Study of the Propagation Center in the Anionic Polymerization of (Meth

Macromolecules 1998, 31, 2731-2743

2731

Study of the Propagation Center in the Anionic Polymerization of (Meth)acrylic Monomers: NMR and MO LCAO Study of the Interaction of Di-tert-butyl 2-Lithio-2,4,4-trimethylglutarate and the Living Poly(tert-butyl methacrylate) Oligomers with Lithium 2-(2-Methoxyethoxy)ethoxide in Tetrahydrofuran C. Zune, P. Dubois,† and R. Je´ roˆ me Center of Education and Research on Macromolecules (CERM), Universite´ de Lie` ge, Sart Tilman B6, 4000 Lie` ge, Belgium

J. Krˇ ı´zˇ ,* J. Dybal, L. Lochmann, M. Janata, and P. Vlcˇ ek Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic

T. M. Werkhoven and J. Lugtenburg Leiden Institute of Chemistry, Gorlaeus Laboratories, University of Leiden, P.O. Box 9502, 2333 CC Leiden, The Netherlands Received September 16, 1997; Revised Manuscript Received February 20, 1998

ABSTRACT: Interactions of lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM) with the model dimer ditert-butyl 2-lithio-2,4,4-trimethylglutarate (A) and the living poly(tert-butyl methacrylate) oligomers (B) were studied in tetrahydrofuran-d8 at 203-273 K using 1H, 13C, 7Li, and 6Li, 1D and 2D, NMR and ab initio SCF 3-21G and MNDO quantum chemical calculations. LiOEEM is shown to have a strong tendency to self-aggregation, producing dimeric, trimeric, and tetrameric aggregates and competing with its mixed aggregation (or complexation) with A and, in particular, B. When dissolved in THF, LiOEEM as well as its mixtures with A form metastable systems which relax in several days at 258 K into equilibrium. Interaction of LiOEEM with A leads to a system of mixed aggregates LiOEEM1A1, LiOEEM3A1, and possibly LiOEEM2A1 along with the original components, in relative populations depending on the LiOEEM/A molar ratio, temperature, time, and probably other factors of preparation. Probable structures of these complexes are proposed, and the nature of the prevalent bonding is suggested. Experimental results indicate that LiOEEM is unable to convert A completely at moderate excess (up to 4/1 mol/mol). Interaction of LiOEEM with B leads to quite analogous complexes but with even lower yields. There appear to be traces of uncomplexed B even at the LiOEEM/B ratio of 10 mol/mol. This is suggested to produce at least two different kinds of active growth centers in the corresponding ligated anionic polymerization of tert-butyl methacrylate and, consequently, the observed bimodality of the polymeric product. The difference with respect to methyl methacrylate, where LiOEEM ensures an almost ideal living polymerization, is suggested to be due to the steric hindrance of efficient complexation exerted by the tert-butyl group.

Introduction work,1

we used NMR spectroscopy and In our previous quantum chemical calculations to clarify the interactions of di-tert-butyl 2-lithio-2,4,4-trimethylglutarate (A), the model living dimer of tert-butyl methacrylate, and of the living poly(tert-butyl methacrylate) (B) oligomers (prepared by organolithium initiation) with LiCl. Our results proved the ability of both the dimer and the oligomer (and, by induction, the propagation center in general) to form an equimolar complex, or mixed aggregate, with LiCl and helped to explain the favorable effect of added LiCl on the living polymerization of methacrylates. The interaction of LiCl with A or B was shown to be quite similar to that of another µ-ligand, lithium tert-butoxide which has been studied by one of our laboratories for some time.2-5 It is thus of utmost interest to extend our study to a recently discovered new

ligand family of lithium polydentate which also promotes living polymerization of methyl methacrylate and acrylates in tetrahydrofuran (THF).6 One of the most interesting members of this family, lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM), contains two different types of ligand groups in the same molecule, i.e., lithium alkoxide (µ type) and polyether (σ type) which both are assumed to interact with the growth center and to be responsible for its favorable effects on the polymerization process. The addition of LiOEEM to lithium ester enolates could thus be expected to display a somewhat specific type of coordination. In analogy with our previous paper,1 we present here a multinuclear NMR and quantum chemical study of the interaction of A as well as B with LiOEEM. To clarify some of its peculiarities, however, we also give some attention to the characterization of LiOEEM itself in its THF solution. Experimental Section

†

Present address: Service des Mate´riaux Polyme`res et Composites, University of Mons-Hainaut, Place du Parc 20, 7000 Mons, Belgium.

Chemicals. Di-tert-butyl 2-lithio-2,4,4-trimethylglutarate was prepared by the addition of tert-butyl methacrylate (tBuMA) to tert-butyl 2-lithioisobutyrate by the method de-

S0024-9297(97)01368-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/17/1998

2732 Zune et al.

Macromolecules, Vol. 31, No. 9, 1998

scribed elsewhere.7 After recrystallization and drying, the product was dissolved in tetrahydrofuran-d8 under purified argon at room temperature, transferred into a cooled NMR tube, and sealed off. If not used directly, the tube was immersed into liquid nitrogen. Solvents 1,1-diphenylethylene and tBuMA were purified as described elsewhere.1 Lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM) was prepared by reacting equimolar amounts of the corresponding polyether alcohol and butyllithium (BuLi) in THF at 0 °C. 2-(2-Methoxyethoxy)ethanol was purchased from Aldrich and dried over CaH2 before being distilled under reduced pressure and stored under inert atmosphere. The anionic polymerization of tBuMA initiated by diphenylhexyllithium ((DPH)Li) was done in THF at -78 °C in the following way. (DPH)Li was prepared in situ from 1,1diphenylethylene (2 equiv) and BuLi (Janssens) (1 equiv) in THF at 0 °C. An appropriate amount of (DPH)Li (1.75 × 10-4 mol) was added to a THF solution of LiOEEM at -78 °C. Subsequently, tBuMA (30 mol/mol of initiator) and, after 10 min, tBuMA-1-13C (ca. 3 mol/mol)8 were added to the mixture under stirring at -78 °C. The resulting solution of the living oligomers (Mw ca. 3000) was transferred into a NMR tube containing 0.1 mL of THF-d8 and sealed off under vacuum. The sample was measured immediately. Size exclusion chromatography. (SEC) was carried out in THF at 30 °C with a Hewlett-Packard 1037A apparatus equipped with a refractive index detector. Polystyrene standards were used for calibrations. NMR measurements. 1H, 13C, 6 Li, and 7Li NMR spectra for the dimer studies were measured at 300.1, 75.5, 44.1, and 116.6 MHz, respectively, with a Bruker Avance DPX 300 spectrometer. For the oligomers studies, the NMR spectra were recorded on a ARX Bruker 400 MHz spectrometer. As external standard for the 7Li NMR spectra, a 0.1 mol/L solution of LiCl in THF at 298 K was used. For 1H and 13C, hexamethyldisiloxane (HMDS) as internal or external standard was used (in THF solution, HMDS has a chemical shift to tetramethylsilane (TMS) of 0.05 and 2 ppm in 1H and 13C, respectively. Most pulse sequences and other measuring methods used were described in previous communications.1-5 The specific parameters of 2D NMR spectra were the following. Homonuclear (COSY, NOESY, and ROESY) spectra were acquired with 1024 points in f2 domain and 256 increments, using quadrature detection (i.e. 512 points in f2) and zero filling (i.e. 512 point in f1), pure sine weighting functions, and symmetrization. The mixing time used in NOESY was 300 ms; in ROESY, 200 ms. Heteronuclear 1H-13C and 1H-7Li (HETCOR, COLOC, and HOESY) spectra were measured with 1024 points in f2 domain (using quadrature detection, i.e., 512 points after Fourier transform (FT)) and 128 increments, zero filling to 256 points in f1. Pure sine weighting functions before 2DFT were used. The polarization transfer delays used were 3.75 ms in HETCOR and 50 ms in COLOC; the mixing time in 1H7Li HOESY was 200 ms. Most of the 2D spectra were measured at 253 K overnight, all of them without sample spinning. 1H-7Li distances were obtained from the corresponding heteronuclear NOESY (or HOESY) cross-peaks in the following way. The expression for the distance rjLi from the j th proton to the lithium nucleus relative to the chosen (e.g. nearest) distance riLi of the i th proton to the same lithium nucleus can be deduced from the known equations for the intensity of NOESY cross-peaks (cf., e.g.9 ref 9). In a slightly simplified form, it reads:

