J . Chem. SOC., Faraday Trans. I , 1982, 78, 3307-3317 Some Aspects of Stereoselectivities and Kinetics in the Ring-opening Polymerization of Norbornenes using Met at hesis Catalysts The Nature of the Metallacarbene Intermediates BY Ho Huu THOI, BOREDDY S. R. REDDY AND JOHN J. ROONEY* Department of Chemistry, The Queen’s University, Belfast BT9 5AG, Northern Ireland Received 16th February, 1982 Stereoselectivities in the ring-opening polymerization of bicyclo[2 .2. l jhept-2-ene and several 5,5- disubstituted derivatives have been extensively investigated using numerous metathesis catalysts at different temperatures and concentrations of monomer. Some studies of the kinetics of polymerization are also briefly reported. For many Group VI catalysts at moderate to high concentrations of monomer a limiting value of ca.50% for the cis content of the polymers is observed. This feature is related to the observation that under these conditions the kinetics of polymerization are zero order with respect to monomer, and is explained by the postulate that propagation is due solely to two mirror-image forms of metallacarbene which may coordinate monomer with equal facility in either a cis or rrans orientation. However, with Group VI catalysts at high dilution of monomer, or with noble-metal catalysts at moderate concentrations, polynorbornenes having a high trans content are obtained. A head-tail structural bias, which increases with dilution, also becomes evident in polymers of the 5,Sdisubstituted derivatives, especially when the substituents are polar and the catalysts are IrCl, or OsC1,.These results are discussed in terms of Michaelis-Menten kinetic theory with the postulate that the chiral metallacarbenes relax to a more stable symmetrical form. The intermediates described and the kinetic theory developed are used to explain several other important aspects of stereospecificities and selectivities in olefin metathesis and ring-opening polymerization. While selectivities and stereoselectivities in olefin metathesis and ring-opening polymerization of cycloalkenes have been frequently studied, comparatively little attention has been given to the kinetics of these processes. For example, stereoselec- tivities in n-alk-2-ene metathesis are widely discussed2 in terms of the energetics of distinct shapes of puckered metallacycles derived from one type of metallacarbene.The distinction in the metallacycles is supposed to depend on the orientations, axial or equatorial, of substituents in cis or trans 1,3-positions. Recently we have questioned3 this postulate, suggesting instead that there may be different types of metallacarbene with distinct kinetic behaviour, the difference arising if the last-formed double bond is cis, because in that case it may still be coordinated to the metal centre while the next propagation step commences. This theory is strongly supported by evidence obtained from 13C n.m.r. spectra of ring-opened polymers, but is obviously applicable to metathesis of n-alk-2-enes as well. The purpose of the present paper is to show that many other aspects of stereosel- ectivities in ring-opening polymerization are also best discussed in terms of kinetically distinct metallacarbenes and Michaelis-Menten kinetic theory. The possible similarity to enzyme systems in this respect was first alluded to as early as 1970 by hug he^,^ who derived kinetic equations based on the now discarded quasi-cyclobutane mechanism.The kinetics of ring-opening polymerization of norbornene (NBE) 3307 107-23308 POLYMERIZATION OF NORBORNENES catalysed by some Ru complexes have recently been inve~tigated,~ but it was erroneously concluded that they were zero order in monomer, whereas the data clearly show that they are first order. Following hug he^,^ a Michaelis-Menten type equation for the rate of propagation, R, = k, K[M] [cat]/( 1 + K[M]), where [MI is the concen- tration of monomer, was also developed but an equilibrium constant, K , for complexation of monomer was used instead of the correct Michaelis-Menten constant.This is a common error which becomes important if the overall reaction is zero order with respect to monomer. During the past five years we have extensively investigated cis contents ( o , ) , ~ cisltrans blockiness ( k , k,)’ and tacticities3 of ring-opened polymers of cyclic