Metathesis polymerization of endo-5- and exo-5-chloromethylbicyclo[2.2.1]hept-2-ene and of 5,5-bis(chloromethyl)bicyclo[2.2.1]hept-2-ene; microstructure of the polymers

The three title monomers were polymerized using a variety of Ru-, Ir-, Mo-, W- and Re-based metathesis catalyst systems to yield polymers having a wide range of cis double bond contents in each case. The ¹³C NMR spectra of the polymers were interpreted in terms of cis/trans, head/tail and m/r microstructures. No head-tail bias was observed in any of the polymers.     

Metathesis polymerization behavior of 2-cyanobicyclo[2.2.1]hept-5-ene

This paper describes the ring-opening methathesis polymerization of homopolymers and copolymers of exo- and endo-2-cyanobicyclo[2.2.1]hept-5-ene (norbornenenitrile) in solution. Polymerization kinetics were followed via ¹H NMR spectroscopy in real time while the final polymer microstructure was determined via ¹³C NMR. While pure endo monomer does not polymerize readily, it copolymerizes with exo monomer, rapidly and to high conversions. The structure of the copolymer appears to be quite similar to that of exo homopolymer. We conclude that exo monomer adds directly to either exo or endo, while endo-endo addition is sterically hindered.     

Mechanism of ring-opening polymerization of 1-methylbicyclo[2.2.1]hept-2-ene using metathesis catalysts

Polymers of the title monomer have been made using 24 different metathesis catalysts, based on Nb, Mo, W, Re, Ru, Os and Ir compounds. Their structure, determined by ¹³C NMR spectroscopy, varies from all-cis (ReCl₅) to all-trans (RuCl₃, OsCl₃) and from fully biased (head-tail) to unbiased. The bias stems partly from an inability to from head-head dyads containing cis double bonds and partly from the dipolar interaction between the propagating metal-carbene complex and the monomer in forming the [2 + 2] transition state. Arguments are presented to show that the formation of biased polymers proceeds largely through a metal carbene species in which the last-added monomeric unit has its methyl group adjacent to the metal centre (P_H). In high-cis polymers the sequences of three double bonds are such as to indicate that a kinetic distinction must be made between species in which the previous double bond formed was cis or trans (P_Hc or P_Ht), consistent with the observation that high-cis polymers of related monomers have a blocky cis/trans distribution.     

Cis/trans reactivity ratios in the ring-opening polymerization of bicyclo[2.2.1]hept-2-ene; the nature of the propagating species in olefin metathesis

The olefinic regions of the ¹³C NMR spectra of numerous ring-opened polymers of bicyclo[2.2.1]hept-2-ene (norbornene) having cis double bond contents in the range σ_c = 0,32–0,86, have been analysed in detail. At least six peaks are resolved corresponding to olefinic carbons defined in terms of the double bond sequences tcc, tct + ccc, cct, ttc, ctc + ttt, ctt; for polymers of intermediate cis-content tct and ccc are also resolved. The relative intensities of the cis-centred triads may be accounted for in terms of a single parameter, r_c, derived from the observed dyad probabilities. The relative intensities of the trans-centred triads, however, show that at higher cis-contents, the sequence cttc occurs with greater frequency than expected, requiring interpretation in terms of two trans-parameters, r_tt and r_tc. These results show that the generally noted feature of cis/trans blockiness at moderately high cis-content is sometimes associated with the presence of three kinetically distinct propagating species rather than two. The first is postulated to be the highly cis-directing olefin-metal-carbene complex 4 in which the previously formed cis double bond remains coordinated to the metal centre; this may have both chiral and achiral forms. The second and third are postulated to be isomeric species formed by decoordination of the previously formed trans double bond from the metal centre but in which the initial ligand geometry is governed by the nature of the penultimate double bond: 3tc and 3tt are postulated to have permanent ligands with square pyramidal and octahedral symmetry, respectively. It is shown how the present results and those from related work may be rationalised in terms of this hypothesis.     