(

)

rjLi aiLiDjLi ) κξ riLi ajLiDiLi

1/6

(1)

where akLi is the integral intensity of the respective cross-peak between the k th proton and lithium resonance and the other symbols have the following meaning:

DkLi ) [(1/4)(1/T1k - 1/T1Li)2 + (ζσ′kLi)2rkLi-12]

(2)

where T1k and T1Li are the longitudinal dipolar relaxation times of the k th proton and lithium, respectively, σ′kLi ) 6J(ωk + ωLi) + J(ωk - ωLi), J being the usual spectral densities for the sum and difference of the respective Larmor frequencies, and ζ ) (hγHγLi/π)2/10, γH and γLi being the gyromagnetic ratios of the respective nuclei. We assume that the term κ ) σ′iLi/σ′jLi can be put to unity, the differences in the proton Larmor frequencies being less than 1 ppm. The remaining part of eq 1 is

ξ ) exp[-(1/T1j - 1/T1i)τc] sinh(DjLiτc)/sinh(DiLiτc) (3) Assuming from the observed behavior of NOE that ωHτc ≈ 1 and τc is of the order of 10-8 s, the approximation ξ ≈ 1 should not lead to a substantial error. For T1k, we assume (in a viable approximation) that the values measured by inversionrecovery experiments correspond to dipolar longitudinal relaxation. In the case of 7Li, only a small fraction of the longitudinal relaxation can be attributed to the dipolar mechanism, most of it being quadrupolar. As the use of T1Li in terms D mainly make correction for the polarization which could not be exchanged due to relaxation, however, we propose using T1Li as it is measured by a standard inversion-recovery experiment. Diffusion coefficients were measured by pulsed field gradient (PFG) NMR spectroscopy using the standard pulse sequence.10 The spectra were obtained using a Bruker AM 300 WB spectrometer with a 8-mm Bruker probe equipped with a Z-gradient coil. The interval δ between two field gradient pulses was 22 s, and the duration time λ was 6 s. The field gradient was calibrated with octanol, which has a diffusion coefficient of 1.4 × 10-10 m2/s.11 Methods of Calculations. The assumed molecular and aggregated structures were studied by ab initio SCF 3-21G and semiempirical quantum-chemical MNDO12 methods using the GAMESS set of programs13,14 running on a Silicon Graphics station Indy. The geometries of the molecules and their complex aggregates were fully optimized using the gradient optimization routine in the program, and all of the stationary states were verified by inspecting their Hessian. The calculations were initially performed in C1 symmetry; if the structures tended to higher symmetry, the refinement was done using group factorization.

Results and Discussion Characterization of LiOEEM) in THF. 1H and 13C NMR spectra of LiOEEM solutions in THF-d8 of increasing concentrations at 253 K are shown in Figures 1 and 2. As expected, five main signals are observed in the 13C NMR spectrum in the range of chemical shift of 50-85 ppm. The two signals for the carbons in the vicinity of the lithium counterion (C17 and C16) present very broad lines at higher dilutions even at room temperature and become narrower and sometimes split to signal pairs under increasing concentration. 1H NMR spectra are composed of broad signals which are partly superimposed on one of the residual signals of THF (the multiplet with the center at 3.65 ppm). The assignment of the 1H and 13C signals corresponds to the superscripts in Chart 1 and was done in the following way. The connectivity between the 1H signals was done by 1H1H COSY spectra and their correlation to the directly attached carbons by 1H-13C COSY (HETCOR). 1H13C long-range COSY (COLOC) was used to reveal twobond connectivities for the following proton-carbon pairs: 13-14, 14-15, 16-17, 17-16. The unambiguous assignment done this way is in accord with the fact that signals 17 and 16 in LiOEEM exhibit a pronounced shift in both 1H and 13C when compared with the corresponding signals of the parent alcohol.


LiOEEM Tendency to Self-Aggregation 2733 Chart 1. Numbering of the Carbons and Protons in A and LiOEEM

Table 1. Chemical Shifts δ (ppm) and T1 Values (ms) in NMR Spectra of LiOEEM in THF-d8 Solution at 253 K and Various Concentrations C (mol/L)

7Li

Figure 1. 300.1 MHz 1H NMR spectra of LiOEEM at 253 K in THF-d8 LiOEEM concentration: (a) 0.2M (b) 0.6M (c) 0.8M (d) 1M (e) 1.6M. 7Li

and 6Li NMR present single signals which are broadened at lower temperatures and higher concentrations. At 203 K and 1.2 mol/L, the 7Li signal assumes a “super-Lorentz” line shape typical of quadrupolar nuclei in bulky, partially ordered structures. As expected, 6Li with its much lower quadrupole moment

Figure 2. 75.5 MHz (e) 1.6 M.

13C

C

δ

T1

C

δ

T1

0.2 0.6 0.8

-0.139 -0.132 -0.126

23.57 23.27 21.05

1.0 1.6

-0.127 -0.126

20.73 21.56

exhibits a broadened signal with a Lorentz-like shape. At 253 K, the slight change in 7Li line shape is paralleled by an equally slight change in chemical shift and T1 value. As shown in Table 1, the change only slightly exceeds experimental error. This suggests some change in the lower part of the concentration range converging to a more or less constant state at larger concentrations. On the whole, the behavior of both 7Li and 6Li can be interpreted in three different ways: (i) there is no substantial change in the aggregation states of dissolved LiOEEM with concentration or temperature, (ii) there exists a very fast exchange of Li between the various aggregated species which surpasses the difference in their spectral behavior, or (iii) different species, e.g. aggregates of LiOEEM, coexist in solution in dependence on both concentration and temperature but they do not substantially differ in the magnetic shielding and the gradient of electric field at the lithium nucleus, i.e., in the basic type of lithium bonding. As

NMR spectra of LiOEEM at 253 K in THF-d8 LiOEEM concentration: (a) 0.2, (b) 0.6, (c) 0.8, (d) 1, and

2734 Zune et al.

Figure 3. 75.5 MHz K.