olefins made using numerous metathesis catalysts under a variety of conditions. Tacticities were determined from the 13C n.m.r. spectra of polymers 2a made from racemic and partially resolved 5,5-dimethylbicyclo[2.2. llhept-2-ene (DMNBE), 1 a. Such spectra show four distinct resonances for the tail-head (TH), tail-tail (TT), head-tail (HT) and head-head (HH) C atoms in both the cis and trans junctions. In most cases there 1 ta T H HT TT TT H H HH ( m dyads only shown ) is no bias towards either HT(TH) or HH, TT structures in 2a, but for certain catalysts and conditions which promote the formation of high-trans polymer a slight bias towards HTunits i~clearlyevident.~ We now report further aspects ofthis stereoselective feature together with some significant information about the kinetics of propagation and novel relevant data on oc values for polynorbornenes. The major aim is to delineate further the different types of metallacarbene intermediates involved, the kinetic conditions under which they are important and their stereoselective characteristics.EXPERIMENTAL AND RESULTS Details of polymerization procedures and systems, methods of extraction of polymer samples and 13C n.m.r.analysis have been given el~ewhere.~’ The rates of polymerization of DMNBE at 353 K in CDCl, initiated by Ph(MeO)C=W(CO), were followed by lH n.m.r. using spectral intensities of the monomer relative to those of CH,Cl, which was added to provide an internal standard. The rate was essentially independent of [MI ([MI = 1.78-0.37 mol dm-3) over the first 20-30% conversion (RP = 1.05f0.13 x loM4 mol dm-3 s-l, o, = 0.55), but then suddenly fell off (fig. 1). Zero-order conversion of monomer was also foundHO HUU THOI, B. S. R. REDDY A N D J. J. ROONEY 0.6- 0.4 0, 3309 A n 0 0 A - 3 0 20 4 0 60 80 time/min FIG. 1 .-Concentration of monomer [MI plotted against time for various initial concentrations [MI,: (a) 1.78, (b) 0.81 and (c) 0.37 mol dm-, in degassed CDCI, solution at 353 K.ph(MeO)C=W(CO),] = 0.26 mol dmP3. 0 0 1 2 3 4 5 6 [Ml,/mol dm-3 FIG. 2.-Effect of initial monomer concentration on oE for ring-opened polymers of nor- bornene. Conversion 20-30%. [WCl,] = 5 x rnol dm-, at 293 K in chlorobenzene solvent. [IrCl,] = 4 x lop2 mol dm-, at 348 K in chlorobenzene/ethanol (5/1). 0, WCl,/Ph,Sn (1/2); A, WCl,/Me,Sn ( l / l ) ; 0, IrCl,.3310 POLYMERIZATION OF NORBORNENES TABLE 1.-Cis CONTENTS IN RING-OPENED POLYMERS OF NORBORNENE~ MADE AT 288 K ~ catalyst g c rt rc WCl, WCl,/Me,Sn (1 /2) WC16/Ph4Sn (1 /2) WCl,/EtAlCI, (1 /4) MoCl, MoC1,/Me4Sn (1 /2) MoCl,/Ph,Sn (1 /2) Ph( MeO)C=W(CO),b MesW(CO),/E/EtAlCl, (1 / 1 / l)c MesMo(CO),/E/EtAlCl, ( l / l / l ) MesCr(CO),/E/EtAICl, (1 / 1 / 1) MesW(CO),/AlCl, ( I /20) ReCl,/EtAICl, (1 /4) MesW(CO),/AlCI, (1 /20)d MesW(CO),/AlCl, (1 /20)e 0.55 0.55 0.56 0.5 0.45 0.46 0.47 0.6 0.46 0.42 0.52 0.42 0.55 0.50 0.55 1.7 2.0 2.1 I .4 1.2 1.6 1.6 1.5 - - a [MI was in the range 1.5-4.0 mol dm-,; the temperature was 353 K; Mes is reactant was cyclopentene; reactant was mesitylene and E is norbornene epoxide; cyclooctene.TABLE 2.-STEREOSELECTIVITES IN THE RING-OPENING POLYMERIZATION OF SOME 5,5-DISUBSTITUTED NORBORNENES [MIU monomer /mol dm-, catalyst TH/TT l a l a l a l c l a l a l a l a l a l c l b I d 3.9 1.6 2.0 1.1 3.3 0.54 2.0 0.4 0.4 1.6 1.6 1.5 ReC1, Ph(MeO)C=W(CO), Ph(MeO)C=W(CO), RuC1, RuC1, IrCl, IrCl,b OsCl, RuC1, RuC1, RuCl, WCl,/Me,Sn (1 / 1) 288 373 288 328 323 323 348 348 348 333 333 333 1 .o 1 .o 0.65 1 .o 0.9 0.7 1 .o 0.9 0.86 0.9 1.1 < 0.05 1.1 0.0 1.2 0.26 1.1 1.1 - 3.0 > 3.0 0.22 1.3 1.2 < 0.05 1.5 0.0 1.5 0.0 1.6 - - - - - - - a Using the noble-metal catalysts the solvents for l a and for l b to Id were, respectively, a 1/5 and a 1/1 mixture (vol./vol.) of chlorobenzene and ethanol.Chlorobenzene alone was used for the other catalysts, except for 1 c and Ph(MeO)C=W(CO),, where the solvent was deuterochloroform. The spectrum was complicated by additional signals due to end groups but the TH/TT ratios were clearly of the size indicated.HO HUU THOI, B. S. R. (REDDY A N D J. J. ROONEY 331 1 for the polymerization of cyclopentene ([MI = 5.4 mol dm-3) at 273 K using the very reactive WCl,/Ph,Sn catalyst in chlorobenzene solvent with analysis by a g.1.c.method to be described el~ewhere.