J. Chem. SOC., Faraday Trans. 1, 1982, 78, 2227-2232 Kinetics and Mechanism of Polymerization of Phenylacetylene Initiated by (Phenylmet hox ycarbene)pentacarbonyltungs ten( 0) BY H o H u u THOI, KENNETH J. IVIN AND JOHN J. ROONEY* Department of Chemistry, The Queen’s University of Belfast, Belfast BT9 5AG, Northern Ireland Received 28th September, 198 1 The kinetics of polymerization of phenylacetylene in CDC1, solution, intiated by Ph(MeO)C=W(CO),, has been followed under various conditions by in situ analysis of monomer (A) and initiator (F) using ‘H n.m.r. spectroscopy. In degassed solution at 332 K the polymerization is zero order with respect to A and fractional order (0.7) with respect to F, the variation of rate with temperature (323-343 K) corresponding to an apparent activation energy of 116 kJ mol-’.The reaction is retarded by the presence of CO, 0, or air, but not by N,. Under 1 atm CO or air the reaction becomes first order with respect to F but remains zero order with respect to A. The mechanism of initiation and propagation is discussed in terms of elementary steps involving formation and reactions of metallacarbenes, metallacarbene-monomer complexes and metallacyclobutenes. The polymerization of phenylacetylene (A) initiated by olefin metathesis catalysts such as WCl,, MoCl, and WCl,/Ph,Sn has been extensively studied by Higashimura and Masuda.l The rate of polymerization and molecular weight of the polymer were investigated as a function of solvent, substituent (on the phenyl group of the monomer), cocatalyst and chain transfer agent; it was concluded that propagation proceeded by a coordination mechanism.Recently Katz2 has reported that metallacarbenes such as the so-called Fischer3 compound (F), Ph(MeO)C=W(CO),, and the Casey4 compound, Ph,C=W(CO),, catalyse the polymerization of phenylacetylene and alkylacetylenes to give high- molecular-weight polymers. This provides convincing evidence for the following mechanism where propagation proceeds via formation and rupture of a metallacyclo- butene intermediate. R1R2C CH R1R2C R1R2C-CH R1R2C=CH [Mt] CR3 [MtI+- 111 [Mtl-CR3 [Mt]=CR3 1 -+ etc. CR3 Furthermore, phenylacetylene in conjunction with F also induces the ring-opening polymerization of cy~loalkenes,~ and we have ourselves observed that it accelerates the polymerization of norbornene. There is as yet comparatively little kinetic information with which to test the mechanisms of these reactions.In the present work we have studied the kinetics of polymerization of phenylacetylene using F as initiator, since this compound has the advantages of being relatively stable at room temperature, insensitive to moisture and giving rates of polymerization that are sufficiently slow to be able to follow the disappearance of both monomer and initiator by lH n.m.r. spectroscopy. The 22272228 POLYMERIZATION OF PHENYLACETYLENE problem of irreproducibility and inhomogeneity which are often encountered with the much more active two-component metathesis catalysts were thereby avoided. EXPERIMENTAL Spectrograde deuterated chloroform was used without further purification.Phenylacetylene (B.D.H.) was distilled under reduced pressure. Ph(MeO)C=W(CO), was prepared according to the method of Fi~cher,~ the crude product being purified by column chromatography (silica gel, 1 % dichloromethane in pentane), recrystallized twice from pentane at 195 K, and stored under argon at 253 K. The reaction mixtures were prepared directly in n.m.r. sample tubes which could be connected to the vacuum line for degassing purposes. A small amount of toluene was added to provide an internal n.m.r. standard. After three freeze-thaw cycles the appropriate gas (CO, N, or 0,) was admitted at 1 atm (when required),* the tubes sealed off and then inserted in a thermostat (323-343 K) to start the reaction. At various time intervals the tubes were quenched and the concentration of monomer estimated from the area of the lH n.m.r.peak due to the acetylenic protons (6 = 3.0 ppm), relative to the area of the peak due to methyl protons of the added toluene. In this way the monomer concentration could