Macromolecules 2000, 33, 261-268
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Articles Oxidation State(s) of the Active Titanium Species during Polymerization of Styrene to Syndiotactic Polystyrene Catalyzed by Cp*TiMe3/B(C6F5)3, Cp*TiMe3/[Ph3C][B(C6F5)4], and Cp*TiCl2,3/MAO Edan F. Williams, Michael C. Murray, and Michael C. Baird* Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L3N6 Received June 23, 1999; Revised Manuscript Received November 10, 1999
ABSTRACT: A combined NMR/EPR investigation of the catalyst systems Cp*TiMe3/B(C6F5)3, Cp*TiMe3/ [Ph3C][B(C6F5)4], and Cp*TiCl2,3/MAO for the polymerization of styrene to syndiotactic polystyrene suggests that evidence for the involvement of EPR-active titanium(III) species as catalysts is ambiguous.
Since the initial report of its discovery in 1986,1 syndiotactic polystyrene (s-PS) has been the subject of intense investigation because of useful properties which include a high melting point (270 °C), a high rate of crystallization, and a high modulus of elasticity.2 This new material also exhibits a low specific gravity and dielectric constant, in addition to general resistance to water and organic solvents at ambient temperature.2b While relatively few compounds of transition metals other than titanium have been found to initiate the polymerization of styrene to s-PS,2 it has been reported that titanium compounds of oxidation states I to IV, but especially III, can behave as initiators or initiator precursors.2 Indeed, there has appeared in recent years a number of reports specifically implicating compounds of titanium(III) as primary initiators, the evidence to a significant extent involving EPR studies of polymerization systems. For instance, EPR resonances attributed to d1 titanium(III) species have been observed in a number of catalyst systems in which a titanium(IV) compound has been treated (“activated”) with an alkyl aluminum compound, i.e., with a type of compound that might well be expected to reduce titanium(IV) to titanium(III).3 Thus, quite strong EPR resonances have been reported for active catalyst systems based on the compounds CpTiCl3, CpTi(OR)3 (R ) Me, Bu, Ph), and Cp*TiCl3, either in solution or supported on silica, after treatment with methylaluminoxane (MAO), the EPRactive compounds being attributed to a variety of types of species.3 While none of these assignments have in fact been confirmed by isolation or further characterization of the putative catalytic species, the possible role of titanium(III) in s-PS formation has apparently also been demonstrated by the greater activity of Cp*Ti(OMe)2/ MAO than of Cp*Ti(OMe)3/MAO3d and, in some cases, by correlations of catalyst activities with intensities of EPR signals.3c Complementary to the above, Grassi et al. have reported EPR investigations of the chemistry involved during reactions of the catalyst precursors Cp*TiMe3
and Cp*Ti(CH2Ph)3 with B(C6F5)3 and [Ph3C][B(C6F5)4].4 These reactions are generally believed to result in alkyl carbanion abstraction and formation of the titanium(IV) complexes [Cp*TiR2]+ (R ) Me, PhCH2),2 which are excellent initiators or initiator precursors for ZieglerNatta polymerization of ethylene, R-olefins, and styrene (to s-PS).2 Interestingly, it was found that the titanium(IV) precursors were converted to significant extents (up to >75%) to EPR-active but MAO-free catalytic systems, results which were interpreted in terms of polymerization of styrene by titanium(III) rather than by titanium(IV).4 The proportions of reduced species were generally somewhat lower than was the case with the MAO-activated systems discussed above, consistent with the fact that no obvious reducing agents were added although the proportions reduced tended to increase on the addition of styrene. Largely on the basis of NMR and EPR studies of isotopically enriched species, the actual initiators of styrene polymerization to s-PS were suggested to be of the type [Cp*TiR]+ (R ) Me, PhCH2) although no such titanium(III) complexes have actually been characterized other than by in situ EPR studies. A kinetics investigation of s-PS formation induced by the compounds Cp*TiR3 (R ) Me, PhCH2) activated by MAO, B(C6F5)3, and [Ph3C][B(C6F5)4] suggested that the catalytic