13C

NMR spectra of 1.2 M solution of LiOEEM in THF-d8 at 253 K: (a) fresh solution; (b) after 48 h at 258

shown below, iii appears to be the most plausible interpretation. The changes in 1H and 13C spectra with concentration as shown in Figures 1 and 2 for 253 K suggest a possible change in aggregation states. However, no intelligible concentration dependence of the relative intensities in the signal pairs 16 and 17 was observed in the range of 203-298 K. When the samples are kept several days at -15 °C, rather dramatic changes in their spectral behavior can happen, as shown in Figure 3 for the 1.2 mol/L solution of LiOEEM: in 13C NMR, the original pair of signals 17 collapses into a single signal and the signal 16 shifts; both new signals preserve their respective positions more or less identically with those originally observed at much lower concentrations. In a similar way, signals 14 and 16 assume, after the “seasoning”, nearly the same position as the analogous signals occupy at much lower concentration. The slowness of this change would suggest a very slow exchange between various states of the solute. However, the gradual coalescence of the split signals in the 13C NMR spectra of highly concentrated LiOEEM solutions (0.8 M and higher) on heating above 273 K to the contrary suggests fast exchange between the two states or species indicated by the signal splittings at lower temperatures. The contradiction of these two sets of observations cannot be resolved merely by assuming a rather high activation energy of the exchange process (which would account for the large difference in exchange rate between 258 and 298 K): fresh samples, after “annealing“ by heating them to 298 K for a short time and cooling back to 253 K do not exhibit a substantial change in spectral behavior. The problem of LiOEEM self-aggregation could be settled by different means. Of the realm of NMR, two independent techniques offer relaxation measurement and self-diffusion experiments. As for the first one, Table 2 compares carbon longitudinal relaxation times for LiOEEM and HOEEM, the parent alcohol of the

Table 2. 1H and 13C T1 Values (s) for Nuclei in Position p in LiOEEM and Its Parent Alcohol HOEEM in THF-d8 at 253 K LiOEEM p

1Hc

13Ca

13Cb

HOEEM 13Ca

13 14 15 16 17

0.89 0.41 0.35 0.23 0.22

1.20 0.27 0.20 0.12/0.07 0.11

1.03 0.31 0.18 0.11 0.12

4.0 2.49 2.35 2.17 1.96

a

0.8 mol/L. b 1.2 mol/L. c 1.0 mol/L.

former. Exact analysis of the data is difficult owing to the present scalar interactions with other protons and with lithium; the striking difference in the relaxation rates of the two substances leaves no doubt that the former forms self-aggregates in solution. Using the same approximative approach as in our recent paper,1 we could conclude that the ratio of the mean radius of the outer rotational profiles of LiOEEM and HOEEM at 0.8 mol/L and 253 K is about 2.2, if we leave out the carbons affected by the vicinity of lithium, i.e., carbons 16 and 17 (carbon 13 is excluded from the comparison due to the anisotropy of methyl motion). This means that the main unit that is decisive for the rate of rotational diffusion of LiOEEM is its dimer (which is necessarily somewhat more extended than the double of HOEEM), which does not preclude the possibility of a tetramer (which could have almost the same rotational profile). Further, the comparison of the carbon relaxation between 0.8 and 1.2 mol/L leaves almost no doubt that the mean self-aggregation state increases with the growing concentration of the solute. Other evidence on self-aggregation can be obtained from self-diffusion experiments. The diffusion coefficients obtained by PFG NMR for a 0.1 M solution of LiOEEM and 2-(2-methoxyethoxy)ethanol (HOEEM) in THF at 298 K were 1.50 × 10-9 and 4.17 × 10-10 m2/s. Their ratio is thus 3.6, which suggests that the main


species present at 0.1 mol/L is still a tetrameric aggregate. This is not very surprising as the aggregation of lithium alkoxides is well-reported in the literature (e.g., refs 2-5 and 15). A similar compound, lithium 2-(ethoxy)methoxide, has been studied by Tomoi et al.16 Cryoscopic measurements in hexamethylphosphoramide (HMPA) solution revealed a high tendency to aggregation of the compound even in such a highly solvating solvent, the mean degree of aggregation reported being 5 for a 0.12 M solution. Although all data on selfaggregation of organometallics have to be taken with caution because of possible unusual behavior, it surely can be concluded that a tetramer can be expected to be a well-populated form of LiOEEM, with some distribution of the dimer, possibly trimer or higher aggregates. This conclusion is in accord with predictions of quantum chemical calculations. Considering the tendency of the semiempirical MNDO method to produce some artifacts when dealing with organometallics of this type,17 we start our calculations on LiOEEM dimers in a more reliable ab initio SCF 3-21G format, and, being sure that the optimum structures obtained by this method and by MNDO are virtually identical (except the known systematic differences in bond lengths and atomic charges), we proceed with MNDO to higher aggregates. LiOEEM dimeric, trimeric, and tetrameric aggregates were all predicted by ab initio SCF 3-21G to be relatively stable, their type of Li bonding being analogous. Figure 4 shows, as an example, the optimum geometry of the tetramer. The stabilization energies per one LiOEEM molecule for these first self-aggregates, calculated by MNDO without respect to possible solvation effects, are 21.1, 23.7, and 24.1 kcal/mol, respectively. The analogous values predicted by ab initio SCF 3-21G are 31.8, 30.0, and 33.0 kcal/mol. The difference between this result and the corresponding one from MNDO is quite usual, its largest part corresponding to correlation energy. The important point here is that the order of stabilization is the same for both methods. There also exists a close analogy between the structures found as most stable by both methods which warrants considering MNDO as a fairly reliable method (except some well-recognized artifacts, cf. ref 17) for extended calculations reported below. As one can see, the tetramer is more stabilized than the lower aggregates. Higher aggregation is not excluded but cannot be expected to be predominant even when entropy and solvation effects are neglected. The calculations (both methods) predict the bidentate (i.e. involving all three oxygens of LiOEEM in the coordination buildup of the aggregate) dimeric structure to be slightly more stable than the monodentate ones and the lithium atoms to be positioned in such a way as to be more or less evenly distanced from their nearest two or three oxygens. In the most stable trimeric and tetrameric structures, however, monodentate structures are preferred. It must be noted that, in contrast to simple alkoxides,17 the core of the aggregate is rather flat and is formed by a hexagonal (trimer) or octagonal (tetramer) structure with alternating lithium and oxygen atoms. The over all type of lithium bonding is again a polycentric electron-deficient bond with a high proportion of electrostatic interactions. These interesting results call for comparison with experiment, namely, with the homonuclear 1H NOESY and heteronuclear 1H-7Li NOESY (HOESY) NMR spectra. 1H NOESY spectra did not offer any nontrivial

