Stereospecific polymerization of methacrylonitrile. III. Some new catalysts for isotactic polymethacrylonitrile

Some classes of organometallic catalysts what induce stereospecific polymerization of methacrylonitrile have been found. They include organolithium aluminum compounds of the type LiAlR₄, Li[R₃AlOAlR₂], and Li[R₃AlN(R)AlR₂], organosodium aluminum compounds of the type NaAlR₄, organolithium zinc compounds of the type LiZnR₃ and Li₂ZnR₄, organomagnesium aluminum compounds of the type RMg[AlR₄] and Mg[AlR₄]₂, and organomagnesium compounds containing an Mg? N bond, such as and their related compounds. One of the features of the polymerization with these catalysts was that the crystalline polymers were formed at moderately high temperatures. Total conversion, solubility index, and molecular weight of the polymer increased with increasing polymerization temperature, as observed in the case of polymerization with diethylmagnesium catalyst. Catalysts with an Mg? N bond were found to be highly effective for the stereospecific polymerization. The acetone-insoluble fractions of the polymers gave x-ray diagrams identical to the crystalline polymer produced with diethylmagnesium. This indicates that the acetone-insoluble crystalline polymers produced with these catalysts have an isotactic structure. The viscosity–molecular weight relationship for crystalline polymer was conveniently determined in Cl₂CHCOOH at 30°C.; [η] = 2.27 × 10^?4 M^0.754.     

Stereospecific polymerization of methacrylonitrile. I. Characterization of crystalline polymethacrylonitrile

Methacrylonitrile (MAN) was polymerized with diethylmagnesium. Acetone-insoluble portions of the polymers are found to be crystalline. Highly crystalline portions can be isolated by further extraction of the acetone-insoluble parts with dimethylformamide (DMF). A film of DMF-insoluble fraction can be oriented uniaxially by hot-press rolling. The crystalline PMAN is insoluble in the usual solvents for amorphous PMAN because of their crystallinity and is easily soluble in CF₃COOH or Cl₂CHCOOH. The viscosity–molecular weight relationship was determined in Cl₂CHCOOH at 30°C. as [η] = 3.24 × 10^?3M^0.520. We found several crystalline bands in the infrared spectra, for example, at 1192 and 885 cm.^?1. Formation of the carbonyl group in the polymer is discussed, and it is concluded that it may be formed by the hydrolysis of conjugated cyclic imine or hydrolysis of the nitrile group in the polymer to acid amide.     

Stereospecific polymerization of methacrylonitrile. II. Influence of polymerization conditions on polymerization and on properties of polymers

Stereospecific polymerization of methacrylonitrile with diethylmagnesium has been studied. Polymerization temperature has an important effect on polymerization. The conversion, stereoregularity, and intrinsic viscosity of the polymer increased significantly with increasing polymerization temperature. Stereoregularity of the polymer improved with increasing the polymerization time and the monomer concentration, but it is independent of the catalyst concentration. Intrinsic viscosity of the crystalline polymer increased with increasing monomer concentration but is independent of the polymerization time and the catalyst concentration. It is suggested that two mechanisms are involved in this polymerization: coordinated anionic polymerization to from the crystalline polymer, and probably conventional anionic polymerization to form the amorphous polymer. It is found that crystalline polymer can also be obtained in homogeneous phase such as in tetrahydrofuran solvent.     

Stereospecific polymerization of methacrylonitrile. V. Formation of the carbonyl in the polymer

The isotactic polymethacrylonitrile prepared by the organometallic catalysts has carbonyl groups in the polymer. Thus, the carbonyl formation during the polymerization by diethylmagnesium was carefully studied by infrared spectrometry. It was found that the imino anion formed by monomolecular termination of the propagating chain end in the cyclization reaction successively attacks the vicinal pendent nitriles in the polymer so as to form the conjugated cyclic imines, which are hydrolyzed to acid amines during the catalyst decomposition with acidic methanol.     