species were in all cases very similar and probably identical and that the concentrations of active species were similar to the concentrations of titanium(III) species derived from EPR data.4c Detailed EPR monitoring of the Cp*Ti(CH2Ph)3/B(C6F5)3 and Cp*TiMe3/ B(C6F5)3 styrene polymerization systems showed that a variety of titanium(III) species may form, depending on the extent of aging and the presence or absence of styrene. However, the major species in catalytically active solutions were believed to be of the type [Cp*TiR]+, amounting to 30% and 10% of the of the total titanium in the Cp*Ti(CH2Ph)3/B(C6F5)3 and Cp*TiMe3/B(C6F5)3 systems, respectively. On the other hand, in many instances not all of the titanium(III) appeared to be catalytically active, as is the case with several of the
10.1021/ma991006f CCC: $19.00 © 2000 American Chemical Society Published on Web 12/28/1999
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MAO-based systems discussed above.3 In contrast to these results, Chien and Rausch et al. could detect no signals in EPR spectra of catalytically active solutions of either the Cp*TiMe3/[Ph3C][B(C6F5)4] or Cp*TiMe3/B(C6F5)3 systems, although treatment of the same precursors with MAO did result in up to 28% reduction.5a More recently, Baird et al. have also reported exploratory EPR investigations of the Cp*TiMe3/ [Ph3C][B(C6F5)4] and the Cp*TiMe3/B(C6F5)3 systems, observing only very weak signals, different from those reported by Grassi et al. and representing 15 000 transients). The resonances at δ 80.7 and δ 44.9 were replaced by resonances at δ 147.9, 110.6, 69.0, 16.8, 10.6, and -4.0 (methane), but no other resonances were observed in the range δ -282 to 2202. In view of the time involved in accumulating the data, the signal:noise ratio was quite high, and we can anticipate that we have indeed observed all compounds, including transient species, formed from the thermal decomposition of the Cp*Ti(13CH3)2(µ-13CH3)B(C6F5)3 during the time of the experiment. Interestingly, although the formation of some insoluble material was apparent during the experiment, the sum of the total intensities of the new species was greater than the sum of the intensities of the original resonances at δ 81.1 and δ 45.6. Thus, some of the observed resonances are indeed reasonably attributed to transients. In a separate experiment designed to obtain “snapshots” of the reaction at relatively short time intervals, a solution of Cp*Ti(13CH3)2(µ-13CH3)B(C6F5)3, freshly prepared at -25 °C, was warmed quickly to 25 °C. A 1H NMR spectrum was run, and then a 13C{1H} NMR spectrum over ∼4 min (53 transients). Both spectra indicated that little change had occurred, although weak methane resonances in the 1H (δ 0.16, JCH 126 Hz) and 13C{1H} (δ -4.0) spectra had appeared. Over approximately the next hour, a J-modulated spin echo13C{1H} spectrum was accumulated (512 transients, ∼40 min), and then a heteronuclear multiple quantum correlation
Polymerization of Styrene to Syndiotactic Polystyrene 263 Table 1. 13C and 1H NMR Spectral Correlations (HMQC) 13C
chemical shifts (δ)
110.8 (s) 81.3 (vs) 80.5 (sh, m) 77.0 (w) 71 (vw) 68 (w)
1H
chemical shifts (δ)
2.40 (s) 1.42 (vs) 1.15 (s) 1.22 (w), 0.97 (w) ∼1.2 (obscured) 1.32 (w)
13C
chemical shifts (δ)
58 (vw) 56 (vw) 55 (vw) 31.5 (vw) 16.5 (s) -4.0 (vw)
1H
chemical shifts (δ)
0.56 (vw) 0.88 (vw) 0.81 (vw) 0.9 (w) 1.55 (m) 0.14 (w)
(HMQC) experiment was carried out (∼30 min), both at 25 °C. As the methane resonance at δ -4.0 was found to have increased somewhat in intensity in the 13C{1H} spectrum, thermal decomposition was clearly occurring during the experiments although the resonances of the starting Cp*Ti(13CH3)2(µ-13CH3)B(C6F5)3 were still dominant. New, weak 13C methyl resonances appeared in the J-modulated spin echo spectrum at δ 110.8, 77.2, 71.5 and 12.3, and these were also present in the HMQC experiment in addition to resonances at δ 68, 58, 56, 55, 31.5, and 16.5. In Table 1, we present the 13C-1H chemical shift correlations and relative intensities of the resonances observed in the HMQC experiment. Although the broad µ-13CH3 resonance of Cp*Ti(13CH3)2(µ13CH )B(C F ) at δ 45.6 and a similarly broad 13CH 3 6 5 3 3 resonance at δ 11.5, possibly attributable