~ A similar abrupt termination after 40-50% conversion was observed, but when the reaction was followed at 3 13 K zero-order behaviour was still noted without any sign of a fall-off in rate up to ca. 75% conversion. The reason for these abrupt terminations is not known and is still being investigated. The effects of initial concentration of monomer on ac values for NBE polymerization catalysed by WCl,/Ph,Sn and WCl,/Me Sn at 293 K and IrCl, at 348 K are further shown in fig. 2. In the higher range, 1-4 mol dm-3, oc is independent of [MI but in the lower range, 0.26-0.1 mol dmP3, ac decreases rapidly with dilution for the WCl,-baSed systems.The value of ac N 0.5 is obviously a limit since it also remained essentially constant for WCl,/Ph,Sn in the range 204-403 K.* Similar values (a, 1: 0.5i were observed using a variety of other catalyst systems at 288 K and they are recorded in table 1. The low rtrc values show that the polymers have essentially a random c5ltran.s distribution.' Polymers were made from 1 a, 1 b, 1 c and 1 d using several catalysts, and ac values together with TH/TT ratios for cis and trans junctions are given in table 2. Three features are noteworthy. (i) The TH/TT ratios are unity when ac is high (non- noble-metal catalysts). (ii) A slight bias in favour of HT units is clearly evident with the noble-metal catalyst (low ac values) and becomes more pronounced at high dilution.(iii) The bias is stronger for monomers with polar substituents. DISCUSSION While the interconversion of olefin-metallacarbene and metallacyclobutane com- plexes is now accepted as the mechanism of olefin metathesis, the precise nature of these species in highly active catalysts is still unknown. We will take the unconventional view here, and justify it later, that a metallacyclobutane proper may only be present as a transition state. Furthermore, for highly reactive olefins, e.g. NBE, it seems likely that the appropriate orbitals of Mt=C and C=C are already engaging each other to form a quasi-metallacyclobutane in the monomer-complexation step. These ideas are summarized in scheme 1 , where [Mt] is the metal ion with permanent ligands and I [ M t]=CH' 6' J P,CH 3 4 5 6 3' SCHEME 1 P, is the polymer chain; the essence of scheme 1, as compared with the analogous scheme 1 previously p~stulated,~ is that there are no longer separate and independent monomer-coordination and metallocyclobutane-forming steps, i.e.a complex with discreet carbene and olefin ligands may often never be realized. This may seem surprising but it is worth remembering that metathesis catalysts are very similar to those for Ziegler-Natta polymerization of a-olefins, where the metal ions involved are knownlO to have very little tendency to coordinate ethylene in comparison with metal * For further details see fig. 5 in ref. (3).3312 POLYMERIZATION OF NORBORNENES ions such as Ag+ and those from Group VIII. The difference is ascribed to the importance of dz 4 p K * back-bonding when the d orbitals are extensively occupied; without this bonding component simple olefins are only very weak bases.Two propagating sequences are now envisaged, one of which involves interaction of monomer with the metallacarbene to give the quasi-metallacyclobutane (3 + M -+ 4) followed by bond switching in 4 (4 -+ 5 ) with rapid orbital disengagement (5 -+ 6), and finally decoordination of the newly formed double bond (6 -+ 3’). The metallacyclo- butane is now only a transition state in step 4 -+ 5. It is likely that in many systems 6 is also a transition state in the conversion of 5 to 3’, but we have retained 6 3 6’ in scheme 1 because there is evidence that for some catalysts the alternative sequence which consists only of 6’+ M + 4 -+ 5 --+ 6 is important where displacement of the newly formed junction by monomer is postulated, but only when the double bond is cis.Furthermore, the entry of monomer into 6’ is believed to be such that the orientation which again gives a cis junction is strongly fav~ured.