Stereospecific polymerization of methacrylates by metallocene and related catalysts

Stereospecific—isospecific, syndiospecific, and diastereospecific—polymerizations of methacrylates using group 4 metallocene and related catalysts produce polymethacrylates with controlled stereo‐microstructures. The versatility and stereospecificity of these cat‐ alysts for methyl methacrylate polymerization were demonstrated not only in solution‐phase polymerization, but also in polymerizations on silica surfaces and inside silicate nanogalleries. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3395–3403, 2004     

Stereospecific oligo- and polymerization with metallocene catalysts

The regio- and stereo-selectivity of metallocene/methylaluminoxane catalysts was shown to be significantly and rapidly enhanced by substitution of the indenyl ligands and by heterogenization of the zirconocenes. By oligomerization of 1-butene with optically active metallocenes, it could be shown that there is a small influence from the growing polymer chain on the isotacticity, the overriding factor in every case being the existence of a chiral active centre. Each insertion step increased the stereospecificity, until the maximum value was achieved.     

Stereospecific polymerization of methacrylonitrile. VI. Effect of esters as a complexing agent with diethylmagnesium catalyst

The stereospecific polymerizations of methacrylonitrile with diethylmagnesium were carefully studied by using various ethers as complexing agents. The complexed ethers exhibit a beneficial effect on the stereoregularity of the resulting polymer, namely, the crystallinity increased by using ethers as a complexing agent. The polymerization rate and the molecular weight of the polymer also increased by using ether-complexed catalysts. The polymerization behavior was studied with the dioxane–diethylmagnesium complex as a typical complexed catalyst. The behavior was mostly similar to that of the diethymagnesium alone, that is, the rate of the polymerization increased in proportion to monomer concentration, and the solubility index increased with increasing monomer concentration. Interestingly, the viscosity of the acetone-insoluble fraction increased with increasing monomer concentration, while that of the acetone-soluble fraction was independent of monomer concentration. This is explained by considering that the catalyst has at least two kinds of catalytic species, one being the species that produces the crystalline polymer by a coordinated anionic polymerization, another being the one from which an amorphous polymer is obtained by a conventional anionic mechanism. The fact that the viscosity of the polymer decreased with increasing the initiator concentration is explained in terms of chain trasfer to the initiator. In case of diethylmagnesium alone, the viscosity of the polymer is independent of the initiator concentration.     

Anionic polymerization of methacrylonitrile—I. Polymerization with n-butyllithium as initiator

The anionic polymerization of methacrylonitrile has been studied at ?75° in toluene and with n-butyllithium as initiator. The kinetics of the polymerization were investigated considering the consumption of both monomer and initiator. BuLi disappears relatively slowly and about 50–60% remains unreacted. A simple kinetic scheme cannot therefore be put forward. All possible side reactions have also been examined. The molecular weight study establishes the living character of this system and gives an initiator efficiency of about 0.2. The contribution of low molecular weight products, typical of the polymerization of polar monomers, is also taken into account. In order to obtain a better understanding of the mechanism of this polymerization, in which unreacted initiator is probably engaged in very stable and inactive mixed associated particles, small amounts of THF (known frequently to break down such aggregates) were added to the system. A rather unexpected slow but complete disappearance of the initiator occurs; the conversion at which the rate of monomer consumption levels off depends on the THF concentration.     

Stereospecific post-metallocene polymerization catalysts: the example of isospecific styrene polymerization

In the context of developing single-site stereoselective post-metallocene catalysts, the case for isospecific styrene polymerization catalysts based on methylaluminoxane-activated group 4 metal bis(phenolato) complexes is summarized. Ligands derived from the 1,4-dithiabutanediyl-linked bis(phenol)s have been found to induce stereochemical rigidity by the presence of the hemi-labile sulfide donor functions. Isospecific styrene polymerization was achieved using easily accessible catalyst precursors of the type [MX₂(OC₆H₂-^tBu₂-4,6)₂{S(CH₂)₂S}] (M = Ti, Zr, Hf; X = Cl, OⁱPr, CH₂Ph). Activating the dibenzyl titanium complex [Ti(CH₂Ph)₂(OC₆H₂-^tBu₂-4,6)₂{S(CH₂)₂S}] with B(C₆F₅)₃ and AlⁱBu₃, controlled isotactic polymerization became possible at lower temperatures. A remarkable dependence of both the activity and stereoselectivity on the ligand substitution pattern was observed. Analogous precursors with the 1,5-dithiapentanediyl-linked bis(phenolato) ligand gave syndiotactic polystyrene with lower activity.     

