Valia Amir-Ebrahimi

Dramatic solvent effects on ring-opening metathesis polymerization of cycloalkenes

Abstract A series of metathesis polymers and copolymers have been formed and their structures were analysed by 13 C NMR spectroscopy. Noble metal and non-noble metal salt catalysts are distinguished by their behaviour in various solvents. Thus, in phenolic solvents, the former class produce alternating copolymers from cyclopentene and norbornene, while the latter are unaffected and produce random copolymers. In contrast, ether solvents have the effect of markedly increasing the cis content of polymers from the latter catalysts while the former are unaffected. The tacticity of various polymers are correlated through their hydrogenated derivatives and found to depend on the type of monomer as well as the catalysts.

Remarkable alternating effect in metathesis copolymerization of norbornene and cyclopentene using modified Grubbs ruthenium initiators

Abstract High trans and mainly cis directing Grubbs Ru initiators, I–III, and IV, respectively have been investigated as catalysts for metathesis copolymerization of norbornene and cyclopentene. When the Lewis acid, MoCl5 or WCl6, is added to these Lewis bases initiators, I–III, a cage effect develops such that the polymers become alternating caused by the severe or almost total denial of access of norbornene to the propagating metallacarbenes. Addition of ethers destroys the cages and removes the effect. In marked contrast the N-heterocyclic ligand Grubbs initiator, IV, does not show the effect, as addition of MoCl5 only seems to retard the overall reaction, but does not change the copolymer composition.

Determination of the tacticity of ring‐opened metathesis polymers of norbornene and norbornadiene by 13C NMR spectroscopy of their hydrogenated derivatives

The 125 MHz 13 C nuclear magnetic resonance (NMR) spectra of the hydrogenated derivatives of ring-opened metathesis polymers (ROMP) of bicydo[2.2.1]hept-2-ene (norbornene) and bicydo[2.2.1]hepta-2,5-diene (norbornadiene) prepared using a range of initiators, have been analysed in detail. The signals due to the methylene carbon atoms in the enchained cyclopentyl rings of these polymers show fine structure which is assigned to m and r ring dyads and mm, mr/rm and rr ring triads, enabling the tacticity of the hydrogenated polymer, and thus of the polyolefin precursor, to be determined for high-cis or high-trans polymers. Novel methods for making high-cis polymers are described and solvent-dependent variations in the stereospecificity of the propagation reaction have been observed.

Hydrogenation of norbornadiene on supported metal catalysts: Evidence for a novel mechanism of hydrogen activation

Abstract Norbornadiene hydrogenates on Cu and Au dispersed on basic supports such as MgO to give significant yields of nortricyclene in addition to norbornene and norbornane, whereas only the latter products are found using Pt on the same supports as catalysts. This novel finding is evidence for a mechanism where surface olefin—metal complexes and not the metal atoms per se are directly involved in activating molecular hydrogen. Dissociation of H 2 is then a step which occurs simultaneously with the addition of the first H atom to the adsorbed olefin and is not a prior independent event, as is thought to be the case in conventional mechanisms. The new mechanism helps to explain why metals with low or negligible capacity for hydrogen chemisorption, e.g. Pt/TiO 2 after high temperature reduction, are still very effective hydrogenation catalysts.

Determination of the tacticities of ring-opened metathesis polymers of symmetrical 5,6-disubstituted derivatives of norbornene and norbornadiene from the 13C NMR spectra of their hydrogenated derivatives

Abstract Ring-opened metathesis polymers were made from 5,6-di-exo-(carbomethoxy)-norbornene and 5,6-di-(carbomethoxy)-norbornadiene using a variety of catalysts and their 13 C NMR spectra analyzed in detail. These polymers were then fully hydrogenated using diimine and found by 13 C NMR analysis to give with complete selectivity the same saturated derivative. This permitted the tacticities to be assigned in detail and the consistency of the analysis further checked with respect to the m (meso) and r (racemic) line orders identified for all the precursor polymers. While OsCl3/phenylacetylene as catalyst gave high cis syndiotactic polymers OsCl3 by itself formed high trans atactic material; RuCl3 afforded the corresponding trans polymer with a substantial isotactic bias. By way of contrast MoCl5/Me4Sn in dioxan is an excellent catalyst for the synthesis of all cis isotactic poly-5, 6-di-(carbomethoxy)-norbornadiene.