LiOEEM Tendency to Self-Aggregation 2735

Figure 4. Structure of the most stable tetrameric aggregate of LiOEEM (two projections) as predicted by ab initio SCF 3-21G calculations.

information. Despite the predominantly quadrupolar relaxation of 7Li, 1H-7Li NOESY brought illuminating results. As shown for 298 K in Figure 5 (the results at 253 K being roughly the same but less convincing), these spectra offered cross-peaks from which mean 1H-7Li distances could be calculated (see Experimental Section). The results are compared with those predicted by SCF 3-21G in Table 3 (for H13-Li, the distance was derived from a selective 1D NOE measurement). Clearly, the experimental results are in better agreement with the predicted geometry of the tetramer or trimer. The differences in the predicted and found distances Li-H14 and Li-H13 are not really serious: although the highly symmetrical monodentate form of the tetramer has been found to correspond to the energy minimum, we cannot be quite sure that the absolute or global minimum of the very complicated many-dimensional energy hypersurface was really found. For instance, its vicinity spanned by the possible conformations of the methoxymethylene groups forms a rather flat energy basin with many shallow dents. Accordingly, there is a high

2736 Zune et al.

Macromolecules, Vol. 31, No. 9, 1998 Table 3. Mean Lithium-Methylene Proton Distances (L) in LiOEEM Self-Aggregates As Predicted by ab Initio SCF 3-21G and Experimentally Found by 1H-7Li HOESY methylene

dimera

trimer

tetramer

expt

17 16 15 14 13

3.31 3.31 3.59 3.18 5.84

3.15 3.33 3.27 4.48 6.48

3.25 3.35 3.56 3.11 5.69

3.25b 3.65 3.97 3.93 5.17

a Structure closest to the experimental one. b Adjusted, the remaining distances relative to it.

Figure 5. 7Li-1H NOESY (HOESY) spectrum of 1 M LiOEEM solution in THF at 298 K.

probability of finding the protons H14 at a somewhat greater-than-predicted distance from Li and protons H13 at a shorter one as our HOESY results indicate. By the same logic, one can expect that various semistable conformations or configurations of the same selfaggregate can be formed with time, differing, e.g., in the magnetic shielding of some protons or carbons but not really very different in basic structural parameters.

One of the consequences of these findings is that the tetramer is clearly the most probable form, although lower and perhaps higher aggregates can coexist with some probability. From these results, one can expect that complexation of other organometallics such as the model dimer will be easy or simple. Complexation of the Model Living Dimer A by LiOEEM. The revelant parts of 1H and 13C NMR spectra of A and its mixtures with LiOEEM (molar ratio LiOEEM/A ) 0/1, 1/1, 2/1, 3/1, 4/1, 8/1) in THF solution at 253 K are shown in Figures 6 and 7, respectively. The assignment of the 1H and 13C signals discussed below corresponds to the superscripts in Chart 1. Most of the 1H NMR spectra of the LiOEEM/A mixtures are complicated by the presence of at least three types of chemically equivalent protons designated a to c in Figure 6. The signal intensities of the same type correlate well in different LiOEEM/A mixtures and at different temperatures in the range of 203-253 K.

Figure 6. 300.1 MHz 1H NMR spectra of A and its mixtures (0.2 mol/L) with LiOEEM at 253 K in THF-d8. LiOEEM/A ratio: (a) 0/1; (b)1/1; (c) 2/1; (d) 3/1; (e) 4/1; (f) 8/1.



Figure 7. 75.5 MHz 13C NMR spectra of A and its mixtures (0.2 mol/L) with LiOEEM at 253 K in THF-d8. LiOEEM/A ratio: (a) 0/1; (b)1/1; (c) 2/1; (d) 3/1; (e) 4/1; (f) 8/1.

In the case of 13C NMR spectra (Figure 7), three types of chemically equivalent carbons are observed if the molar ratio LiOEEM/A reached 4/1, whereas the situation is much more complicated at lower ratios. Apart from this intensity correlation, the signal assignment was done using coherence transfer of several kinds. Correlation of protons with the directly attached carbons (C3, C6, C7, C8, C10, C12) can be done by 1H-13C COSY (HETCOR) and the correlation with the nonprotonated carbons (C1, C2, C4, C9, C11) is obtained by its longrange variant, i.e., the COLOC-2D experiment. It can be demonstrated (using, e.g., selective INEPT) that all chemically equivalent carbons of the A part have at least three (or more) signals, some of them being obscured by the signals of the ligand or solvent. Comparison of the 13C spectra shows gradual disappearance of the two sets of signals, attributed respectively to the dimeric aggregated form and the nonaggregated intramolecular complex form of A, at the expense of several new sets of signals. At LiOEEM/A > 3 (e.g. 4), three new sets of signals appear with a substantial shift of C1, C2, C5, and C6 compared to those of A observed under the same conditions. As no detectable chemical change in A is observed, these signals must correspond to a complex of A and LiOEEM (the same applies to other new signals obtained at lower ratios). If the solutions are kept at -15 °C for several days, the relative intensities of the signals gradually change with time, as shown for the carbonyl region in Figure 8. The time developments of the relative intensities of the aggregation-sensitive signals C1 and C5 in the mixture LiOEEM/A confirm that an equilibrium is reached after several days at -15 °C. Comparison with the spectra in Figure 7 shows that such an equilibrium

state is very close, with respect to C1 or C5, to that reached almost immediately at higher LiOEEM/A ratios. Combining 1H and 13C spectra, it can be said with certainty that the prominent signal B observed either after long storage at the 4/1 ratio or at higher ratios corresponds to the LiOEEM/A 3/1 complex aggregate. The intensity ratios suggest, however, that even signals A and C could correspond either to the same stoichiometry but different structure of the aggregate or to the 2/1 aggregate. Unfortunately, the signals of complexed LiOEEM are not sufficiently resolved and their integration using the deconvolution procedure is not precise enough to give a confident lead in this matter. The argument for the latter possibility is that A, along with the original signal of A and two other signals, already appears at the 2/1 LiOEEM/A ratio but not at the 1/1 ratio. Thanks to a slow exchange process between free and complexed LiOEEM on the 13C NMR time scale, the measurement of the relative intensity ratio of free to complexed LiOEEM peaks can provide some information on the cumulative amount of the alkoxide involved in the various types of mixed aggregates. Deconvolution of the better resolved signal C13 shows that in a 4/1 LiOEEM/A mixture, 44% of the LiOEEM present is free, while in the 8/1 solution its content grows to 65%. This leads to the respective mean LiOEEM/A ratios in the complexes 2.4/1 and 3.8/1. Inspecting the A part of 13C NMR spectra of the same systems, one can conclude that the main mixed aggregate in both cases must be LiOEEM3A; the minor one, LiOEEM2A. However, the result for the 8/1 mixture (and generally for mixtures in the 5/1 to 8/1 ratio, cf. Table 4; the discrepancy from 3 cannot be explained merely by deconvolution error)

2738 Zune et al.


Figure 8. Carbonyl region of the 75.5 MHz (b) after 48 h, and (c) after 5 days at 273 K.