~ The division of 6 into two general types, Pc and pt, where the subscripts define whether the newly formed double bond is cis or trans, respectively, is a necessary distinction, even though pt is never regarded as a propagating species, in contrast to Pc. Also three different forms of each of these, a mirror-irlage pair and a kinetically symmetrical form, have to be considered in order to explain the results of the tacticity s t ~ d i e s . ~ A good example which illustrates this important conclusion is the hexacoordinated W-complex which seems to be the propagating speciesf1 when Casey’s compound, Ph,C=W(CO),, is used as catalyst.The following octahedral geometries seem reasonable for the mirror-image forms and the kinetically symmetrical form, respectively. Several forms of 3 = 3’ also seem to be important. Thus as the newly formed double bond decoordinates in 6 the corresponding pentacoordinated species immediately obtained, P, can also be regarded as essentially octahedral with a vacancy 0. The following three geometries are also envisaged for P H P\$ II/ ’I C -Mt--O P H \ $ II/ ’I C -Mt--O which may then isomerize to more stable forms, P,, where square pyramidal or trigonal bipyramidal structures of the following types seem to be reasonable approximations. H P \ / C II / -yt\ The above discussion by no means exhausts the symmetry possibilities because in the hexacoordinated/pentacoordinated manifold all the permanent ligands, e.g.C1-HO H U U THOI, B. S. R. REDDY A N D J. J. ROONEY 3313 or CO, may not be identical, and corresponding pentacoordinated/tetracoordinated geometries would have to be considered for propagating species such as >C=WOCl,. However, for the purposes of the present paper and for any given catalyst system it is only necessary to consider three kinetically distinct propagating species P,, P and P,, and two relaxation processes, P, + P and P -+ P,, as shown in scheme 2, in order to account for the results over the full range of IT^ values (1 .O-0.0).By definition, all p, species must relax to P. Since each catalytic cycle consists of a series of steps, as detailed in scheme 1, the rate constants in scheme 2 are not those for propagation k2c Pc + M - pc f k1 I + P + M ’ L P, k4 / p, + M--!J%+ PC SCHEME 2 but in subsequent discussion will be regarded as rate constants for the monomer- complexation steps. The kinetics of propagation can be considered in terms of Michaelis-Menten theory which, when applied to scheme 1 (3 + M + 4 -+ 9, leads to three distinct limits as follows. Type (1): Step 4 -+ 3+ M is negligible compared with 4 -+ 5, which is the rate-controlling step, so polymerization is zero order in [MI. Type (2): Step 4 -+ 3+M is also negligible compared with 4 + 5, but with dilution 3 + M -+ 4 becomes slower than 4 -+ 5 and first-order behaviour in [MI is eventually obtained.Type (3): Step 4 -+ 3+M is always significant and in the limit is at equilibrium with the complexation step, such that the rate of polymerization is first order with respect to monomer. One extreme behaviour predicted by scheme 2 is that there is no relaxation of any sort so propagation is confined to the sequence 6’+M -+ 4 + 5 + 6, giving all-cis polymer, as found for NBE or DMNBE at room temperature using ReCI, as ~atalyst.~ In the range 1.0-0.5 the IT, values and cisltrans blockiness’ are then governed by the effect of variables such as temperature and monomer concentration on the ratio k2 [M]/I?,.~ Blockiness demands participation of at least two kinetically distinct propagating species, e.g.P, and P. A second important limiting case, often noted for the most active catalysts, is when IT, 21 0.5, even at 204 K for NBE polymerization using WCl,/Ph,Sn, a fact which supports the view that 6 is now only a transition state and not an intermediate. Furthermore, the essentially random nature of the polymers (table 1) confirms that there is only one type of propagating species which must be assigned as P. The 0, values can then be explained by Michaelis-Menten theory as applied to eqn (1) and (2), where [PM] denotes 4 and P is 3. (2) Steady-state analysis shows that the competition ratio, R,,,, for the formation of cis and trans junctions is given by 4, k2t P+ M - [PM], + Pt. k-It (3) klCk2, 1 kltk2t (k-1, + k 2 d V - l t +k2t) ’ Re,, =3314 POLYMERIZATION OF NORBORNENES When the kinetics are type (1) (cf. fig.l), kWlc and k-,, are negligible compared, respectively, with k,, and k2t, so eqn (3) reduces to R,,, = klCIklt. (4) The temperature independence ofo, and the values (ca. 0.5) are then in accord with the conclusion that for a highly reactive olefin such as NBE monomer complexation with P is non-activated and therefore random, in contrast to the analogous reaction of