Activity of butadiene polymerization centers at in situ formation of titanium catalysts

Directional action on the activity of macromolecule propagation centers and their typical set in synthesis of stereoregular polydienes in the presence of polycentered Ziegler-Natta catalytic systems allows control over the molecular characteristics of the polymers obtained and over the service characteristics of products thereof.     

The nature of active sites in butadiene polymerization catalyzed by organotitanium(III) derivatives

The stereospecificity of benzyl derivatives of trivalent titanium (R_n TiX_3–n, where X = Cl, I, n = 1–3) in butadiene polymerization was studied. It was found that dibenzyltitaniumiodide is an efficient catalyst of the 1,4-cis-polymerization of butadiene and that tribenzyltitanium forms 1,2-units. In both cases all the titanium-benzyl bonds participated in the initiation reaction and the active sites were polymeric analogues of crotyl derivatives of Ti(III); namely, bis-π-oligobutadienyltitaniumiodide and tris-π-oligobutadienyltitanium. These sites are stable at room temperature. The nature of the active sites in the polymerization of butadiene with Ziegler's 1,4-stereo-specific systems Til₄ (or Til₂Cl₂) + AIR₃ are described.     

Reducing consumption of titanium catalyst in stereospecific polymerization of butadiene

The possibility of reducing the consumption of titanium catalyst in butadiene polymerization by using a tubular turbulent prereactor of the diffuser-confuser design in the step of mixing the reaction mixture components was examined.     

Polymerization by transition metal derivatives. XII. Factors controlling activity and stereospecificity in the 1,4 polymerization of butadiene by monometallic nickel catalysts

In the course of investigations of polymerization of diolefins by transition metal derivatives, we have synthesized various monometallic nickel coordination catalysts. The complexes were prepared by reacting 2,6,10-dodecatriene-1,12-diyl nickel with protonic acids; they were shown to initiate the stereospecific polymerization of 1,3-butadiene. The study of these catalysts showed the strong influence of the nature of the counteranion used on the stereospecificity and the polymerization rate. Moreover, by adding various ligands, we were able to modify the behavior of the catalytic systems and to prepare either pure cis-1,4 or pure trans-1,4 or cis–trans equibinary polybutadienes, starting from the same complex and keeping a high 1,4 specificity. Some of these modifications were shown to be reversible.     

Structural and kinetic continuum in the butadiene polymerization with titanium catalysts

The short-time polymerization of butadiene induced by heterogeneous titanium catalysts was studied for the first time. In the 0.1?0.3 s time interval, polymerization is characterized by a considerable decrease in the chain propagation rate constant and 1,4-stereospecificity of the catalysts. A structure kinetic continuum model was proposed in which the initiation step and the first propagation steps are kinetically continuous. The violation of this continuity changes the catalyst stereospecificity and creates conditions for chain transfer reactions.     

Stereoregularity and regioregularity of active centers in propene polymerization

The relation between stereoregularity of active centers on a MgCl₂/TiCl₄ catalyst and functions of inside donor (ID) and outside donor (OD) was investigated in the case of ethyl benzoate (EB)/methyl p-toluate (MPT) as an ID/OD pair. The results indicate that stereregularity depends merely on the amount of MPT supported on the catalyst. On the other hand, regioregularity of active centers was investigated with a MgCl₂/TiCl₄/dioctyl phthalate(DOP)-Et₃Al/diphenyldimethoxysilane(DPDMS) catalyst system. Regio-irregular inserted units were detected only in end groups of PP. It indicates that regio-irregular insertion leads to dormant centers with respect to propene insertion, though such centers are active for hydrogen transfer.     

Temperature dependence of stereospecificity of active sites in heterogeneous catalysts for propylene polymerization

Temperature dependences of the stereoregularity parameters of the most stereospecific active centres of α-TiCl₃-AlEt₃ catalytic system from ?25 to 120 and of VCl₃-AlEt₃ catalytic system from ?15° to 90° have been measured. It was found that this temperature dependence could be represented by a curve with a minimum at 20–50. The results could be explained by a two-step mechanism of isotactic chain growth (propagation) with preliminary co-ordination of monomer on the active centres.     

Electronic structure of active centers and kinetic stereocontrol in polymerization of butadiene under the influence of a catalytic system based on neodymium chloride

On the basis of quantum-chemical calculations of models of active centers, a suggested mechanism of kinetic stereocontrol of the microsctructure in polymerization of butadiene under the influence of the catalytic system NdCl₃ · 3TBP-HA1 (i-C₄H₉)₂ has been verified experimentally.Institute of Chemistry, Bashkirian Scientific Center, Urals Branch, Russian Academy of Sciences, Ufa. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 6, pp. 741–745, November–December, 1991. Original article submitted August 9, 1990.     