13C

NMR spectra of the 4/1 LiOEEM/A mixture in THF-d8 (0.2 M for A): (a) fresh,

Table 4. Mean Number of Equivalents n of LiOEEM Incorporated in Any Form of Mixed Aggregate with A under Various Conditions LiOEEM/A (mol/mol)

storage at 258 K (days)

n

4/1 4/1 4/1 5/1 8/1

0 1 4 0 0

2.38 2.58 2.69 3.18 3.83

shows that a larger excess of LiOEEM must lead to the formation of additional mixed aggregates with LiOEEM contents larger than 3. The structure of these additional aggregates cannot be very different in the part involving A as no new signals corresponding to them appear in the 13C or 1H NMR spectra. They also cannot be simply the self-aggregates of LiOEEM as the C13 signal is not sensitive to self-aggregation (cf. Figure 2). Hence, the only plausible explanation appears to be that the additional mixed aggregation occurs at the outer rim, i.e., on the LiOEEM part of the LiOEEM3A complex. At low LiOEEM/A ratios, i.e., 1/1 to 3/1, the signals of uncomplexed A still appear. Their traces can be observed even at much higher ratios in fresh samples but gradually disappear under storage at 258 K. At 1/1 and even 2/1 ratio, they survive even prolonged storage of many days, showing thus that, in contrast to lithium tert-butoxide and even LiCl, the complexation of LiOEEM with A is perceptibly reversible. In contrast to the higher LiOEEM/A ratios, much more complicated sets of relevant signals appear in 13C as well as 1H NMR spectra which are difficult to precisely assign. Several types of LiOEEM-complexed species coexist with free A in these systems, most of them built with a 1/1 LiOEEM/A ratio, according to the relative signal intensity of the complexed LiOEEM. 7Li and 6Li NMR spectra confirm some of the above suggested conclusions. Figure 9 shows the 7Li NMR spectra of A (0.2 M) and its mixtures with gradually

Figure 9. 116.6 MHz 7 Li NMR spectra of A (0.2 mol/L) and its mixtures with LiOEEM at 253 K in THF-d8. LiOEEM/A ratios (a) 0/1 (b) 1/1 (c) 2/1 (d) 3/1 (e) 8/1.

increasing amounts of LiOEEM at 253 K. It can be seen again that free A persists in the system up to the molar ratio 3/1, above which only one broad signal is detected. As expected, 6Li NMR spectra offer quite an analogous pattern, suggesting, thus, that either the magnetic shielding of lithium nuclei in the various species emerging at larger excesses of LiOEEM does not perceptibly differ or a rapid lithium exchange between them causes coalescence of their respective signals. The former possibility is backed by the fact that cooling of the system down to 203 K merely causes signal broadening adequate to the lowering of the T2 value while no signal splitting occurs. Three other types of observation, however, back the latter possibility without excluding the former one: (i) with a larger excess of LiOEEM, the chemical shift of the main signal moves downfield



Table 5. 7Li NMR Chemical Shifts (ppm) and T1 Longitudinal Relaxation Times (ms) of A and Its Mixture with LiOEEM at 253 K in THF-d8 (Concentration of A ) 0.2 M)

a

LiOEEM/A

δ (ppm)

T1 (ms)

1/0a 8/1 5/1 4/1 3/1 2/1 1/1

-0.139 -0.143 -0.145 -0.149 -0.151 -0.154/-0.292 -0.163/-0.292

23.57 25.68 27.71 27.72 28.81 30.70/35.30/81.33

toward the value observed with LiOEEM alone at equivalent concentration (see Table 5); (ii) the 7Li cumulative T1 values behave with the same logic as i (see also Table 5); (iii) any attempt to “burn a hole“ in a wing of the broad signal by selective irradiation or to reveal a signal structure under it by T2-filtering failed. Assuming fast exchange, the chemical shift of the 7Li resonance should be a weighted mean of the exchanging sites; i.e., it should follow the relation δ ) ∑δixi, δi being the chemical shift and xi the molar fraction of the i th site. Assuming that this relation can be partitioned into the form

∑δjxj ) δ1 (1 - x2) + δ2x2

ligand LiCl LiOtBu LiOEEM

0.2 M LiOEEM in THF solution.

δ ) δ1x1 +

Table 6. Comparison of the 13C Chemical Shifts (ppm) of A in THF at 253 K and in the Presence of Various Types of Ligand (L)

(4)

where the subscript 1 is related to the LiOEEM3A complex (δ1 ) -0.163 ppm) and subscript 2 to the free LiOEEM (with δ2 corrected for the concentration and x2 calculated from the assumed stoichiometry); the values δ obtained are always higher (in the negative sense) than the observed ones. This again indicates the presence of one or several additional species with their own chemical shifts somewhere between those of LiOEEM3A and LiOEEM, the whole system being in fast exchange. The parallel development (Table 5) of the T1 values which should follow an additive relation analogous to eq 1 points to the same conclusion. Due to their poor or missing resolution, 7Li or 6Li NMR spectra offer only scarce supporting evidence on the system time evolution, based mainly on the change in the chemical shift. Combined with the abovereported 1H and 13C NMR spectra, they lead to the following plausible conclusions: (1) up to the LiOEEM/A ratio 3, the mixed aggregation is reversible or rather incomplete, leaving a substantial part of A in the original state of dimeric self-aggregate; (2) the exchange of any part of A2 with the emerging LiOEEM2A2, LiOEEM2A, and LiOEEM3A mixed aggregates is very slow, if any, at 253 K and below; (3) the ratio of the various mixed aggregates (and probably of their various structural and conformational variants) changes very slowly even at 258 K but arrives eventually at a stable equilibrium, especially at higher LiOEEM/A ratios; (4) the mixing of LiOEEM and A THF solutions leads to various metastable systems, which includes the fact that, for the same LiOEEM/A ratio, a somewhat different starting ratio of the mixed aggregates is obtained. As the probability fluctuations of various types of encounters must be smoothed-out by statistics on the molecular level, the sample variations and metastabilities are probably determined by subtle differences in the mixing procedure causing short-lived differences in the local concentrations of the components. These findings can be of utmost importance for the polymerization control, but formation of metastable

L/A (mol/mol)