M with P, using ReCl,. When [MI is decreased and the kinetics are type (2), eqn (4) and therefore o, II 0.5 should still hold if dilution of monomer has no other consequences. However, the data in fig. 2 show that at low [MI o, rapidly decreases. We believe that the reason for this change is that the metallacarbenes P are now isomerizing to the more stable and therefore less reactive form P,.Complexation is now an activated process and because [PM], is lower in energy than [PM],, as a consequence of steric factors associated with substituents on the carbene C atom and on the double bond in M as 4 is formed, P, is trans directing with El, > El, > 0 (cf. El, z El, II 0 for P, but El, < El, > O for P,). El, and El, are the activation energies for monomer complexation in a cis and trans orientation, respectively. A steady-state analysis of the relevant part of scheme 2, neglecting the minor step (5c), gives which predicts that o, may change with dilution from independence of to first-order dependence on [MI, as found for the WC1,-based catalysts (fig. 2). The noble-metal catalysts tend to give polymers with rather low o, values, and the kinetics for NBE polymerization using RuCl, are first order5 even at [MI = 5.0 mol dm-3.Therefore it is quite possible that this is an example of type (3) behaviour, in which case k-l, % k,, and k-,, % k,,, so that eqn (3) becomes Rclt = KCk,,/Ktk,t (6) where K, and K, are, respectively, klc/k-lc and klt/k-lt, the equilibrium constants for monomer complexation. K, < Kt should hold since [PM], is higher in energy than [PM],, so o, < 0.5 is expected by this analysis (k,, e k,, should be true). However, type (2) kinetic behaviour even at high monomer concentrations may be more likely with the noble metals since rapid decomplexation of NBE may never be feasible. This would imply that complexation is activated (3+M + 4 slower than 4 + 5), so El, > El, > 0 is reasonable for P and therefore its trans directing character.In any event, first-order rates in [MI imply that the trans directing P, should be a significant propagating species, and indeed the fall in o, with dilution of NBE noted for the IrCl, catalyst (fig. 2) supports its intervention. The increasing loss of tacticity in polymers of DMNBE with monomer dilution using RuC1, as catalyst had previously led to the same concl~sion.~ We also know from the tacticity studies3 that P, is not involved in Ph(MeO)C=W(CO), catalysed polymerization of DMNBE, where the rates are zero order in [MI (fig. 1) and o, = 0.55. In this case P seems to be the dominant carrier and in agreement with the kinetic theory should not, and does not, relax to P,.HEAD-TAIL STRUCTURAL BIAS I N POLYMERS OF 1 A head-tail structural bias is never observed for ring-opened polymers of 1 a when P, and P are the likely propagating species3 (the substituents act merely as a label), but a slight bias (table 2) is noticed, especially with noble-metal catalysts at highHO HUU THOI, B. S. R. REDDY A N D J. J. ROONEY 3315 dilution of monomer where P, is believed to be a significant carrier. In order to confirm this theory, DMNBE was polymerized using some of the W catalysts described in table I , but with [MI decreased by a factor of ca. 10 in order to bring it within the range (cf. fig. 2) where it is expected that P, should be a significant chain carrier. In agreement with prediction, oc decreased substantially and a head-tail bias in the junctions became evident.12 This is a key observation in conjunction with the type (2) kinetic behaviour expected of such W catalysts at high dilution of M.Thus even cyclopentene polymerization (less strongly complexing monomer than NBEY is zero order in [MI at higher concentrations using the WCl,/Ph,Sn catalyst. Clearly if monomer coordination and metallacycle formation were independent consecutive steps, and the reversibility of the former is negligible, the head-tail bias, which must be due to partial enantiomeric selection,12 would have to be entirely associated with discrimination between the enantiomers in the coordination step (addition is confined to the exo face of norbornenes). This is where scheme 1 has great advantages because all the polar and steric factors required for such discrimination are immediately operative in the complexation step (3+ M -+ 4).Since P, has either a head or