the use of C02 as the continuous phase. 

First, new "living" initiators have been discovered (although not always as efficient), which respond to other mechanisms, i.e. cationic (5) or even radical ones (6), and can accordingly accomodate other types of monomers. A recent typical example is the coordination polymerization of butadiene by bis (n3-allyl-trifluoro-acetato-nickel) to yield a "living" pure 1.4 cis-poly-butadienyl-nickel, able to initiate in turn the polymerization of monomers like isoprene or styrene (7). 

Early studies (1 ) of the kinetics of polymerization of styrene, isoprene and butadiene in hydrocarbon solvents indicated a half-order rate dependency on growing chain concentration, although there were conflicting data at that time (10, 11) which suggested even lower fractional orders for the dienes. Since the apparent half-order dependency could not be rationalized, as in the case of the polar media, by an ionic dissociation mechanism, some other form of association-dissociation phenomenon offered a possible answer. In view of the known tendency of organolithium compounds to undergo molecular association in non-polar media, the following scheme was proposed by us (l) ... 

So, it seems that there exists a discrepancy about the mechanism of polymerization. Until this is resolved the proposed mechanism of the reaction must be in doubt. Therefore, we have realized a systematic study (UV, NMR, kinetic) of the influence of several factors determining the reactivity of active centres in the case of the polymerization of isoprene (17, 18, 19). 

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