C1

C2

C5

1/3 1/3 1/4

159.7/170 160 155/170 160.2/159.5

70.9 72.5 77 69/64.5

179.3/178 180 181/177 177.9/177.4

systems as such is not uncommon with compounds of this type. As it has been shown in previous studies2 of A complexed with lithium tert-butoxide, the ratio of the products formed in a first encounter with the complexing agent can be governed by shear local probability rather than by the overall free energy balance. Proceeding now to a more detailed examination of the structure of A incorporated into a mixed aggregate, we find that, as in our earlier studies,2-5 the respective downfield and upfield shifts of signals C2 and C1, compared to the neutral dimer, are in full accord with the prevailing lithium ester enolate structure. However, in particular, one of the C2 signals is perceptibly upfield shifted in the 3/1 complex in comparison to bare A (Table 6). This is at variance with other ligands investigated by us,1-5 in particular with lithium tertbutoxide where the shift is quite the opposite. As in previous cases, this spectral feature cannot be intepreted simply in terms of an increase in p-hybridization of the C2 atom due to an increase in R-C-lithium ester character, as the C1 resonance does not exhibit the complementary downfield shift. However, C3 is also displaced from its initial position at 43 ppm to the region from 45 to 43 ppm accompanied by the attached H3 which also exhibits a downfield shift, whereas C5 and C11 are upfield-shifted. This indicates that the structure of LiOEEM/A complexes must be perceptibly different than that created by the other two ligands. This conclusion is backed by both theoretical calculations and relaxation measurements as shown below. In the LiOEEM part of the complex, all carbon signals except that of the methyl group are shifted upfield, which might indicate a change in the charge density at the R and β oxygens. The signals of the free and complexed LiOEEM remain resolved up to 273 K. No fast exchange between both forms is going on even at such relatively high temperature, which suggests a rather strong interaction between the ligand and the lithium ester enolate. The same holds for the parts A of the spectra which remain virtually unchanged between 203 and 273 K, indicating the existence of a rather stable complex. It should be noted in this connection that the stability of A against self-decaying reactions is markedly improved by the presence of LiOEEM even in comparison to the use of a µ ligand such as lithium tert-butoxide or lithium chloride.2,6 The quest for better understanding of the main mixed aggregate structure would not be complete without examining relaxation and related effects, such as nuclear Overhauser effect (NOE). Unfortunately, due to its probable structure and molecular volume, it can be expected to approach the critical condition τcω = 1, and, consequently, a very weak dipolar exchange between the incident protons can be expected. Actually, no detectable or a very weak NOE both in the laboratory and rotating frame was detected for the part A of the complex in a 4/1 LiOEEM/A mixture at 253 K. In the LiOEEM part, weak NOESY cross-peaks between the proton pairs 14-17, 16-17, and ROESY cross-peaks

2740 Zune et al. Table 7.

1H

and


13C

Longitudinal (T1) Relaxation Times (s) of the LiOEEM/A Mixture in THF-d8 at 253 K (Concentration of A ) 0.2 M) 13C

atom 1 2 3 4 5 6 7,8 9 10 11 12 13 14 15 16 17

neutral dimer

LiOEEM

LiOEEM/neutral dimer ) 4/1

LiOEEM/A ) 4/1

1.03 0.27 0.18 0.11 0.11

10.87 1.20 0.62 8.0 9.3 0.55 1.8 10.5 0.69 9.58 0.69 1.29 0.33 0.22 0.12/0.11 0.11/0.10

1.85/1.72 1.81/1.76/1.64 0.18/0.16 0.99/1.19 1.58/1.83 0.34 0.18 2.65 0.15/0.10/0.23 2.89 0.251/0.37/0.17 0.708 0.11/0.19 0.14 0.157/0.105 0.10/0.11

11.18 1.14 0.57 8.4 8.5 0.51 0.50 0.64 0.64 1.2 0.31 0.20 0.12/067 0.12

between protons 13-17, 13-15, 14-17, 14-16 have been observed. To confirm these weak effects and the absence of NOE in part A of the complex, we measured the nonselective 1H longitudinal relaxation times (T1) under selective irradiation of each proton signal. No reliable effect on the T1 of proton signals in parts A of the spectra were observed, confirming the almost vanishing NOE. In contrast, important changes in the T1 values of the alkoxide protons were observed. However, as all of these three kinds of NOE resemble those obtained with free LiOEEM, we suspect that they are mainly related to the always present uncomplexed part of the alkoxide. The same could be said about the heteronuclear 7Li-1H NOESY spectrum of the same system with the decreasing observable intensity of the cross-peaks between 7Li and H17, H16, and H14 (however, we do not exclude the possibility of the geometries of LiOEEM in a self-aggregate and a mixed aggregate being similar). The 7Li T1 relaxation times shown in Table 5 for different LiOEEM/A ratios clearly have to be considered in a way similar to the corresponding chemical shifts, i.e., with respect to fast Li exchange between the complex and the excess LiOEEM. The gradual decrease in the T1 value as the system approaches a state similar to a free alkoxide shows, however, that the electric field gradient and/or its asymmetry grow in the order A2 < LiOEEMxA < LiOEEMy, x and y meaning mostly 3 and 4, respectively. From this, it could be tentatively conjectured that the coordination bond strength grows in the same order, i.e., that the cause of coordination between LiOEEM and A is mainly the weakness of the coordination bond in the self-aggregate of A rather than the lability of the LiOEEM self-aggregation. This possibility is corroborated by the observation3 of free nonaggregated molecules in the THF solutions of A. 13C longitudinal relaxation can be used to obtain some information about the geometry of the main complex present in the system. T1 values for a LiOEEM/A mixture are compared with those of the neutral dimer in Table 7. In the following discussion, we chose the methodics quite analogous to that of our previous study (cf. ref 1). Using the well-known approximative formula for the correlation time of rotational diffusion

τceff ) 4πr3ηeff/3kT

(5)

1H LiOEEM/A ) 4/1

0.22/20 0.56/0.38 0.19/0.21/0.214 0.22 0.32/0.37/0.373 0.62/0.61 0.28 0.30 0.22 0.21

(for the meaning of the symbols, see ref 6) and assuming ω0τc ≈ 1, i.e., for 13C under proton decoupling

1/T1 ) (1/20)K2[J0(ωH - ωC) + 3J1(ωC) + 6J2(ωH + ωC)] (6) with K ) ∑γHγCh/πrCH3, J0(ωH - ωC) ) 2τC, J1(ωC) ) 2τc/[1 + (ωCτc)2], J2(ωH + ωC) ) 2τc/{1 + [2(ωH + ωC)τc]2}, we get for the ratio of the respective radii of the outer rotational profiles rc and rd of the complex and A

rc/rd ) (τc/τd)1/3 ) (T1d/T1c)1/3{10/[ 1 + 3/(1 + ωC2τc2) + 6/(1 + 4(ωH + ωC)2τc2)]}1/3 (7) or, approximatively, κ ) rc /rd ) 1.21(T1d/T1c)1/3. Taking the values for C4 from Table 7, we arrive at a minimum value κ ) 2.35. The complex is thus probably substantially more bulky than that of A with LiCl (κ ) 1.781.98; cf. ref 1). Hence, a markedly different structure is expected. This is in full accord with the predictions of MNDO. In our calculations, we have explored all reasonable possibilities of the lowest mixed aggregates AxLiOEEMy, x ) 1, 2 and y ) 1, 2, 3, starting with those core structures which proved stable in the case of µ-ligands and, in a complementary approach, those typical of LiOEEM self-aggregates. After full energy optimization, substantially different global minima were found which, for x ) 1 and y ) 1, 2, 3, are shown in Figures 10-12, respectively. As it can be seen, very symmetrical structures with even-numbered compact cores are not predicted. The core structures are rather tetraor pentagonal, hexagonal, and octagonal and rather planar or of a shallow basin shape. Consequently, the mean radius of the outer profile of isotropic rotation has to be about 1.4 times that predicted for an A(LiCl)3 complex (compare with the ratio 1.32 obtained above from relaxation measurements). No high stabilization can be expected for such rather loose structures. Compared with the heats of formation predicted for LiOEEM2 and A2 self-aggregates, the stabilization energies of the lowest mixed aggregates predicted by MNDO are given in Table 8. As shown, only A1LiOEEM1 is predicted to be really stabilized if compared to self-aggregation of the components. However, the destabilization of the 1/2, 1/3, and 2/1 mixed



Figure 10. Most stable structure of the LiOEEM1A1 mixed aggregate as predicted by MNDO calculations.