to a small amount of [B(13CH3)(C6F5)3]- displaced by the solvent, were observed in the 13C{1H} spectrum run prior to the HMQC experiment, neither was apparent in the HMQC experiment. This is possibly a result of quadrupolar relaxation by the boron nuclei, since the anticipated shortening of the carbon T2 values would render the HMQC experiment unrealizable for these carbons given the experimental parameters utilized. We also did not observe a cross-peak for the Cp* methyl resonance, presumably because of the relatively low concentration of 13C (natural abundance) at this position. The 1H and 13C{1H} (J-modulated spin echo, 512 transients) NMR spectra of the same sample were run again after it had been maintained at 25 °C for a further 14 h. The only observable 13C resonance arising now from positions containing 13C-enrichment was that previously observed at δ 110.8. The corresponding 1H resonance at δ 2.4 (JCH 126 Hz) was also observed, as were the 1H and 13C resonances of methane. The intensity of the resonance at δ 110.8 was significantly lower than that of the original starting material, in part at least because of the formation of both methane and a small amount of insoluble material. These types of NMR experiments could not be carried out in the presence of styrene because of the proclivity of s-PS to precipitate from solution. The EPR spectrum of a freshly prepared solution containing equimolar amounts of Cp*TiMe3 and B(C6F5)3 ([Ti] ) 0.01 M) in chlorobenzene at room temperature exhibited a weak doublet centered at g ) 1.994 with a hyperfine coupling constant a ) 8.4 G (Figure 2a). The coupling constant and observed g value lead to the reasonable assignment of this doublet to a titanium(III) hydride species similar to those reported previously for homogeneous CpTiCl3/MAO (g ) 1.989, a ) 7.4 G; g ) 1.995, a ) 4.5 G)3a and CpTi(OBu)3/MAO (g ) 1.989, a ) 7 G)3b systems and a supported CpTiCl3/MAO (g ) 1.989, a ∼ 7.5 G)3f system. On the basis of comparisons with TEMPO standards, the concentration of EPR observable species present in the sample was found to be ,0.01% of the initial titanium concentration in the sample. These results, which are very reproducible
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Figure 2. EPR spectra of the room-temperature reaction of Cp*TiMe3 with B(C6F5)3 (a) in chlorobenzene and (b) the same with added styrene.
(three experiments), differ markedly with those reported for the same system elsewhere,4b i.e., a sharp singlet at g ) 1.977 with intensity accounting for almost 1% of the initial titanium in the sample. The addition of 0.1 mL of styrene to the EPR tube produced a spectrum exhibiting a doublet at g ) 1.994 (Figure 2b); this eventually disappeared over ∼5 min and was replaced by a broad singlet at g ) 1.978. During this time, the initially bright orange reaction mixture turned very dark with some heating and within 2-3 min became a solid mass of s-PS. Again, contrary to the results previously reported,4b the concentration of EPRactive titanium(III) was not observed to increase upon addition of styrene and still remained ,0.01% of the initial titanium present in the sample. The collapse of the initially observed doublet would be consistent with monomer insertion into a Ti-H bond, which would eliminate the apparent coupling to hydrogen. Our inability to reproduce the EPR experiments reported in the literature for the Cp*TiMe3/B(C6F5)3 system4 led us to wonder whether small differences in sample preparation had led to the differing results. Since irradiation of many monocyclopentadienyl complexes and metal alkyl compounds by both UV and visible light has been shown to produce EPR observable paramagnetic species,14a,b it was thought that inadvertent exposure to bright sunlight14c might be the cause for the high titanium(III) concentrations reported by Grassi et al.4 To investigate this possibility, solutions containing equimolar amounts of Cp*TiMe3 and B(C6F5)3 were irradiated with sunlight in an attempt to reproduce the literature results. Two experiments were carried out, with one solution being irradiated continuously for 1 h and another sample being irradiated five times for 1 min at a time. Upon EPR analysis of the irradiated samples, it was