Stereospecific polymerization of α-methylstyrene by friedel-crafts catalysts

The relationship between stereoregularity and polymerization conditions of α-methylstyrene has been studied by means of NMR spectra. The effects of solvents and various Freidel-Crafts catalysts have been investigated. The stereoregularity of poly-α-methylstyrene increased with increased polymer solubility in the solvent used and with decreasing polymerization temperature. This behavior is completely different from the stereospecific polymerization of vinyl ethers and methyl methacrylate in homogeneous systems. This may be due to the strong steric repulsion exerted by the two substituents in the α-position of α-methylstyrene. For example, with BF₃ · O(C₂H₅)₂ as catalyst at ?78°C., atactic polymer is obtained in n-hexane, a nonsolvent for α-methylstyrene, whereas highly stereoregular polymer is produced in toluene or methylene chloride, good solvents for the polymer. However, the polarity of the solvent and the nature of the catalyst hardly affect the stereoregularity of the polymer.     

Radiochemical studies of free radical vinyl polymerization—V. Polymerization of methacrylonitrile in dimethylsulphoxide solution

[¹⁴C]-Azoisobutyronitrile was used to initiate free radical polymerization of methacrylonitrile at 60° in dimethylsulphoxide solution. Approximately two initiator fragments were found combined with each polymer molecule, indicating that termination was almost exclusively by combination of pairs of polymer radicals.     

Anionic polymerization of methacrylonitrile—II.: Polymerization with 1,1-diphenyl-n-hexyllithium and sec-butyllithium as initiators

The anionic polymerization of methacrylonitrile has been investigated at ?75° in toluene with 1,1-diphenyl-n-hexyllithium and sec-butyllithium as initiators. With both initiators, initiation was fast and no unreacted initiator was left. The efficiency was much higher for 1,1-diphenyl-n-hexyllithium than for sec-butyllithium probably for steric reasons. With both initiators, a second order dependence on initiator was found. To check that this is not an artifact resulting from measurements in a medium starting to gel at a point which is difficult to detect precisely, kinetic measurements were also carried out with 2-polystyryl-l,l-diphenylethyllithium as an initiator. Second order kinetics were again obtained; the propagation rate constants, taking into account the efficiencies of initiators, were the same within experimental error for all three initiators. Viscosity measurements performed on a methacrylonitrile “capped” 2-polystyryl-l,l-diphenylethyllithium initiator showed the living polymethacrylonitrile chains to be associated.     

Anionic heterogeneous polymerization of methacrylonitrile by n-butyllithium

The anionic heterogeneous polymerization of methacrylonitrile by butyllithium in petroleum ether was investigated. The polymerization was of the “living” type, as seen from the linear dependence of the molecular weights on [MAN]/[BuLi]. This behavior was further supported by block polymerization experiments in which the monomer was added in two portions and the molecular weights obtained were directly proportional to the total monomer concentration. The initiator efficiency was low, and initiator consumption was only about 2%. This fact, together with the results of the block polymerizations showed that there was preferential addition of monomer to the growing chain ends rather than to the initiator. The molecular weights were independent of the rate of monomer addition. This as well as the “living” behavior of the polymerization of methacrylonitrile on a wide range of monomer and catalyst concentrations and the absence of chain transfer to monomer was essentially different from that of the similar heterogeneous polymerization of acrylonitrile by butyllithium previously investigated. This is due to the absence of an α-acidic hydrogen in methacrylonitrile.     

Stereospecific polymerization of isobutyl vinyl ether by AlR3–VCln catalysts. I. Influence of preparative conditions of the catalysts

The polymerization of isobutyl vinyl ether by the VCl_n–AIR₃ system was carefully studied. The vanadium components were prepared by the reaction between VCl₄ and AlEt₃ or n-BuLi as a reducing agent. VCl₃·LiCl and VCl₂·2LiCl are the effective catalysts for the stereospecific polymerization of isobutyl vinyl ether. When VCl₃·LiCl is combined with AlR₃, a new catalytic system is formed. The effect of the preparative conditions of the various vanadium component in the AlR₃–VCl_n system shows that the effective vanadium component is trivalent. In the polymerization by VCl₃·LiCl–Al (i-Bu)₃ system, a change of the polymerization mechanism may occur at Al(i-Bu)₃/VCl₃·LiCl ratio at around 5. When the ratio is lower than 5, a cationic polymerization by VCl₃·LiCl takes place predominantly, while at ratios higher than 5, it is suggested that the polymerization proceeds by means of a VCl₃·LiClA–Al(i-Bu)₃ complex by a coordinated anionic mechanism. The polymers obtained by these catalysts are highly crystalline. Styrene was also polymerized by using the same catalysts. VCl₃·LiCl and VCl₃·LiCl–THF complex yielded amorphous polymer by cationic polymerization. When VCl₃·LiCl was combined with 6 mole-eq of Al(i-Bu)₃, the resulting polystyrene was highly crystalline and had an isotactic structure, while the VCl₂·2LiCl–Al(i-Bu)₃ (1:6) system yielded traces of polymer of extremely low stereoregularity. The results indicate that the effective vanadium component at Al/V ≧ 6 is trivalent and that the mechanism is a coordinated anionic one.     