Figure 12. Most stable structure of the LiOEEM3A1 mixed aggregate as predicted by MNDO calculations. Table 8. Stabilization Energies ∆E (kcal/mol) Predicted for the Mixed Aggregates AxLiOEEMy by MNDOa

a

x

y

∆E

x

y

∆E

1 1 1

1 2 3

0.6 -5.7 -4.0

2 2

1 2

-5.7 -28.4

Compared to heats of formation of A2 and LiOEEM2.

Table 9. Data for the Oligomerization of tBuMA in THF Initiated with (DPH)Li at -78 °C in the Presence of Various Amounts of LiOEEM

Figure 11. Most stable structure of the LiOEEM2A1 mixed aggregate as predicted by MNDO calculations.

aggregates is lower than 2 kcal/mol when calculated for a single molecule which is below the confidence region of the method. Accordingly, even these mixed aggregates can be considered as possible, in agreement with the experimental results. We can be sure, however, that all simplest mixed aggregates should have only weak stabilization, which explains the experimental fact that a large excess of LiOEEM is needed to complex most of A in the system. When inspecting the MNDO results more closely, we can conclude that LiOEEM indeed acts as a µ/σ ligand in the sense that other oxygen atoms of its molecule are involved in an electron-donating coordination bond to the lithium atoms in the system. As in the case of pure µ ligands, the system of electron-sharing multicenter bonds forms only a minor part of the total bonding energy, its major part being due to Coulombic interactions. Interaction of LiOEEM with a Living Oligomer of tert-Butyl Methacrylate. There are two main reasons for caution in extrapolating the above results

tacticitya (%)

LiOEEM/ (DPH)Li (mol/mol)

Mn (SEC)b

MWD

iso

hetero

syndio

0 1 2 10 10c

3600 3180 3170 3120 9012

1.88 1.53 1.46 1.14 1.23

17 11.2 19

52 35.2 48.6 33 43

48 46.8 40.2 48 57

a Determined by quantitative 13C NMR. b Determined by SEC in THF at 30 °C with a polystyrene standard calibration. Theoretical value of Mn ca. 3000 (tBuMA/initiator ) 20/1 mol/mol). c Theoretical value for M ) 10 000 (tBuMA/initiator ) 70/1). n

about interaction of LiOEEM with A directly to polymer systems. First, the analogy between the dimeric model and the living (PtBuMALi) chains (B) is incomplete considering the steric and possibly other factors coming from the chain. This is clearly illustrated by Lochmann and Mu¨ller’s work where striking changes in reactivity in the first several propagation steps are reported.18 Second, the partial symmetry of A does not allow the detection of any diastereoisomerism in the lithium ester enolate site. As in our previous work, we therefore extended our study to living tert-butyl methacrylate oligomers 13C-enriched in the C1 position in several last monomer units (B in the following text means a living oligomer, specifically its growth center in the quantitative expressions). Characterization of these products is shown in Table 9. Figure 13 shows 7Li NMR spectra of the living oligomers prepared in the presence of LiOEEM at its

2742 Zune et al.


Figure 14. 100 MHz 13C NMR spectra of B (0.01 mol/L) and its mixtures with LiOEEM at 220 K in THF-d8. LiOEEM/B ratios: (a) 0/1; (b) 1/1; (c) 3/1; (d) 10/1. Chart 2. Numbering of the Carbons in the Living Oligomeric Chains

Figure 13. 155 MHz 7 Li NMR spectra of B (0.01 mol/L) and its mixtures with LiOEEM at 220 K in THF-d8. LiOEEM/B ratios: (a) 0.25/1; (b) 1/1; (c) 3/1; (d) 10/1.

various ratios to B at 220 K. When increasing LiOEEM/B in the range 0-4, the signals characteristic of bare B gradually disappear while virtually a new signal appears at -0.12 ppm. The assignment of this new signal to a mixed complex of LiOEEM with B is straightforward. As clearly seen in Figure 13c, traces of uncomplexed B remain in the system up to LiOEEM/B ) 3. On the other hand, at LiOEEM/B ) 4 or higher, i.e., close to the ratio which has proved to be critical in changing the course and outcome of the anionic polymerization of tBuMA, there is only one single signal left, its position gradually shifting toward that of LiOEEM at higher LiOEEM/B ratios. This indicates fast exchange between free LiOEEM and the complex. The overall behavior closely resembles that observed with LiOEEM/A mixtures. We can thus conclude that a complexation of LiOEEM with B takes place in a close analogy to the interaction of LiOEEM with A. Carbonyl parts of 13C NMR spectra of living oligomers B prepared in the presence of various amounts of LiOEEM and carbonyl-13C enriched in several (ideally 3) last monomer units are shown in Figure 14. Due to the stochastic nature of monomer addition, a distribution of lengths of the 13C-labeled block (which are those mainly visible in the spectra) is always produced. In Figure 14, signals at 161-164 ppm have been assigned to the living end group (C1 in Chart 2) and those between 178 and 183 ppm to the penultimate carbonyl (C2). One of the signals in the C1 group can be assigned to uncomplexed B in agreement with 7Li NMR spectra

of the same product. The analogy with the 13C NMR spectra of LiOEEM/A is not complete as there are always three different signals in the C1 region here. At a low LiOEEM/B molar ratio, 0.25, a broad signal is observed at 161 ppm. This signal has the same chemical shift and shape as the one observed in the 13C NMR spectrum of pure B in THF solution. Already at LiOEEM/B ) 3, two other signals are observed at 164 and 161 ppm. Increasing LiOEEM excess only modifies the relative intensities between these three signals, the major one at the molar ratio 10/1 being at 164 ppm. The corresponding C2 signals (cf. Figure 14) form two groups centered at 182 (D1) and 179 (D2) ppm, respectively, with relative intensities depending on the LiOEEM/B ratio. At 0.25, only one broad signal at 182 ppm is observed, with position and shape exactly the sames as those observed with B in pure THF. At LiOEEM/B > 3, the main signal in the D1 group shifts to 179.5 ppm, the other one remaining near 181 ppm. The disappearance of the D1 (along with A-C) signals after hydrolysis indicates their belonging to the penultimate unit. Their width can be explained by a length distribution of the oligomeric chains assuming that their conformation can be slightly length-dependent. There are mostly two broad D1 signals (D1a,D1b) correlated to three C1 signals (A-C). Of these, one pair (D1a, B) is identical, both in position and shape, with that observed with additive-free B. From this, one could conclude that a



