Butadiene polymerization mechanism

In spite of the assortment of things discussed in this chapter, there are also a variety of topics that could be included but which are not owing to space limitations. We do not discuss copolymers formed by the step-growth mechanism, for example, or the use of Ziegler-Natta catalysts to regulate geometrical isomerism in, say, butadiene polymerization. Some other important omissions are noted in passing in the body of the chapter. 

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. 

Currently, more SBR is produced by copolymerizing the two monomers with anionic or coordination catalysts. The formed copolymer has better mechanical properties and a narrower molecular weight distribution. A random copolymer with ordered sequence can also be made in solution using butyllithium, provided that the two monomers are charged slowly. Block copolymers of butadiene and styrene may be produced in solution using coordination or anionic catalysts. Butadiene polymerizes first until it is consumed, then styrene starts to polymerize. SBR produced by coordinaton catalysts has better tensile strength than that produced by free radical initiators. 

The lanthanide oxide cations [LnO]+ and the bare lanthanide ions Ln+ react differently with butadiene (162). Some bare Ln+ ions (La, Ce, Pr, Gd) activate butadiene but their oxide cations are inert toward butadiene. The lanthanides with weak M-O bonds, EuO and YbO, react by oxygen transfer to the butadiene. The oxide cations of Dy, Ho, Er, and Tm activate butadiene, whereas the bare metals of these lanthanides are unreactive with butadiene. The [HoO]+ ion has been studied in detail and is able to polymerize butadiene the mechanism of this reaction has been discussed. 

While both solution and solid-state NMR has been routinely applied to polymers for many years, there have been a few recent applications of HRMAS to polymer systems, analyzing polymerization mechanisms and characterizing the resulting polymers in the swollen state. The vulcanization of butadiene rubber by cyclic disulfides was shown to follow two different mechanisms with two different classes of sulfur compounds - cross-linking progressed... 

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... 

Scheme48 Butadiene polymerization with Nd[N(SiMe3)2]3/[NHMe2Ph] [B(C6F5)4]/R3A1 and proposed activation mechanism [202]...

Mechanism of Butadiene Polymerization Initiated by ir-allylic Complexes... 

As stated earlier, S/DPE copolymers are compatible with GPPS up to a DPE content of about 15wt%. This means that on modifying S/DPE(>15) with S-B(H)-S the impact modification decreases owing to decreased compatibility between the S/DPE matrix and the styrene blocks of the S-B(h>-S impact modifier. S-DPE, however, is prepared anionically and using this polymerization mechanism S/DPE-butadiene S/DPE block copolymers can be prepared. Thus the S/DPE blocks can be tailor-made to be compatible with the S/DPE polymer matrix. For compatibility, the DPE content of the blocks and the matrix should not differ by more than about 15 %. As with the S-B(H)-S block copolymers, the double bonds of the butadiene phase should be removed by hydrogenation. 

A classical radical polymerization mechanism has been applied to butadiene polymerization with azo initiators32). Transfer reactions are absent. 

The polymerization mechanisms for vinyl chloride and acrylonitrile (and also styrene) with coordination catalysts are also uncertain [222] and the copolymerization of butadiene/acrylonitrile (q.v.) also shows some features suggesting the formation of free radicals (or possibly radical-ions from charge transfer complexes). As these polar monomers can react, or form strong complexes, with the organo-metal compound it is likely that the kinetic schemes will be complex. As with styrene there is a good deal of scatter in the experimental kinetic data with these monomers which detracts from the certainty of the deductions, and much work will be required to put their polymerization by coordination catalysts on a sound mechanistic and kinetic basis. 

Development of the elucidation of the catalytic reaction mechanism and the structure-reactivity relationships proceeded much more slowly. By the mid-1960s Wilke [17], Porri [18], and Dolgoplosk [19] had already shown that allyl-transition metal complexes can catalyze the butadiene polymerization stereoselectively and quite probably represent the real catalysts. In particular the allylnickel(II) complexes [Ni(C3Hs)X]2 (X = I [20], CF3CO2 [21]) and more recently the cationic complexes [Ni(C3H5)L2]PFe, with L = P(OPh)3, etc. [22, 23], were also used to explore the catalytic reaction mechanism. 