Ring-opening metathesis polymerization of endo- and exo-5-methoxymethylbicyclo[2.2.1]hept-2-ene; microstructure of the polymers

The title monomers 1 and 2 were polymerized by Ru-, Ir-, Mo-, W- and Re-based metathesis catalysts to yield polymers with cis contents ranging from 10% to at least 90% for 1 and 23–60% for 2 . Assignments of the ¹³C NMR spectra were made. No significant head-tail bias was observed in either polymer. A high-trans polymer made from the optically active endo monomer ((?)- 1 ) was atactic, while high-cis polymers could be either atactic or biased towards syndiotactic, depending on the catalyst.     

13C NMR spectra of hydrogenated polymers of exo-5-methyl-endo-5-methyl-, and 5,5-dimethyl-derivatives of bicyclo[2.2.1]hept-2-ene prepared by ring-opening metathesis polymerization

¹³C NMR spectra of the three title polymers have been determined for polymers of different tacticities. Assignments to the various possible HH/HT/TT and m/r dyad structures were made. These were facilitated by the use of optically active monomers in the case of 1 and 3 . In the precursor polymers the cis double bonds in the HH dyads were less readily hydrogenated than those in the other types of dyad. Methyl substitution parameters are summarized for the hydrogenated polymers and their unsaturated trans precursors.     

13C NMR spectra of polymers made by ring-opening polymerization of (±)-endo-5-methylbicyclo[2.2.1]hept-2-ene using metathesis catalysts

A range of olefin metathesis catalysts has been used to prepare ring-opened polymers of (±)-endo-5-methylbicyclo[2.2.1]hept-2-ene ( 1 ), having cis double bond contents of 22–100%. The ¹³C NMR spectra of these polymers are interpreted in terms of TH, TT, HH, HT and tt, tc, ct, ccstructures (T = tail, H = head, referring to the methyl group orientation; t = trans, c = cis, referring to the double bond configuration). The fine structure of the olefinic region at 75 MHz indicates that the cis/trans double bond distribution is blocky when the cis content is more than 50%. The chemical shifts of the ring carbons indicate puckering of the cyclopentane rings. The cis relationship of the methyl carbon and the adjacent olefinic carbons causes an upfield shift in both cases.     

13C NMR spectra of polymers made by ring-opening polymerization of optically active and racemic 5,5-dimethylbicyclo[2.2.1]hept-2-ene using metathesis catalysts

Ring-opened polymers of racemic and optically active 5,5-dimethylbicyclo[2.2.1]hept-2-ene ( 6 ) (5,5-dimethylnorbornene), with cis double bond contents of 0–100%, were prepared using various Mo-, W-, Re-, Os-, Ru-, and Ir-based olefin metathesis catalysts. The ¹³C NMR spectra of these polymers are interpreted in terms of TH, TT, HH, HT structures (T = tail, H = head, referring to the orientation of the methyl groups), also tt, tc, ct, cc structures (t = trans, c = cis double bonds), and m, r dyads (m = meso, r = racemic, referring to the relative configuration of adjacent cyclopentane rings in the chains). m/r splitting is observed for both the olefinic carbon (C³) and the adjacent ring carbon (C⁴) in the HH structures of the all-trans polymer made from racemic monomer using RuCl₃ as catalyst. All-cis polymers are fully syndiotactic while all-trans polymers have 58% isotactic (m) and 42% syndiotactic (r) dyads. A consistent set of substitution parameters is derived, the effect of two methyl substituents on the ¹³C chemical shifts being generally a little less than the sum of the effects of the individual substituents.     

Ring-opening metathesis polymerization of syn- and anti-7-methylbicyclo[2.2.1]hept-2-ene initiated by Mo(CHCMe3)(NC6H3-2,6-i-Pr2)(OCMe3)2