be determined with a precision of kO.02 mol dm-3. The formation of polymer was indicated by the appearance of a broad peak at 6.3-7.5 ppm. The concentration of F was likewise estimated from its methoxy proton peak (6 = 4.6 ppm); in this case the limit of precision was kO.01 mol dm-3. The formation of carbon monoxide was confirmed by high-resolution mass spectroscopy and that of tungsten hexacarbonyl by 13C n.m.r. RESULTS Typical plots for the conversions of monomer (A) and initiator (F) against time are shown in fig.1 for a degassed solution. Note that the rate of removal of A remains constant up to 75% conversion while the rate of removal of F suffers a sharp fall after 8% conversion. After 120 min the ratio of monomer removed to initiator removed is > 20, indicating a chain reaction. 100 I I I 1 0 0 50 100 150 time/min FIG. l.--Conversion-time plots for (a) monomer (A) and (b) initiator (F) in degassed CDCl, solution at 332 K. [A],, = 0.90, [F], = 0.24 mol dm-3. * 1 atm = 101 325 Pa.H. H. THOI, K. J. IVIN AND J. J. ROONEY 2229 0 100 200 300 4 00 0 50 100 150 200 ti me/ m in FIG. 2.-Concentration of monomer [A] against time for various initial concentrations [A],: (a) 4.9, (b) 1.48, (c) 0.87, ( d ) 0.53 mol dm-, in degassed CDCI, solution at 332 K.[F], = 0.155+0.005 mol dm-,. - 9 -1 0 h - I I/? 0 ,-- .-. 0 0 - E D. -11 . s f: - -12 1 p’ -4 -3 -2 -1 0 FIG. 3.-Logarithmic plot of the rate of polymerization, R,, against the initial concentration of initiator, [F],, in chloroform solution at 332 K: (a) degassed, (b) 1 atm air present, (c) 1 atm CO present, ( d ) 1 atm N, present, (e) 1 atm 0, present. [A], = 0.9 0.1 rnol dm-3. The zero order with respect to monomer is confirmed by the results shown in fig. 2 for experiments at different initial monomer concentrations : the rate of polymeriza- tion R, is constant at (9+2) x lod5 mol dm-3 s-l, over a range of [Ale from 0.5 to 4.9 rnol dmP3, using [F],, = 0.155 & 0.005 mol dm-3. R, as a function of [Fl0 is shown in fig. 3 for the degassed solution [line (a)] and for the reaction in the presence of 1 atm air [line (b)], 1 atm CO [line (c)], 1 atm N,2230 POLYMERIZATION OF PHENYLACETYLENE [point (d)] and 1 atm 0, [point (e)].The reaction is retarded several-fold in the presence of air, CO or O,, but is unaffected by N,; it is evident that the active component of air is 0,. The order with respect to initiator is 0.7 for the degassed solution but unity in the presence of 1 atm air or CO. Although the initial rates of polymerization under 1 atm 0, and 1 atm CO are about the same the reaction slows up under 0, and eventually stops after ca. 3 h. The concentration of initiator decreases much more rapidly under oxygen than under vacuum and falls to nearly zero afer 30 h. Under 1 atm CO or air the order with respect to monomer remains zero.The apparent activation energy for the reaction in the degassed solution is 1 16 kJ mol-l(323-343 K). DISCUSSION The key observations are the zero order with respect to monomer, the fractional order with respect to initiator, and retardation by CO, which is also a product. These observations can be accounted for in terms of the following mechanism. F + I+CO (1) 1+A -+ IA (2) IA-+(IA) -+Pl (3) P,+A e P,A (4), (-4) Pl A -+ ( P A ( 5 ) I denotes Ph(MeO)C=W(CO),, formed from F by loss of one molecule of CO. IA denotes I with one molecule of A coordinated to the vacant site, while (IA) is the intermediate metallacyclobutene. P, is the product Ph(MeO)C=CPh-CH=W(CO), [or Ph(MeO)C=CH-CPh=W(CO)J, formed by intramolecular reaction of IA as indicated in the Introduction.P,A is the species formed by coordination of P, with another molecule of A, while (P,A) is again the corresponding intermediate metallacyclobutene. It is clear that if, over the experimental range of [A], the equilibrium (4), (-4) is well to the right, then for long chains the rate of removal of A will be governed by the rate of step ( 5 ) or (6) and will be independent of [A], as observed. Step (7) represents a termination step of the 16-electron species (P,A) with CO, involving ultimately the formation of a pentacarbonyl species