tail structure with respect to the positions of the 5,Sgroups and the metal, the formation of the different types of junction, HH, HT, TH and TT, for one type of double bond, e.g. trans, can be considered in terms of the following set of equations, where MH and MT are enantiomers: An analogous set of equations can be written for cis junctions, and a head-tail bias will develop if PH is the dominant carrier with kHT > kHH, or if P, is dominant and kTH > kTT. The reason why P, shows such a bias, but not P, can again be attributed to the fact that P, is the more stable isomeric form and this increases the energy of activation for the step (3+ M + 4) to the point where P, becomes enantioselective as well as trans directing.While the discrimination against the cis orientation is attributed to unfavourable cis- 1,2-~ubstituent interaction in the appropriate complex [PM],, the head-tail bias seems to be determined mainly by the polarities of the Mt=C bond and the C=C bond in the monomer, in agreement with the facts that the bias is more evident for polar substituents in 1 (table 2) and is more pronounced for 0 s and Ir than for the Ru catalyst although the latter is more trans directing at the same dilution. There is good evidence13 that Mt=C is the direction of polarization associated with high metathesis activity, and such polarization is expected to increase down a triad of metals in the transition series as a consequence of the dramatic increase in ligand field effects on the dn orbitals and thus on the dn-pn bonding component in the Mt=C bond.The same theory has been usedl to explain the trends in the ratios of degenerate to productive metathesis of n-alk-1 -enes using Group VI metal catalysts. 6- 6+3316 POLYMERIZATION OF NORBORNENES - If the polarized Mt=C bond is represented by the canonical form Mt-C it is also easy to see by analogy with carbonium-ion chemistry why scheme 1 is also mechanistically realistic (scheme 3). Indeed mechanisms of this type are necessary for olefin metathesis catalysed by All4 and P15 complexes. TC-i c+ c c-c [ M t l - C [ M t l - + C [Mtl-C SCHEME 3 GENERAL ASPECTS For high cis directing catalysts monomer complexation is an activated process.It is not surprising therefore that such catalysts are highly selective in copolymerizationl6? l7 and poor at cross-metathesis with n-alk- 1 -enes,l* because the less strongly complexing olefin will not compete very well. The highly active catalysts (a, = 0.5; cf. table 1) have low selectivities and are good at cross-metathesisl69 l7 since olefin complexation is apparently almost non-activated and therefore rather non- selective. However the high trans directing noble-metal catalysts, such as RuCl,, are again highly selective in copolymerization16 in favour of the more strongly complexing olefin. In view of the first-order kinetics with respect to NBE c~ncentration,~ complexation strengths are again important.The stereoselective feature referred to as ‘formation of cis from cis and trans from trans’ sometimes found in the metathesis of pent-2-enes2* has been much discussed. Here we put forward an entirely novel explanation. It could well be related to cisltrans blockiness in ring-opened polymers and indeed is readily accounted for by two kinetically distinct metallacarbene species, one cis and the other trans directing. Such a pair analogous to P, and P or, better still, to P, and P,, is readily envisaged for the acyclics, cis-pent-2-ene having to displace cis-but-2-ene or cis-hex-3-ene in the complexation step, and therefore being forced to do so largely in the orientation which again gives an all-cis metallacycle (e.g. the behaviour of P, in ring-opening polymerization).For trans-pent-2-ene, however, the propagating metallacarbene is equivalent to P, and the only steric factor operating in the complexation step is the avoidance of an unfavourable cis-l,2-orientation of substituents, so the net result is that ‘trans is made from trans’. Puckering19 of the metallacycles is not therefore of primary importance as to the type of junction, cis or trans, which is ultimately formed, but may instead be of considerable significance in conjunction with group and orbital movements in the acts of formation and fission of the metallacyclobutanes (cf. 4 -+ 5). 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