Copolymerization of cycloalkenes as a probe of the propagation steps in olefin metathesis

Abstract A range of ring-opened metathesis copolymers of norbornene and cyclopentene have been prepared and 13 C NMR spectroscopy used to analyse in detail the nature of the homo and heterodyad units. This has provided significant new information on the sensitivity of the [2 + 2] cycloaddition step in metathesis to the steric and electronic factors associated with the [Mt]C and CC moieties involved. Novel very high cis directing catalyst systems, have also been developed for the homopolymerization of norbornene using various ethers with Mo and W-based catalyst systems or chelating dienes and phenylacetylene with RuCl 3 and OsCl 3 systems. The general feature of cis/trans blockiness at high cis content is investigated in much greater detail. A clearer description of the associated relaxation processes of the propagating species emerges from this work. The metathesis polymerization of the matched pair, benzonorbornadiene and 7-oxa-benzonorbornadiene has been investigated using Ru and Os-based initiators. The results illustrate the subtle way whereby the presence of the proximate 7-O-atom may facilitate [2 + 2] cycloadditions. This constitutes a valuable novel method of studying key electronic factors in the mechanism of the metathesis reaction.

13C NMR spectra of tactic and atactic hydrogenated ring-opened polymers of enantiomeric and racemic endo,exo-5,6-dimethylnorbornene

Polymers 1 of the title monomer, prepared using well-defined molybdenum carbene complexes as catalysts, have been hydrogenated and the structures of the resultant polymers 2 examined by 13 C NMR spectroscopy. The hydrogenated polymer made from the all-cis isotactic polymer of (+)-monomer showed a single set of 13 C NMR lines as expected for an NX sequence of endo (N) and exo (X) substituents. The hydrogenated polymer made from a cis isotactic polymer of (±)-monomer showed additional fine structure arising from the random incorporation of both enantiomers in the isotactic polymer chain: four equal lines for C-9 (orientational triad sensitivity), two equal lines for C-3, C-4, C-5, and C-1 (dyad sensitivity), but single lines for C-8, C-2, C-7 and C-6 (insensitive to the relative orientation of adjacent repeating units). The hydrogenated polymer made from a trans atactic polymer of (+)-monomer showed fine structure due to the presence of both m and r dyads. That made from a trans atactic polymer of (±)-monomer contains 16 possible triad sequences and gave a more complicated spectrum. A complete assignment was made for the first three polymers and a partial assignment for the fourth. Polymers made using non-carbene catalysts were also examined. Hydrogenation of an all-trans precursor made from (±)-monomer using RuCl 3 as catalyst gave an atactic polymer, confirming previous observations. Hydrogenation of a 61% cis, cis/trans blocky precursor, made from (+)-monomer using OsCl 3 /PhC≡CH as catalyst, gave a syndiotactic-biased stereoblocky polymer, indicating a c/r, t/m correlation in the precursor polymer.

Metathesis polymerisation of norbornene and its derivatives. Relative stability of tungstacyclobutane intermediates and kinetics of their ring opening to form tungsten–carbene complexes

A family of tungstacyclobutane intermediates in the catalytic ring-opening polymerisation of norbornenes have been studied by 1H n.m.r.; their rates of rearrangement into tungsten–carbene complexes are reported and compared.

Methylaluminoxane as a novel catalyst and cocatalyst for ring-opening metathesis polymerisation of norbornene

Abstract Methylaluminoxane (MAO) alone acts as a weak catalyst for the ring-opening metathesis polymerisation of norbornene, thereby providing good evidence that some transient [Al]C metallacarbenes are formed and propagate the reaction. A novel general mechanism of initiation involving generation of metallacyclopentanes, followed by ring contraction to metallacyclobutanes, is proposed. Methylaluminoxane is also a very effective cocatalyst for Mo and W chlorides, and especially for non-ionic carbonyl compounds of these two metals. Remarkably cis/trans highly stereo-blocky polymers are obtained which for the first time show clear resolution (m/r) on some of the 13C NMR lines.

Free radicals in dimethyl ether on H-ZSM-5 zeolite. A novel dimension of heterogeneous catalysis

By using a spin trapping reagent with e.s.r. analysis, free radicals have been detected in reactions of dimethyl ether over H-ZSM-5 zeolite thereby providing important novel evidence concerning the initial C–C bond forming step which is crucial for production of alkenes and aromatics; the formation of such radicals at moderate at moderate temperatures on surfaces seems to be quite general and adds a new dimension to heterogeneous catalysis.