Interactions of living oligomers of tert-butyl methacrylate (tBuMA) with a Li counterion and of the model living dimer di-tert-butyl 2-lithio-2,4,4-trimethylglutarate (A) with LiCl (B) were studied in tetrahydrofuran-d 8 (THF-d8) solution by 7 Li, 6 Li, 1 H, and 13 C, 1D and 2D, NMR spectroscopy. Di-tertbutyl 2-lithio-2,4,4-trimethylglutarate is shown to form a 1:1 complex with lithium chloride in THF. 1 H, 7 Li NMR results suggest that a rapid equilibrium between free LiCl and the mixed complex takes place when the molar ratio LiCl:lithiated dimer is higher than 1. From 13 C relaxation times, a dimeric aggregation (i.e. A2B2) of the complex is deduced, in full accord with MNDO calculations. Preliminary results are presented for living tBuMA oligomers prepared by initiation with diphenylhexyllithium in the presence of various amounts of lithium chloride. Fast-acquired 13 C NMR spectra of the living ends were recorded using 13 C-enriched monomer added at the end of the polymerization. Their structure is shown to be affected by the amount of LiCl added and to correspond to an equilibrium between free lithium chloride and a complexed end group. A higher excess of LiCl apparently does not change the nature of the complexed living end. The oligomers exhibit marked self-termination in THF at 273 K. Finally, a correlation of the structure of the living chain ends with the macromolecular parameters of the resulting polymer is attempted.
A synthetic scheme has been developed to prepare isoprene specifically 13 C-labelled at all different positions and in any combination of positions, from methyl methacrylate and [a] 2113 methyl iodide. According to this scheme (4-13 C)isoprene was prepared with high label incorporation (99%).
Interactions of lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM)
with the model dimer di-tert-butyl 2-lithio-2,4,4-trimethylglutarate (A)
and the living poly(tert-butyl methacrylate) oligomers
(B)
were studied in tetrahydrofuran-d
8 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
LiOEEM
1
A
1
,
LiOEEM
3
A
1
,
and
possibly
LiOEEM
2
A
1
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.
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