Structure and reactivity in cationic polymerization of butadiene derivatives. III. 2-phenylbutadiene

Cationic polymerization of 2-phenylbutadiene (2-PBD) has been investigated. Polymerization were performed by SnCl₄·TCA, WCl₆, and BF₃·OEt₂ as catalysts in methylene chloride. 2-PBD polymerized easily and gave low molecular weight polymers. The polymerization proceeded to give a polymer having 1,4-structure without 1,2- or 3,4-structure. The double bonds of the polymer were partially consumed, probably owing to cyclization and chain-transfer reactions. 2-PBD was 0.66 times as reactive as styrene and 1.2 times as reactive as isoprene in the copolymerization at ?78°C by SnCl₄·TCA in methylene chloride. Reactivities of ring-substituted 2-PBD obeyed the Hammett relation with ρ⁺ = ?2.04. The ¹³C chemical shift of ring-substituted 2-PBD was measured. Chemical shift values for C₁ and C₃ were correlated with Hammett σ, but those for C₂ and C₄ were almost unaffected by the substituents. On the basis of experimental results, the transition state of the cationic polymerization of 2-PBD was depicted as a benzylic cation rather than a phenylallylic one.     

Structure and reactivity in cationic polymerization of butadiene derivatives. IV. 1-alkoxybutadienes

The reactivity of trans-1-alkoxybutadienes in cationic homopolymerization and copolymerizations and structure of the polymers produced were investigated. 1-Ethoxybutadiene is polymerized easily at ?78°C by various acidic catalysis. The reactivity of 1-ethoxybutadiene was similar to that of ethyl vinyl ether. The polymers produced possessed molecular weights of several thousands, and were composed of 70–95% 1,4 structure and 5–30% 3,4 structure. In the copolymerization of ethyl vinyl ether (M₁) with 1-ethoxybutadiene at ?78°C in toluene by boron trifluoride diethyl etherate, r₁ = 1.15, r₂ = 2.62. From the Hammett plot of the relative reactivities of alkoxybutadienes (alkoxy: CH₃O, C₂H₅O, i-C₃H₇O), the reaction constant _p^* was determined to be ?2.9. Results of the present study were compared with those of various butadiene derivatives.     

Structure and reactivity in cationic polymerization of butadiene derivatives. II. 1-phenylbutadiene

The reactivity of 1-phenylbutadiene (1-PBD) in cationic polymerization and the monomer structure were investigated. 1-PBD polymerized at ?78°C in several solvents initiated by cationic catalysts such as stannic chloride and tungsten hexachloride. The polymerizations proceeded predominantly via 3,4-type propagation mode, and gave low molecular weight polymers. More than one double bond of 1-PBD was consumed during the polymerizations, probably due to transfer and cyclization reactions. 1-PBD was several times as reactive as styrene and trans-1,3-pentadiene in copolymerizations. The Hammett plots of reactivities of ring-substituted 1-PBD in cationic polymerization gave the p-value of -1.20, which is 0.6 times that of styrene. The ¹H and ¹³C NMR chemical shifts of ring-substituted 1-PBD were measured and discussed in relation to the reaction mechanism.     

The nature of active sites and stereospecificity of their action in diene polymerization catalysed by chromium-containing systems

A study has been made of the nature of active sites, stereospecificity of their action and the regularities of diene polymerization catalysed by chromium-containing systems. All possible polymer structures with high stereospecificity can be produced for butadiene and isoprene with π-allyl chromium compounds. Tris-π-allyl chromium produces polybutadiene predominantly of 1,2-units. Cis-polybutadiene is formed when the electronegative group (Cl^?, CCl₃COO^?) is substituted for one or two π-allyl groups in Tris-allyl chromium or in the catalytic system (π-C₃H₅)₃CrAl₂O₃. A catalyst obtained through interaction of (π-C₃H₅)₃Cr with silica-alumina or silica gel produces 1,4-trans-polybutadiene and 1,4-trans-polyisoprene. The rate of butadiene polymerization in the presence of Tris-π-allyl chromium is given by k[Cr]², and in polymerization of isoprene with the catalytic system (π-C₃H₅)₃Cr-silica-alumina, by k[Cr].[M]². Polymerization of dienes catalysed by (π-C₃H₅)₃Cr-silica-alumina system or supported chromium oxide catalyst proceeds according to a type of living system. A study was made of copolymerization of butadiene and isoprene in the presence of supported chromium oxide catalyst and with that produced by the reaction of (π-C₃H₅)₃Cr with silica-alumina. The constants of copolymerization for the systems were equal. A conclusion has been drawn regarding the similar mechanisms for diene polymerization under the action of supported chromium oxide catalyst or of catalyst formed in the reaction of (π-C₃H₅)₃Cr with silica-alumina or silica gel.     

