Kenneth J. Ivin

Mechanism for the stereospecific polymerization of olefins by Ziegler–Natta catalysts

A mechanism for the stereospecific polymerization of olefins by Ziegler–Natta catalysts is developed which differs significantly from previous mechanisms in the proposal that it proceeds via a 1,2-hydrogen shift from the α-carbon of the polymer chain and formation of metallocycle and carbene intermediates.

Microstructure and mechanism of formation of the ring-opened polymers of SYN- and AATI-7-methylbicyclo-[2.2.1]hept-2-ene initiated with metathesis catalysts

Abstract A 51:49 mixture of syn- and anti-7-methylbicyclo[2.2.1]hept-2-ene was subjected to a number of Ru-, Os-, Ir-, W- and Re-based catalysts, and the structures of the polymers and their hydrogenated analogues determined by 13C NMR spectroscopy. Except in one case, the polymer consisted exclusively of ring-opened units of the anti isomer; the syn isomer could be recovered in good yield. This confirms that the exo face of norbornenes is normally highly reactive whereas the endo face is relatively inert towards metathesis. The exception was the highly reactive (mesitylene)W(CO)3/ EtAlCl2/epoxide catalyst, where a small proportion of syn units was found in the soluble part of the polymer. The same catalyst also polymerises the syn isomer in the presence of a small amount of anti isomer and effects its copolymerisation with norbornene. It will also polymerise bicycio [2.2.2]-oct-2-ene by ring-opening, but not 1,7,7-trimethylbicyclo[2.2.1]hept-2-ene. The ring dyad tacticity with respect to both cis and trans junctions could be determined directly from the spectra of the ring-opened polymers of the anti isomer, and the overall tacticity from the spectra of the corresponding hydrogenated polymers. The fractions of m and r dyads for trans and cis junctions respectively are generally in the range 0.5–1.0, but are not always equal, with noble metal catalysts having a tendency (which is especially marked in the case of initiation by the Ru-TFA complex) for trans junctions to be atactic and cis junctions syndic tactic. The cis/trans double bond distribution is often blocky. These results confirm that, in general, two types of metal-carbene complex are important propagating species; one in which the last formed unit (cis) is still coordinated, thereby sterically biasing the next monomer insertion in favour of another cis unit, while in the second the last formed unit (trans) has detached from the metal ion before the metal carbene reacts with monomer, which it then does with a strong bias towards the formation of a trans junction. Various possible chiral and achiral forms of these two species together with appropriate relaxation processes are briefly discussed.

Ring-opening polymerization of endo and exo-dicyclopentadiene and their 7,8-dihydro derivatives

A range of metathesis catalysts have been used to prepare polymers from the title monomers. The fraction of cis double bonds, σc, in the polymers was determined by 13C NMR spectroscopy. The spectra of polymers containing mainly cis or trans double bonds in the main chain, as well as polymers derived from these by hydrogenation, were analysed in detail in terms of orientational and tacticity effects. With endo-dicyclopentadiene, the catalyst RuC3·3H2O produced a high-cis polymer. This anomalous result is believed to be due to a steric effect at the catalyst site caused by endo-dicyclopentadiene also acting as a permanent ligand because of its unique potential, among these monomers, to chelate in a bidentate fashion.

Tactiticy and stereochemistry in the ring-opening polymerization of 5,5-dimethylbicyclo[2.2.1]hept-2-ene initiated by metathesis catalysts

Abstract Ring-opened polymers of optically active 5,5-dimethylbicyclo[2.2.1]-hept-2-ene (5,5-dimethylnorbornene DMNBE), having cis double bond contents of 0 – 100%, were prepared using various olefin metathesis catalysts based on Mo, W, Re, OS, Ru and Ir compounds. The ring dyad tacticities in these polymers, with respect to both cis and trans double bonds, were determined from 13C NMR spectra. Cis double bonds are always asociated with 50 – 100% r dyads (syndiotacticity (σr)c = 0.5 – 1.0), while trans double bonds are always associated with 50 – 100% m dyads (isotacticity (σm)t = 0.5 – 1.0). The polymers may be divided into four groups: I, those of high tacticity, with (σr)c) ∼ (σm)t ∼ 1.0; II, those of intermedia tacticity, with (σr)c ∼ (σm)t ∼ 0.6 – 0.9; III, those with (σr)c > (σm)t; IV, those of low tacticity, with (σr)c ∼ (σm)t ∼ 0.5. These is no correlation between tacticity and cis content, but all-cis polymers are generally highly syndiotactic and all-trans polymers slightly isotactic when prepared from 3 M monomer at 20 °C. The tacticity falls with increasing preparation temperature but not always with decreasing monomer concentration. The results are interpreted in terms of sequential formation of a metallacarbene-monomer complex, 4, a metallacyclobutane, 5, a metallacarbene with the newly-formed double bond first coordinated to, 6, and achiral ‘symmetrical’ form of 6; there are corresponding mirror-image and symmetrical forms of 3. The first type of rupture accounts for cases of high tacticity or tacticity which falls with monomer dilution; the second explains how the tacticity can be independent of monomer dilution. Propagation is assumed to occur either by addition of monomer to 3 to form a cis or trans double bond, or by displacement of coordinated cis double bond from 6 by monomer to form a new cis doubled bond. With these assumptions a complete mechanism is developed (Scheme 6) to account not only for the results presented here but also for some more general features of olefin metathesis, especially stereospecificity and selectivity in cross-methathesis.