Stereospecific polymerization of isobutyl vinyl ether. III. Polymerization with VCl3 • LiCl

The polymerization of isobutyl vinyl ether by vanadium trichloride in n-heptane was studied. VCl₃ ? LiCl was prepared by the reduction of VCl₄ with stoichiometric amounts of BuLi. This type of catalyst induces stereospecific polymerization of isobutyl vinyl ether without the action of trialkyl aluminum to an isotactic polymer when a rise in temperature during the polymerization was depressed by cooling. It is suggested that the cause of the stereospecific polymerization might be due to the catalyst structure in which LiCl coexists with VCl₃, namely, VCl₃ ? LiCl or VCl₂ ? 2LiCl as a solid solution in the crystalline lattice, since VCl₃ prepared by thermal decomposition of VCl₄ and a commercial VCl₃ did not produce the crystalline polymer and soluble catalysts such as VCl₄ in heptane and VCl₃ ? LiCl in ether solution did not yield the stereospecific polymer. It was found that some additives, such as tetrahydrofuran or ethylene glycol diphenyl ether, to the catalyst increased the stereospecific polymerization activity of the catalysts. Influence of the polymerization conditions such as temperature, time, monomer and catalyst concentrations, and the kind of solvent on the formed polymer was also examined.     

Polymerization of isocyanates. IV. Polymerization by aqueous initiator systems

Isocyanates were polymerized by aqueous solutions of numerous alkali salts at a low temperature. Polymer could be obtained even in the presence of a large excess of water to monomer in the polymerization system. The initiating species was proposed to be hydroxide ion.     

Stereospecific polymerization of o-methoxystyrene by anionic initiators

o-Methoxystyrene was polymerized with n-butyllithium (n-BuLi), Na naphthalene, and K dispersion as initiators in tetrahydrofuran (THF) and toluene. The stereoregularity of the polymer was investigated by means of the NMR spectroscopy. The methoxy resonance of the spectrum split into ten components due to the tactic pentads. It was found by x-ray examination that the polymer obtained by n-BuLi in toluene at ?45°C was crystalline and highly isotactic. In THF, the stereospecificity of the polymerization was independent of the initiator, and the isotacticity of the polymer increased with increasing reaction temperature. In toluene, the stereospecificity depended on the initiator; i.e., n-BuLi gave a polymer with higher isotacticity than that given by phenylsodium. The fraction of isotactic triad of the polymer obtained by n-BuLi in toluene at ?78°C was more than 90%, but 50% at 50°C. The presence of ca. 1% THF in toluene led to a steep decrease in the isotacticity even at ?78°C. The tacticity of the polymer given by Na naphthalene was not affected by the existence of NaB(C₆H₅)₄ in THF. The polymerization in THF could be explained by Bovey's “single σ” process, while a penultimate effect was observed in the polymerization by n-BuLi in toluene.     

Cationic polymerization of α,β-disubstituted olefins. XV. Stereospecific polymerization of butenyl ethers by cationic catalysts

Methyl, ethyl, and isopropyl butenyl ethers, CH₃CH₂CH?CHOR, were polymerized with homogeneous catalysts at ?78°C. Toluene, methylene chloride, and nitroethane were used as solvents, and BF₃O(C₂H₅)₂ and SnCl₄·CCl₃CO₂H were used as catalysts. The stereoregularity of the polymers were compared by x-ray diagrams and infrared absorption ratios. The stereoregularity of polymers increased with increasing content of the trans isomer in the monomer and with increasing polarity of the solvent. In the polymerization of methyl and ethyl butenyl ethers, crystalline polymers were obtained from both the trans and cis isomers. The crystalline polymer prepared from the trans isomer and that from the cis isomer had the same steric structure. This behavior is quite different from that observed in the polymerization of propenyl ethers. It is concluded that the bulkiness of the group on the olefinic β-carbon plays an important role in the stereospecific polymerization of α,β-disubstituted olefins.