If the rate of anti-syn isomerization is relatively low, then the cis-trans selectivity can be determined by the formation of the anti- or the 5y/i-butenyl structure, for example from the t] -cis or the if-trans coordinated butadiene, in the catalyst complex. This is the mechanism of stereoregulation which was suggested in the mid-1960s by Cossee and Arlman [34, 35] for titanium-catalyzed butadiene polymerization, and which was reconsidered more recently for the allylne-odymium complex catalysts to explain their cis-trans selectivity [39], But it is also possible that the difference in reactivity between the anti and the syn structure of the catalytically active butenyl complex can determine the cis-trans selec-... 

It was discovered that the addition of 1,3-cyclohexadiene to the Rh -catalyzed reactions increased the rate of butadiene polymerization by a factor of over 20 [20]. Considering the reducing properties of 1,3-cyclohexadiene, this effect could be due to the reduction of Rh to Rh and stabilization of this low oxidation state by the diene ligands. With neat 1,3-cyclohexadiene, Rh is reduced to the metallic state. These emulsion polymerizations are sensitive to the presence of Lewis basic functional groups. A stoichiometric amount of amine (based on Rh) is sufficient to inhibit polymerization completely. It was also discovered that styrene could be polymerized using the Rh catalyst. However, the atactic nature of the polymer, along with the kinetic behavior of the reaction, indicated that a free-radical process, rather than a coordination-insertion mechanism, was operative. 

A free-radical polymerization mechanism can be excluded on the basis of the polymer microstructure and experiments with radical inhibitors. Rhodium(I)-spe-cies, formed by reduction of Rh " salts used as catalyst precursors by butadiene monomer, have been suggested as the active species. The catalyst is stable during the aqueous polymerization for over 30 h [23]. Catalyst activities are moderate with up to ca. 2x10 TO h [24, 25]. By contrast to industrially important free-radical copolymerization, styrene is not incorporated in the rhodium-catalyzed butadiene polymerization [26]. Only scarce data is available regarding the stability and other properties of the polymer dispersions obtained. Precipitation of considerable portions of the polymer has been mentioned at high conversions in butadiene polymerization [23, 27]. 

Polymerization of cis, cis-1,4-dideuterio-l, 3-butadiene by several transition metal catalysts has been studied. The existence of non-stereo-specific bond forming events is postulated to signal the involvement of allyl isomerization in the polymerization mechanism. Trans-1,4-polymers are accompanied by complete scrambling of deuterium stereochemistry, contrasting with a more specific process to form cis polymers. Allyl isomerization is thus implicated as a key event in the formation of trans, but not cis, polymer. 

We have previously used this method to establish allyl isomerization events in nickel catalyzed cyclodimerization of butadiene, and describe here our application to the polymerization mechanism. 

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. 

Butadiene polymerization studies with HMTT/CH2Li catalysts have given results which are directly contrary to those expected from the Hay mechanism. Activity at 0.5 HMTT/CH2Li was double that at 1 1 ratio whereas the reverse should have been obtained if the tetramine solvated lithium compound were the active species. Our lithium catalyst studies suggest that all of the known TMED complexes are active for butadiene polymerization with activity increasing roughly in the order (RLi)4 TMED < (RLi)2 TMED < RLi TMED << RLi(TMED)2. The equilibrium to form RLi(TMED )2 is believed to be unfavorable except when R" is a highly delocalized carbanion. 

The polymer formed from a diene such as 1,3-butadiene contains vinyl branches. Propose an anionic polymerization mechanism to account for the formation of these branches. 

FT-NIR spectroscopy in combination with a fiber-optic probe was successfully used to monitor living isobutylene, ethylene oxide and butadiene polymerizations using specific monomer absorptions. In the case of EO a temperature dependent induction period was detected when 5ec-BuLi/ BuP4 were used as an initiating system. This demonstrates the usefulness of this technique because this phenomenon had not been observed so far by other methods. We have also successfully conducted experiments in controlled radical polymerization. Then we were able to monitor the RAFT polymerization of A -isopropylacrylamide (NIPAAm). Thus it can be expected that with the help of online NIR measurements detailed kinetic data of many polymerization systems will become available which will shed more light onto the reaction mechanisms. Consequently, FT-NIR appears to be a method, which can be applied universally to the kinetics of polymerization processes. 

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