13C n.m.r. spectra of polymers made by ring-opening polymerization of (±)- and (+)-exo-5-methylbicyclo [2.2.1] hept-2-ene using metathesis catalysts

Abstract A range of olefin metathesis catalysts has been used to prepare ring-opened polymers of (±)- and (+)-exo-5-methylbicyclo [2.2.1] hept-2-ene, (l), having cis double bond contents of 11–100%. The 13 C n.m.r. spectra of these polymers are interpreted in terms of TH, TT, HH, HT and tt, tc, ct, cc structures ( T = tail, H = head, referring to methyl groups; t = trans , c = cis , referring to double bonds). The all- cis polymer has a fully-syndiotactic ring sequence, but polymers with less than 55% cis double bonds have an atactic ring sequence. The substitution shift parameters indicate that the cyclopentane rings in the polymer chain adopt the puckered conformation which minimizes non-bonded repulsion between the cis -1,3-olefinic substituents.

Some recent applications of 13 C NMR spectroscopy to the study of the ring-opening polymerization of cycloalkenes and related reactions initiated by metathesis catalysts

Recent results on the application of 13C n.tn.r. spectroscopy in the field of ring—opening polymerization of cycloalkenes, initiated by metathesis catalysts, and the reactions of alkynes initiated by the same catalysts, are reviewed and extended, especially in relation to the polymerization of 5,5—disubstituted derivatives of bicyclo[2.2.1]hept—2—ene. Direct and indirect observations of the tacticity with respect to both cis and trans double bonds in these polymers is reported. Applications to the detection of end groups, cis/trans isomerization of double bonds in the polymer chain, and double bond migration are also reviewed. INTRODUCTION The ring—opening polymerization of cycloalkenes is a particular case of the olefin metathesis reaction. Such reactions are initiated by a variety of catalyst systems, generally based on a compound of one of the following transition metals: Ti, V, Nb, Ta, Mo, W, Re, Ru, Os, Ir. It is generally desirable, though not always essential, to use a co— catalyst; typical combinations include WC16/EtA1C12, MoCl5/R4Sn (R = Me, Bu, Ph), and TiCl4/LiA1R4. It was proved many years ago by isotopic labelling (2H or 14C), that the double bond itself is broken during the metathesis reaction of both acyclic (Ref. 1 & 2) and cyclic olefins (Ref. 3). More recently it has been shown that the same or similar catalyst systems can induce two types of reaction in alkynes, namely metathesis itself, again demonstrated (Ref. 4) by isotopic labelling (13C), and polymerization (Ref. 5). That the mechanisms of the reactions of alkenes and alkynes induced by these catalysts are intimately connected is shown by the fact that small amounts of alkynes sometimes act as cocatalysts for the reactions of cycloalkenes (Ref. 6). There is now a considerable body of evidence to show that olefin metathesis reactions (and possibly also the above polymerization reactions of alkynes) are propagated by a metalla— carbene generated from the catalyst system, sometimes with the assistance of the substrate (Ref. 7). The relatively stable metallacarbenes Ph2C=W(CO)5 and Ph(MeO)C=W(CO)5 can also be used as initiators of ring—opening polymerization of cycloalkenes, but sometimes need an acetylenic cocatalyst (Ref. 6). The propagation reactions, as represented below for the polymerization of cycloalkenes and alkynes, are thought to involve metallacyclobutane and metallacyclobutene intermediates respectively, though it must be said that there is no direct spectroscopic evidence either for these intermediates or for the metallacarbenes. EMt] denotes the transition metal surrounded by various permanent ligands. PCH PCH PCH=C}1 + n n [Mt] [Mt] [Mt]=CH (a Mt]=CHP1) P CR CH P CR — CH P CR=CR nil + ill n n Mt CR Mt1 — CR [Mt]=CR

Structural sequences in ring-opened polymers and copolymers of cycloalkenes as a guide to the mechanism of olefin metathesis