Enhancement of stereospecificity in isoprene polymerization with alkylaluminum–titanium chloride catalysts through use of carbon disulfide

The effect of CS₂ on isoprene polymerization with triisobutylaluminum-titanium tetrachloride catalysts was studied at Al/Ti ratios of optimum (0.9) and higher values. In the absence of CS₂, appreciable amounts of low molecular weight oils (“extractables”) were formed at the expense of cis-1,4-polyisoprene with higher than optimum Al/Ti ratios. Small amounts of CS₂ were found to prevent extractables formation and allow attainment of higher yields of cis-1,4-polyisoprene. The optimum CS₂/Ti chloride molar ratio (0.1) was independent of the Al/Ti ratio of the catalyst. Polymer microstructure and dilute solution viscosity were unaffected by CS₂. The results support the theory that the catalyst surfaces hold two types of active sites: p-sites, which initiate polymerization, and o-sites, which lead to oligomerization. CS₂ appears to enhance polymerization by coordinating selectively at the o-sites. The predominance of oligomerization at the higher Al/Ti ratios was attributed to a destruction of p-sites by excess trialkyl-aluminum.     

The (butadiene)zirconocene route to active homogeneous olefin polymerization catalysts

(Butadiene)zirconocene is observed to exist as an equilibrium mixture of (s-cis-) and (s-trans-η⁴-C₄H₆)ZrCl₂ isomers. The system adds a variety of unsaturated organic reagents to form metallacyclic (allyl)metallocene products. In some cases, a second equivalent of a reagent is taken up, which forms the basis of a variety of useful template coupling reactions of butadiene at the bent metallocene framework. The Lewis acid B(C₆F₅)₃ adds to (butadiene)zirconocene and to a great variety of butadiene complexes of ansa-metallocenes and related systems to give zwitterionic metallocene-butadiene-borate betaines. Most of these systems are active homogeneous Ziegler-Natta olefin polymerization catalysts, that do not require additional activation. Catalyst activities are often in a similar range to those observed for other catalyst activation procedures used in this chemistry. In the case of the (butadiene)metallocene/B(C₆F₅)₃ systems we can often observe the first olefin insertion step. This feature was utilized to carry out a variety of mechanistic studies of the essential carbon-carbon coupling steps that take place at such bent metallocene catalyst systems. Even reactions with the functionalized monomer methylmetacrylate could be followed in this way. Some chelate ligand late metal systems were also included in these studies. However, these systems mostly behaved differently, even in cases where some structural similarities were observed.     

Electron spin resonance study of polymerization of butadiene with tris(acetylacetonato)titanium and triethylaluminum

ESR spectra of homogeneous catalyst derived from tris(acetylacetonato)titanium(III) and triethylaluminum were observed at several temperatures from ?78°C, to +25°C, at molar ratios of aluminum to titanium of 1–108. At ?78°C, this catalyst yields a violet complex which shows an ESR signal with a g value of 1.959 and is associated with the first intermediate. At ?40°C to ?30°C, this signal decreases, and two signals with g values of 1.947 and 1.960 are observed. The latter two signals diminish at ?5°C to +10°C, while two kinds of new signals with g values of 1.965 and 1.969 appear overlapping each other. The structures of the species corresponding to these five signals are discussed on the basis of the ESR spectra, the intensity change, and the unpaired spin distribution. A new signal with a g value of 1.978 is found in the presence of butadiene at 25°C at Al/Ti > 8 and is assigned to a growing end of polybutadiene with this catalyst. The polymer yield increases remarkedly at Al/Ti molar ratios greater than 10. The microstructure of the resulting polymer consists almost completely of 1,2 units. The structure of the growing end is proposed to be a titanium (III) species containing two 1-substituted allyl groups, by comparison with the structure ascribed to the growing end of polybutadiene with n-butyl titanate-triethylaluminum catalyst.