The ring-opening metathesis polymerization of syn- and anti-7-methylbicyclo[2.2.1]hept-2-ene (M_s and M_a) by Mo(CHCMe₃)(NC₆H₃-2,6-i-Pr₂)(OCMe₃)₂ has been followed by ¹H and ¹³C NMR spectroscopy in C₆D₆ and CD₂Cl₂ as solvents. The anti monomer M_a polymerizes first, followed much more slowly by the syn monomer M_s, leading to a block copolymer. The reaction is considerably faster in CD₂Cl₂, where it proceeds to completion, than in C₆D₆. In C₆D₆k_pa/k_ia = 9, as determined from the proportion of residual initiator. The double bonds formed are mainly trans (80–90%) in both blocks and the diads embracing the trans double bonds in the M_a blocks have an isotactic bias: (σ_m)_t = 0,69 in C₆D₆, independent of monomer concentration; the diads in the M_s bloks have a slight tactic bias, probably isotactic. The tacticity is interpreted in terms of control of addition of monomer by the configuration of the chiral carbons in the C₅ ring nearest to the molybdenum centre in the propagating metal-carbene complex.     

1H NMR study of the kinetics of metathesis polymerization of 5- and 5,6-methoxycarbonyl derivatives of bicyclo[2.2.1]-hept-2-ene,initiated by

The ring-opening metathesis polymerization of 5-endo-methoxycarbonyl ( 1 ), 5-exo-methoxy-carbonyl ( 2 ), 5,6-endo,endo-bis(methoxycarbonyl) ( 3 ), 5,6-exo,exo-bis(methoxycarbonyl) ( 4 ) and 5,6-endo,exo-bis(methoxycarbonyl) ( 5 ) derivatives of norbornene, initiated by the tungsten cyclopentylidene complex , have been followed by ¹H NMR in situ at 290 – 320 K. The initial metal-carbene adduct P ₁ is usually distinguishable from the subsequent adducts P _n(n > 1). Initiation is generally faster than or as fast as propagation. The propagation rate constants for the monosubstituted derivatives 1 and 2 are two to four times greater than those for the 5,6-disubstituted derivatives 3 – 5 . Arrhenius parameters were determined for the propagation reactions of 1 and 3 .     

The ring-opening metathesis polymerization of (±)-endo-bicyclo[2.2.1]hept-5-en-2-yl acetate; microstructure of the polymer

The monomer (±)-endo-bicyclo[2.2.1]hept-5-en-2-yl acetate ( 1 ) was polymerized by Re-, W-, Mo- and Ru-based metathesis catalysts to yield polymers with cis double bond contents ranging from 76% to 16%. A complete assignment was made of the lines in the ¹³C NMR spectra. No significant head-tail bias was observed and no splittings due to tacticity were detected.     

Synthesis and cationic ring-opening polymerization of 2-methylene-7-oxabicyclo[2.2.1]heptane

A new unsaturated bicyclic ether, 2-methylene-7-oxabicyclo[2.2.1]heptane ( 4 ) was synthesized by successive Diels-Alder addition of furan to acrylonitrile, hydrogenation of cyano compound 5 , and Hofmann degradation of the quaternary ammonium hydrate 7 derived from the amine 6 . Cationic polymerization of the monomer 4 was carried out in CH₂Cl₂ using BF₃ · OEt₂, PF₅, SbCl₅ and SnCl₄ as catalysts in the temperature range of ?20 to ?25°C. White or slightly yellow powdery polymers of low molecular weight were obtained with PF₅ and BF₃ · OEt₂ as catalysts, while no polymer was obtained with SbCl₅ and SnCl₄ as initiators. The IR, ¹H and ¹³C NMR spectra of the polymers indicated that they consist of three kinds of repeating units, that is, two unsaturated units caused by ring-opening polymerization and one addition unit. Using proton NMR data of the polymers, the distribution of repeating units was calculated. A polymerization mechanism is proposed. The thermal stability of the obtained polymers is discussed.     

