3. Table S1. Commentary on the Stationary points on the potential surfacea for the overall mechanism, illustrated using the RR,SS stereoisomer.
Storyboard:

  1. In this terminology, RR (of RR,SS) refers to the configuration of the two chiral centres of the previously ring-opened lactide unit whilst SS refers to the newly co-ordinated lactide unit which is about to be ring-opened (see Int1 window).

  2. The lowest energy of the first lactide (RR) is that of a 5-ring coordinated species: .

  3. The reactant is solved by one molecule of THF , but the solvent is then displaced by a second (SS) lactide monomer. The coordination of the THF solvent is closer to equatorial rather than axial.

  4. The geometry of the resulting intermediate (Int1) is mediated by a weak O...C=O electrostatic interaction (3.01Å).
  5. The first transition state involves O...C=O bond formation in which the weak O...C=O electrostatic interaction is strengthened (2.88Å).
  6. The geometry of this transition state is controlled by the stereoelectronic antiperiplanar orientation of the incoming O...C bond and the C-C bond
  7. The original alkyl-oxygen coordination to Mg is preserved in this transition state
  8. as it also is in the resulting tetrahedral intermediate.
  9. The tetrahedral intermediate undergoes a (rocker-switch like) reorganisation of the Mg-O coordination, accompanied by a conformational change. Thus the original alkyl-oxygen...Mg bond breaks and a new alkyl-oxygen...Mg bond forms
  10. This change in conformation now sets up the final C-O bond cleavage
  11. which after further Mg...O de-coordination, and re-coordination by the terminal C=O group
  12. results in the product with extruded polymer chain
  13. The rate-determining transition state has an imaginary normal mode Enable Java 1.4.2_5 or later which reveals both O-C cleavage and concomitant C=O...Mg de-coordination with a computed wave number of 49 cm-1. This unusually low value for a transition mode arises in part because of the highly correlated nature of the vibration, in which most atoms of the reaction centre participate, and hence the relatively large mass-weighting of this vibration.
  14. TS2 has Wiberg bond indices in the NAO basisb; C-Oacyl 0.15 , Oacyl-Mg 0.09 , Ocarbonyl-Mg 0.03 , Ocarbonyl-C 1.65) with considerable ionic nature (natural charge on Mg +1.68) .
  15. The product re-coordinates a solvent THF molecule in such a manner as to minimise repulsions to the methyl group . This means in effect that the THF solvent and the polymer chain have to be coordinated di-equatorial to the Mg. This methyl group thus avoids clashing with the isopropyl group . The extruding (and conformationally flexible) polymer chain folds back across the same face as the THF solvent. The energy reported here is considered an upper bound, since not all the conformational space of this chain has been explored. Alternative modes of coordination such as having the THF and the polymer chain effectively di-axial at the Mg centre are significantly higher in energy .
  16. The thermochemistry of the overall mechanism is (respectively, as difference in total energy gas phase, with IEFPCM solvation correction for THF,b and for ΔG gas phase):
    1. (TS2,THF) - (RR-reactant+THF,lactide): 17.1, 16.5, 18.9 kcal mol-1.
    2. TS2 - Int1(RR-reactant+lactide): 18.3, 13.7, 20.2 kcal mol-1
    3. (RR-reactant,lactide) - (Product): -16.9, -7.7c, -0.9,
    4. (RR-reactant+thf,lactide) - (Product+thf): -11.4, -2.6, +2.3
Total energies (Hartree)/Relative energies (kcal-1)
[Corrected for ΔG298 (Hartree)/relative ΔG298, kcal-1], {IEFPCM solvation model}b
RR-Reactant SS-Lactide
-2077.83601 [-2077.06000] {-2077.78925} -534.35128 [-534.24493] {-534.35364}
Total: 2612.18729 [2611.30493] {2612.14289}
Enable Java 1.4.2_5 or later
RR-Reactant+THF THF
-2310.29905 [-2309.40424] {-2310.25064} -232.44540 [-232.35690] {-232.44402}
Total: 2612.18729 [2611.30493] {2612.14289}
Enable Java 1.4.2_5 or later
Int1 (=Reactant + lactide - thf) TS1
-2612.20735/0.0 [-2611.29448/0.0] {-2612.155852} -2612.18938/11.3 [-2611.27550/11.9]
Enable Java 1.4.2_5 or later
TI1 TI2
-2612.18950 [-2611.27569] -2612.19235 [-2611.27650]
TS2 Product
-2612.17810/18.4 [-2611.26220/20.2] {-2612.134079} -2612.21424 [-2611.30626/-7.4] {-2612.155228}
Product.thf Product.thf isomer
-2844.66852 [-2843.645407] 2844.65185 [-2843.63036]

aThese calculations are all based on the B3LYP density functional procedure, employing a 6-311G(3d) basis for Mg, 6-31G(d) for the lactide and core ligand units, and STO-3G for the 2,6-di-isopropyl aryl ligand substituents. The Gaussian 98 and 03 programs were used; a) Gaussian 03, Revision C.02, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., Wallingford CT, 2004. b Reed, A. E.; Curtiss, L. A.; Weinhold, F., Chem. Rev., 1988, 88, 899 - 926. b M. T. Cances, B. Mennucci, and J. Tomasi, J. Chem. Phys., 1997, 107, 3032. c Errors in convergence may render this value suspect.