F’ analogous to F. This will also account for retardation by CO; further reactions of F’ can lead to the observed W(CO),. Reaction (1) may be slightly reversible, but not sufficiently so that it competes significantly with step (2), otherwise there would be an appreciable dependence of rate on [A].Competition between CO and A for (P,A) or P, can be omitted for the same reason. The mechanism therefore avoids postulating 20-electron metal complexes as intermediates, but makes a distinction between the various types of 16-electron metal complexes in terms of their relative reactivities towards A and CO or 0,. There is apparently no significant competition between A and CO for reaction with the metallacarbenes, I or P,, since the kinetics are still zero order in A even under 1 atm CO. However, we have to postulate completely opposite behaviour for the metallacyclobutenes, (P, A), which must coordinate CO or 0, readily in the presence of A if the poisoning effects of these diatomics are to be explained.This distinction in the behaviour of the two types of 16-electron metal complexes may lie in the differences between the electron-donor-electron-acceptor properties of A and these gases.H. H. THOI, K . J. I V I N A N D J. J. ROONEY 223 1 Assuming that termination reactions of P, or (P,A) other than those with CO are insignificant, a steady-state treatment, with step (6) rate determining, leads to the following expression which, when CO is present initially in sufficient concentration [CO],, reduces to where R, = k[F] k = k6kl/k7[C0],. (9) The fractional order (0.7) with respect to F in the degassed solution can be explained as follows. In the initial stages a steady-state concentration of CO builds up [reactions ( I ) and (7)]. This concentration of CO is obviously a function of the initial concentration of the Fischer compound, [F],.The denominator in eqn (8) is therefore a function of [Fl0 such that the rate of polymerization has a fractional power dependence. When CO is initially present in much greater amount than that produced in the reaction then a simple first-power dependence on [Fl0 is to be expected. These conditions appear to obtain in the presence of 1 atm CO. The activation energy of 116 kJ mol-1 is unusually high for this type of polymeri- zation but a value of this magnitude is expected if step (1) is of major importance in rate control. The fall-off in the rate of polymerization at very low [A] [fig. 2, line (d)] may be attributed to a significant shift of equilibrium (4), (-4) towards the left.Note that an analogous reaction to step (1) has been postulateds as the first step in the metathesis reactions of alkenes, initiated by Ph,C=W(CO),, and that the metallacarbene analogous to I has been detected by lH n.m.r. The present mechanism is based on the consecutive formation of the 16-electron complexes P,, the 18-electron complex P, A, and the 16-electron metallacyclobutene (P, A). An insertion reaction (lo), of the type shown to occur in the reactions of polar acetylenes with F and assumed to proceed via a zwitterion,, OMe OMe I (CO),Mt CPh (10) 6- s+,OMe I d + + 6- Ph - + I - II ll (CO),Mt=C, (CO),M t-CPh YCECZ Y=C=CZ YC-cz (Y = NEt,, OEt; Z = Me, H; Mt = Cr, W) is ruled out in the present case by the zero order with respect to A. A displacement reaction (1 1) of the type F+A+IA+CO (1 1) can be ruled our for the same reason. In any case reactions (10) and (1 1) do not allow any explanation of the retardation caused by CO. We thank the S.R.C. for financial support.2232 POLYMERIZATION OF PHENYLACETYLENE T. Masuda and T. Higashimura, Macromolecules, 1979, 12, 9 and references cited therein. T. J. Katz, S. J. Lee, M. Nair and E. B. Savage, J . Am. Chem. Soc., 1980, 102, 7940. E. 0. Fischer and A. Maasbol, Chem. Ber., 1967, 100, 2445. C. P. Casey, T. J. Burkhardt, C. A. Bunnell and J. C. Calabrese, J . Am. Chem. SOC., 1977, 99, 2127 T. J. Katz, E. B. Savage, S. J. Lee and M. Nair, J . Am. Chem. SOC., 1980, 102, 7944. C. P. Casey and A. J. Shusterman, J. Mol. Cutal., 1980, 8, 1. ’ H. Fischer and K. H. Doetz, Chem. Ber., 1980, 113, 193. (PAPER 1 / 1 505)