the former. Further investigations in this direction are under way. Acknowledgment. The authors would like to thank the Grant Agency of the Academy of Sciences of the Czech Republic for its support given under Grant A4050606. The Belgian authors are indebted to the Belgian Fonds National de la Recherche Scientifique (FNRS) for a fellowship (C.Z.) and to the “Services Fe´de´raux des Affaires Scientifiques, Techniques et Culturelles” in the frame of the “Poˆles d’Attraction Interuniversitaires 4-11: Chimie et Catalyze Supramole´culaire” for financial and scientific support. P.D. is research associate with the Belgian FNRS. Supporting Information Available: Figures showing the COSY(HETCOR) NMR spectrum of the 1 M solution of LiOEEM in THF-d8 at 253 K, structures of the two most stable dimeric aggregates of LiOEEM as predicted by ab initio SCF 3-21G calculations, structures of the two most (and almost equally) stable trimeric aggregates of LiOEEM as predicted by ab initio SCF 3-21G calculations, the 1H-13C COSY(HETCOR) NMR spectrum of the mixture of A (0.2 mol/L) with 4 equiv of LiOEEM in THF-d8 at 253 K, and the 1H-13C longrange COSY (COLOC) NMR spectrum of the mixture of A (0.2 mol/L) with 4 equiv of LiOEEM in THF-d8 at 253 K (5 pages). Ordering information is given on any current masthead page

1H-13C

Figure 15. SEC chromatogram of poly(tert-butyl methacrylate) initiated by (DPH)Li in THF at 195 K in the presence of 10 equiv of LiOEEM.

minor part of the growing center either always remains uncomplexed even at high LiOEEM/B ratios or forms a LiOEEM-poor complex which resists higher complexation. Compared to A, formation of the complexed growth center of B (D2, A and C) apparently has an even less favorable equilibrium. This can be due to steric effects, possibly enhanced by a self-solvation of the center by its own oligomeric chain. In accord with the found inability of the growth center to be fully coordinated and thus shielded by the ligand, the self-termination process detected by its ketonic product proceeds at 273K with an appreciable speed. As shown in Table 9, the influence of a varying excess of LiOEEM on the stereoregularity of the mainly syndioand heterotactic oligomeric products is rather complex, but generally weak. In contrast, its favorable influence on the polydispersity of molecular weights is unquestionable. The same is observed with polymers of higher molecular weights. The livingness of the polymerization in THF at 195 K is evidenced by (i) the linear relationship between the theoretical molecular weight and the experimental value determined by SEC (LiOEEM/B ) 10), (ii) the linear relationship of the molecular weight versus conversion, and (iii) the rather narrow distribution of molecular weights. The bimodal chararacter of the distribution (cf. the SEC chromatogram in Figure 15) is attributed to the presence of at least two kinds of growth centers with different rates of propagation. We propose to attribute the narrow part of the distribution to be produced by the LiOEEM-complexed growth center because it increases and narrows at higher LiOEEM/B ratios, i.e., at an increased population of the complexed form of the center (signal A in Figure 14). As a concequence, the polydispersity of the product decreases under the same conditions (cf. Table 9). The existence of two or three growth centers propagating with different rates, persisting even at relatively high LiOEEM/B ratios, indicates that the ligand is not quite efficient in the complexation and thus in promoting an ideal living polymerization of tBuMA. In contrast, LiOEEM is reported to be efficient in promoting an almost ideally living polymerization of MMA characterized by a very narrow MWD of the product (polydispersity 1.05).19 The difference between tBuMA and MMA lies very likely in the much larger steric hindrance of the complexation caused by the tert-butyl group of

References and Notes (1) Zune, C.; Dubois, P.; Je´roˆme, R.; Krˇ´ızˇ, J.; Dybal, J.; Lochmann, L.; Janata, M.; Vlcˇek, P. Macromolecules 1998, 31, 2744. (2) Krˇ´ızˇ, J.; Dybal, J.; Janata, M.; Vlcˇek, P. Magn. Reson. Chem. 1994, 32, 58. (3) Krˇ´ızˇ, J.; Dybal, J.; Janata, M.; Lochmann, L.; Vlcˇek, P. Macromol. Chem. Phys. 1996, 197, 1889. (4) Krˇ´ızˇ, J.; Dybal, J.; Vlcˇek, P.; Janata, M. Macromol. Chem. Phys. 1994, 195, 3039. (5) Krˇ´ızˇ, J.; Dybal, J.; Lochmann, L.; Janata, M.; Vlcˇek, P. Macromol. Chem. Phys. 1995, 196, 3005. (6) (a) Bayard, Ph.; Fayt, R.; Teyssie´, Ph.; Varshney, S. K. Fr. Pat. 91, 09172 (1991). (b) Bayard, Ph.; Je´roˆme, R.; Teyssie´, Ph.; Varshney, S. K.; Wang, J.-S. Polym. Bull. 1994, 32, 381. (7) Lochmann, L.; Rodova´, M.; Petra´nek, J.; Lı´m, D. J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 2295. (8) Werkhoven, T.; Lugtenburg, J. J. Labelled Compd. Radiopharm., to be submitted for publication. (9) Neuhaus, D.; Williamson, M. The Nuclear Overhauser Effect in Structural and Conformational Analysis; VCH Publishers: New York, Weinheim, Cambridge, 1989; p 123. (10) (a) Stilbs, P. Prog. NMR Spectrosc. 1987, 19, 1-45. (b) Haner, R. L.; Schleich, T. Methods Enzymol. 1989, 176, 418-447. (11) Stilbs, P. Prog. NMR Spectrosc. 1987, 19, 1. (12) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (13) Dupuis, M.; Spangle, D.; Wendolski, J. J. Program QG01. NRCC Software Catalog 1980; University of California: Berkeley, 1980; p 1. (14) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. J.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347. (15) (a) Jackman, L. M.; Lange, B. C. Tetrahedron 1977, 33, 2737. (b) McGarruty, J. F.; Ogle, C. A. J. Am. Chem. Soc. 1984, 107, 1807. (16) Tomoi, M.; Sekiya, K.; Kakiuchi, H. Polym. J. 1974, 6, 438. (17) (a) Dybal, J.; Krˇ´ızˇ,, J. Collect. Czech. Chem. Commun. 1995, 60, 1609. (b) Dybal, J.; Krˇ´ızˇ,, J. Collect. Czech. Chem. Commun. 1994, 59, 1699. (c) Dybal, J.; Krˇ´ızˇ,, J. Macromol. Theory Simul. 1997, 6, 437. (18) Lochmann, L.; Mu¨ller, A. H. E. Makromol. Chem. 1990, 191, 1657. (19) Bayard, P.; Je´roˆme, R.; Teyssie´, Ph.; Varshney, S.-K.; Wang, J.-S. Polym. Bull. 1994, 32, 381.

MA971368S

Study of the Propagation Center in the Anionic Polymerization of (Meth

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