Abstract Recent work on the determination of various types of pair sequence in ring-opened polymers and copolymers of cycloalkenes is reviewed. Four types are discussed: (1), cc, ct, tc, tt double bond pairs; (2) m and r ring-configurational dyads in norbornene polymers; (3) TH, TT, HH, HT dyads (H = head, T = tail) in polymers of 5-substituted norbornenes; and (4) M1M1, M1M2, M2M1, M2M2 compositional dyads in copolymers. All these may be determined from 13C NMR spectra. For polymers of norbornene and its derivatives there is a trend from blocky to random cis/trans double bond distribution and from fully syndiotactic (r) to atactic (m/r) ring dyads as the cis content decreases from 100% to 35%. This is interpreted in terms of competition between addition of monomer to a particular configuration of the propagating metallocarbene and the relaxation of this configuration to one or more different configurations. Published work on the effect of temperature and monomer concentration on the cis content of polypentenamer supports this mechanism. Ring-opened copolymers of cycloalkenes have either a random or blocky compositional distribution. The copolymer composition is sensitive to the catalyst system.

Kinetics of initiation and propagation of the metathesis polymerization of the exo diels-alder adduct of cyclopentadiene and maleic anhydride initiated by the tungsten-carbene complex W[H2](OCH2CMe3)2Br2

Abstract The title reaction was followed by 1H NMR at 277 – 314 K. The initiator (I) adds one molecule of monomer (M) to give a tungsten-carbene species P1 which then adds further monomer molecules to give Pn (n > 1). P1 and Pn (n > 1) can be distinguished by their (OCH2CMe3)2 signals at 4.44, 4.39 and 4.46, 4.41 ppm respectively; the two neopentoxy ligands are non-equivalent in each case. [P1] builds up to a maximum (65% of [I0]) and then declines. At 294 K the initiator is all consumed after 40 min and [M] then decays according to a first-order law (t 1 2 = 74 min for [Ptotal] = [I0] = 5.0 × 10−2 mol dm−3). The activation energy for the propagation reaction is about 51 kJ mol−1. The initiation rate constant is 3.8 ± 0.2 times as large as the propagation rate constant. The 13C NMR spectrum of the exo, exo-dimethyl ester polymer derived from the polymer of M indicated that the polymer contained 28% cis double bonds. The endo adduct of cyclopentadiene and maleic anhydride was also found capable of adding to the living chain; this is the first report that the endo monomer can undergo metathesis polymerization.

The detection of ‘living’ propagating tungsten–carbene complexes in the ring-opening polymerization of bicycloalkenes

The tungsten–carbene complex W(CHCMe3)(OCH2CMe3)2Br2, when mixed with GaBr3, adds various bicyclo[2.2.1]hept-2-enes to form ‘living’ propagating carbene complexes which may be characterized by 1H n.m.r. spectroscopy, and used to make block copolymers.

Ring-Opening Metathesis Polymerization of 7-tert-Butoxybicyclo[2.2.1]hepta-2,5-diene Initiated by Well-Defined Molybdenum and Ruthenium Carbene Complexes

The ROMP of 7-tert-butoxynorbonadiene initiated by Mo(=CHCMe 2 Ph)(=NC 6 H 3 -2,6-i-Pr 2 )(OR) 2 [1, OR = OCMe(CF 3 ) 2 ; 2, OR = OCMe 3 ] and Ru(=CHPh)Cl 2 (PCy 3 ) 2 (3) has been studied at ambient temperature by NMR in CDCl 3 , CD 2 Cl 2 and C 6 D 6 . The reaction with 1 as initiator is extremely fast, but with 2 and 3 reaction proceeds at measurable speed, being fastest in CDCl 3 and slowest in C 6 D 6 . The polymer formed in CD 2 Cl 2 using 1 contains 47% syn units and mainly cis double bonds; that formed using 2 contains 34% syn units and about 17% cis double bonds; and that formed using 3 contains less than 7% syn units and 32% cis double bonds. The carbene proton of the propagating carbenes derived from 1 and 2 give two groups of 1 H NMR doublets, attributed to species having syn and anti units adjacent to the Mo center, but that derived from 3 gives only a single group of signals in which the fine structure is independent of spectrometer frequency and therefore not due to spin coupling. With 3 as initiator, after polymerization is complete, the intiator is partiallu regenerated at the expense of the propagating species indicating the occurence of secondary metathesis reactions at the chains ends, leading to a decrease in the number of chain ends in the polymer. Under conditions where the chain length is low, the shorter chains may be detected by GPC and NMR. Such reactions occur intermolecurlarly resulting in an increase in the average molecular weight; they may also occur by ring-closing metathesis, though this has not been proved. k p /k i values were derived and in three cases individual values of k p and k i were determined. With 3 as initiator in CD 2 Cl 2 the presence of a five-fold excess of PCy 3 caused a marked diminution in the rate of ROMP showing that the reaction proceeds mainly via addition of monomer to monophosphine complexes, Ru(=CHR)Cl 2 (PCy 3 ).