determined that these photolysis experiments were unsuccessful in increasing spectral intensity or in producing any of the species previously reported.4 The discrepancies between the extremely low concentration of titanium(III) species observed here and the much higher proportions reported elsewhere4 led us to also consider the possibility that titanium(III) species were formed initially but had then taken part in secondary processes that would render them EPRinactive. The [CpTi(III)]2+ ion can coordinate up to five electron pair donor atoms,15a and dimerization of titanium(III) compounds via bridging ligands is known to result in many cases in spin-pairing of the d1 ions to give EPR silent and even diamagnetic products.11c,15b Alternatively, spin exchange between titanium(III) centers in solution could result in severe EPR line broadening, and it thus seemed possible that either type of behavior might somehow account for the differences in our results and those reported elsewhere.4 Certainly the EPR resonance of Cp*TiCl2 in THF solution is
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significantly broadened when at higher concentrations (see below). To assess these possibilities, excess amounts of the potential ligands pyridine and PPh3 were introduced into solutions containing equimolar amounts of Cp*TiMe3 and B(C6F5)3 in chlorobenzene at 25 °C. If the reaction solutions contained ligand bridged dimers that can dissociate and coordinate styrene, then coordination of better ligands should also occur and should produce monomeric titanium(III) species which should be EPR active and might even exhibit hyperfine couplings to 31P and 14N. In the same way, coordination of bulky ligands would hinder intermolecular spin exchange interactions between titanium(III) complexes in solution, and rates of electron exchange would also be altered significantly. However, while the above-mentioned doublet disappeared on the addition of PPh3, the spectrum exhibited no new EPR resonance. Furthermore, the addition of pyridine to a second sample resulted in neither significant changes in the spectral features nor a measurable increase in spectral intensity. We note a previous study in which PMe3 was added to a solution containing equimolar amounts of Cp*TiMe3 and B(C6F5)3 in chlorobenzene at -20 °C.4d A new EPR resonance was observed, superimposed on the resonance previously noted, but no mention was made of change in intensity. While the differences in g values discussed here are generally in the range of experimental errors (perhaps (0.003), our spectra are all carefully calibrated with respect to internal DPPH and are quite reproducible. It is very important to note, moreover, that the multiplicities that we observe are different from those reported in the literature and that we never observe the high concentrations of EPR active species which are reported elsewhere. These results are important because they make it clear that s-PS can be formed by the Cp*TiMe3/B(C6F5)3 system in the absence of the EPR active titanium(III) species reported elsewhere4b and, indeed, in the absence of significant amounts of any detectable titanium(III) species. For this reason, the necessity of titanium(III) for s-PS formation is clearly called into question. EPR Monitoring of the Reaction of Cp*TiMe3 and [Ph3C][B(C6F5)4] in Chlorobenzene-d5. The reaction of Cp*TiMe3 and [Ph3C][B(C6F5)4] in CD2Cl2 has been shown5c to proceed as in eq 2, producing the dititanium compound [(Cp*TiMe2)2(µ-Me)][B(C6F5)4] (B) rather than the compound [Cp*TiMe2][B(C6F5)4] earlier presumed by Grassi et al.4
2Cp*TiMe3 + 2[Ph3C][B(C6F5)4] f [(Cp*TiMe2)2(µ-Me)][B(C6F5)4] + [Ph3C][B(C6F5)4] + Ph3CMe (2) The dititanium compound is thermally labile at ambient temperatures, however, decomposing to give hitherto unidentified products exhibiting a number of resonances in the 1H NMR spectrum.5c An EPR spectrum of a solution containing equimolar amounts of Cp*TiMe3 and [Ph3C][B(C6F5)4] in chlorobenzene ([Ti] ) 0.01 M) at 23 °C is shown in Figure 3a. Again a doublet, similar to that in Figure 2a, was observed (g ) 1.992, a ) 8.2 G) and is tentatively attributed to a titanium(III) hydride. The titanium(III) concentration was again apparently negligible, however, with
