Butadiene polymerization, reaction scheme

The (butadiene) ansa-zirconocene-derived complexes 130 (RCp, R = H, Me, cyclohexyl, isopropyl, tert-butyl) are active catalysts for the polymerization of methylmethacrylate.149 The isotacticity of the resulting PMMA increases rapidly with increasing bulk of the attached substituents.150 The initial intermediate (156) of the polymerization reaction was observed by NMR spectroscopy124,149 (Scheme 54). 

To explain these changes in selectivity, a reaction scheme for allyllithium-catalyzed butadiene polymerization is formulated (Scheme 4), which is derived tentatively on the basis of experimental and theoretical investigations of the structure and reactivity of the allyllithium compounds. 

Scheme 4. Reaction scheme for the allyllithium-catalyzed 1,4- and 1,2-polymerization of butadiene in a weakly or noncoordinating solvent. L, donor ligand for the butadiene only the jr-coordination in the single cis form is shown.

There are many possible schemes for addition reactions of diene monomers from electronical and steric viewpoints. Because the monomer molecules arrange along the direction of the channels, a,co-addition may selectively take place in one-dimensional inclusion polymerization. Therefore, conjugated polyenes, such as dienes and trienes, may selectively polymerize by 1,4- and 1,6-addi-tion, respectively. 1,3-Butadiene polymerized via 1,4-addition exclusively in the chaimels of urea and perhydrotriphenylene. while the same monomer polymerized via both 1,2- and 1,4-additions in the channels of deoxycholic acid and apocholic acid. Moreover, we have to evaluate head-to-tail or head-to-head (tail-to-tail) additions in the case of dissymmetric conjugated diene monomers such as isoprene and 1.3-pentadiene. 

It is known that CoCl2/Al(C2H5)Cl can polymerize 1,3-pentadiene but not the cis isomer. This suggests that a two-point coordination is required. Several reaction schemes that provide for an attack at either Ci or C4 position were proposed over the years. One mechanism for polymerization of butadiene suggests that complexes of the catalysts in solvents of low dielectric constant will either act as ion pairs or as independent solvated entities. Also, the growing chain may be bound by either a tt or a a linkage, and it is suspected that a continuous a- n isomerism is possible ... 

Dupont and co-workers studied the Pd-catalyzed dimerization [108] and cyclodimerization [109] of butadiene in non-chloroaluminate ionic liquids. The biphasic dimerization of butadiene is an attractive research goal since the products formed, 1,3,5-octatriene and 1,3,6-octatriene, are sensitive towards undesired polymerization, so that separation by distillation is usually not possible. These octa-trienes are of some commercial relevance as intermediates for the synthesis of fragrances, plasticizers, and adhesives. Through the use of PdCl2 with two equivalents of the ligand PPhj dissolved in [BMIM][Pp6], [BMIM][Bp4], or [BMIM][CF3S03], it was possible to obtain the octatrienes with 100 % selectivity (after 13 % conversion) (Scheme 5.2-23) [108]. The turnover frequency (TOP) was in the range of 50 mol butadiene converted per mol catalyst per hour, which represents a substantial increase in catalyst activity in comparison to the same reaction under otherwise identical conditions (70 °C, 3 h, butadiene/Pd = 1250) in THF (TOP = 6 h ). 

The monomer addition scheme, shown at the top, requires an initiator which is capable of removing a hydrogen atom from the allylic position of the butadiene, resonance stabilization of the radical from AIBN does not permit this initiator to effect this reaction while benzoyl peroxide is capable of reaction to remove a hydrogen atom and initiate the reaction. On the other hand the polymeric radical addition scheme requires that homopolymerization of the monomer be initiated and this macroradical then attack the polymer and lead to the formation of the graft copolymer. Huang and Sundberg explain that the reactivity of the monomer... 

The free-radical kinetics described in Chapter 6 hold for homogeneous systems. They will prevail in well-stirred bulk or solution polymerizations or in suspension polymerizations if the polymer is soluble in its monomer. Polystyrene suspension polymerization is an important commercial example of this reaction type. Suspension polymerizations of vinyl ehloride and of acrylonitrile are described by somewhat different kinetic schemes because the polymers precipitate in these cases. Emulsion polymerizations aie controlled by still different reaetion parameters because the growing macroradicals are isolated in small volume elements and because the free radieals which initiate the polymerization process are generated in the aqueous phase. The emulsion process is now used to make large tonnages of styrene-butadiene rubber (SBR), latex paints and adhesives, PVC paste polymers, and other produets. 

Metallo-ene reactions involving the transfer of palladium and nickel have been described since the early 1960s, mostly in cormection with their crucid role in the Pd- and Ni-catalyzed polymerization of butadiene." Hence, insertions of 1,3-dienes into allylpalladium compounds were extensively studied. Thus preformed allylic complexes (49) underwent the metallo-ene reactions (49) —> (51) at 20 C (20 h) or 35 °C (<5 min) or 70 °C 0 h) the reaction rate depended on the substituents R and as well as on the ligand L and decreased in the order R = Cl > H > Me R = H > Me L = Feacac > acac > Cl. Further diene insertion into the resulting allylpalladium product (51) (polymerization) was generally slower than the initial step (49) —> (51) and again relies on the nature of the ligand L (Scheme 11). ... 

Butadiene is polymerized by rhodium compounds in aqueous or alcoholic solution [178]. It is generally accepted that the active species is a TT-allyl rhodium complex of low valency [28, 179] which is not rapidly terminated by reaction with water or alcohol. No clear kinetic pattern was observed in the earlier papers but a recent investigation [180] has shown the rate and molecular weight data to be accommodated by a scheme involving monomer transfer and physical immobilization of the active centres in precipitated polymer. In the initial stages the polymerization is first order in rhodium and, at constant monomer concentration, is (pseudo) zero order E = 14.8 kcal mole" ). This is followed by a declining rate which is almost independent of temperature. Molecular weights rise slowly to a maximum value with time (ca. 4000 after 22 h at 70°C). 

Scheme 5. Tentative reaction mechanism for the allylcobalt(I) complex-catalyzed syndiotactic 1,2-polymerization of butadiene.

Scheme 7. The catalytic reaction mechanism of trans-1,4 polymerization of butadiene with the bis(triphenylphosphite) complex [Ni(7 -RC3H4)(P(OPh3)2]PF6 as the catalyst.

In Scheme 8 the reaction mechanism for the cis-lA polymerization of butadiene with the polybutadienylnickel(II) cation as the catalyst is shown. 

---