On the mechanism of polymerization of methyl substituted 7-oxabicyclo-[2.2.1]-heptanes

The polymerization of endo- and exo-2-methyl-7-oxabicyclo-[2.2.1]-heptane with phosphorus pentafluoride catalyst in methylene dichloride and nitrobenzene has been found to proceed with increasing yield and molecular weight with decreasing temperature in the range +25°C to ?30°C, but at ?78°C there was no polymerization. The ring-opening appears to proceed exclusively through nucleophilic attack at C-1 in the alkyloxonium ion, leading to only one type of structural unit in each type of polymer. The polymers of the exo-2-methyl isomer are completely soluble in THF, whilst those of the endo-2-methyl isomer are insoluble. The solubility appears to be connected with flexibility of the chain molecule associated with the possibility for interconversion between different chair conformations for the cyclohexane ring-structures. By ring-closure of 2.6-dimethylcyclohexane-(1.4)-diol two isomers have been prepared and identified. The endo,exo-2.6-dimethyl-7-oxabicyclo-[2.2.1]-heptane gives insoluble, highly crystalline polymers with phosphorus pentafluoride catalyst in methylene dichloride. The failure of the corresponding exo,exo-2.6-dimethyl isomer to polymerize under identical conditions is explained by conformational analysis.     

Metathesis polymerization of exo,exo-, endo,exo- and endo,endo-5,6-dimethylbicyclo[2.2.1]hept-2-enes; microstructure of the polymers and their hydrogenated products

Polymers of exo,exo-, endo,exo, and endo,endo-5,6-dimethylbicyclo[2.2.1]hept-2-ene monomers were prepared using a selection of eight metathesis catalysts and their structures examined by ¹³C NMR spectroscopy. Tacticities could be determined for the polymers of the endo, endo and endo,exo monomers and for all three hydrogenated polymers. RuCl₃ gave hightrans atactic polymers while ReCl₅ gave high-cis syndiotactic polymers, but WCl₆/Bu₄Sn gave a high-cis atactic polymer of the endo,endo monomer. The endo,exo monomer was unusual in giving polymers of relatively low cis content (fraction of cis double bonds σ_c ≤ 0,3), and with strong exo,endo (XN) bias except in the case of RuCl₃. The parallel with the head, tail (HT) biased polymers of 1-methylbicyclo[2.2.1]hept-2-ene is noted. Polymers of the exo,exo and endo,endo monomers have a blocky cis/trans double bond distribution at moderate cis content. The results are discussed in terms of a mechanism involving propagation by metal-carbene, metal-carbene-olefin and metallacyclobutane complexes.     

Polymerization of bicyclic acetals, 8. Cationic polymerization of 6,8-dioxabicyclo[3.2.1]oct-2-ene

Poly(5,6-dihydro-2H-pyran-2,6-diyloxymethylene) ( 6 ), a new polyacetal having a dihydropyran ring in its repeating unit was prepared by selective ring-opening polymerization of 6,8-dioxabicyclo[3.2.1]oct-2-ene ( 5 ) at temperatures between ?60 and ?90°C in dilute methylene chloride or toluene solutions using boron trifluoride etherate as initiator. The resulting polymers, having a number average molecular weight up to 1,5 · 10⁴, melted at 110 – 130°C and decomposed at ≈ 170°C in air. The isotactic dyad content of the polymers, estimated by ¹³C NMR spectroscopy, was found to be in the range of 70 to 77 mole-%, somewhat lower than that of its saturated analogue, poly(tetrahydropyran-2,6-diyloxymethylene), ( 2 ). The preferred formation of the isotactic sequence along the polymer chain is rationalized in terms of the enantiomer selection by the growing chain end control mechanism as previously proposed for the polymerization of 6,8-dioxabicyclo[3.2.1]octane, ( 1 ).     

Enthalpy of polymerisation of 7-oxabicyclo[2.2.1]heptane,and exo- and endo-2-methyl-7-oxabicyclo[2.2.1]heptane

The enthalpies of combustion of poly(oxy-1,4-cyclohexylene) formed from 7-oxabicyclo[2.2.1]heptane ( 1 ) and of its derivatives formed from exo- and endo-3-methyl-7-oxabicyclo[2.2.1]heptane ( 2a and 2b ) have been measured. From literature values for the enthalpies of combustion of the liquid monomers, the enthalpies of polymerisation have been derived: ?ΔH(1→c)=44,3±1,9, 49,7±2,6, and 45.4±3,1 kJ mol^?1, respectively. The results indicate a minimum strain energy in these polymers of ca. 15 kJ mol^?1. The estimated entropies of polymerisation yield ceiling temperatures of 320, 240, 200°C respectively for the three bicyclic ethers.—The reasons for the discrepancy between our present ?ΔH(1→c) for the endo compound and that previously reported², are explained.     

Anionic polymerization of exo-3,4,5-trithiatetracyclo[5.5.1.O2,6.O8,12]tridec-10-ene (dicyclopentadiene trisulfide)

The anionic polymerization of exo-3,4,5-trithiatetracyclo[5.5.1.O^2,6.O^8,12]tridec-10-ene ( 1 ) in bulk and/or in aromatic solvents (benzene, toluene) was studied. The polymerization was initiated with sodium benzenethiolate (sodium cation complexed with dibenzo-18-crown-6). Polymers with high-molecular weights were obtained (M _n ≈ 10⁵, osmometrically). The polymerization was found to be living and reversible; the equilibrium monomer concentration increases with the temperature. The ceilling temperature was estimated as 167°C. The thermodynamic data of the polymerization in toluene was determined and compared with those of the polymerization of exo-3,4,5-trithiatricyclo[5.2.O^2,6]decane. The standard enthalpy ΔH = ?(6,6 ± 0,6)kJ · mol ¹ and entropy ΔS = ?(29,3 ± 2,1)J · mol ¹ · K ¹ of the polymerization of 1 were evaluated from the temperature dependence of the equilibrium monomer concentration, determined dilatometrically.     

Enthalpy of polymerisation of endo-2-methyl-7-oxabicyclo[2.2.1]heptane

The enthalpy of combustion of poly(oxy-2-methyl-1,4-cyclohexenylene) prepared from endo-2-methyl-7-oxabicyclo[2.2.1]heptane has been determined by oxygen-bomb combustion calorimetry. The calculated standard enthalpy of formation of the polymer, ΔH (298,15K)= —283,5±3,0 kJ (base-mol)^—1, has been combined with that of the monomer to give the enthalpy of polymerisation of liquid monomer to solid polymer as ΔH = —33,3±3,7 kJ mol^—1.     

Polymerization of bicyclic acetals, 4. Cationic polymerization of 1,3,3-trimethyl-2,7-dioxabicyclo[2.2.1]heptane

The polymerization of 1,3,3-trimethyl-2,7-dioxabicyclo[2.2.1]heptane ( 1 ) was carried out in methylene chloride, toluene, and 1-nitropropane at temperatures between ?78 and 0°C. Borontrifluoride etherate, triethyloxonium tetrafluoroborate, antimony pentachloride, and iodine were used as initiators. Irrespective of the solvents and initiators used, the products at 0°C were white powders with melting points of 50—55°C, while those obtained at lower temperatures were sirups. The number average molecular weights of the unfractionated products ranged from 400 to 600. The molecular weight distribution of the oligomers prepared at 0°C was broad, in contrast to the relatively narrow distribution for those obtained at ?40°C. On the basis of the determination of the hydroxyl group and unsaturated bond of the oligomers, along with the hydrogenolysis of 1 , the structure of the oligomers and the mechanism of their formation are discussed.     

Polymerization of bicyclic acetals, 5. Cationic polymerization of 6,8-dioxabicyclo[3.2.1]oct-3-ene

The polymerization of 6,8-dioxabicyclo[3.2.1]oct-3-ene ( 1 ) was carried out in methylene chloride at temperatures ranging from ?78 to 0°C. Boron trifluoride etherate, triethyloxonium tetrafluoroborate, and iodine were used as initiators. Completely or nearly completely soluble polymers were obtained at ?78°C in dilute monomer solution, whereas crosslinking occurred to an appreciable extent at higher temperatures or in concentrated monomer solution. ¹H NMR and ¹³C NMR analyses of the soluble polymer, together with its catalytic hydrogenation, disclosed that the cationic polymerization of 1 proceeded, without isomerization of its dihydropyran ring, through the selective cleavage of the C⁵? O⁶ bond to yield a polyacetal having a 5,6-dihydro-2H-pyran ring in the repeating unit.