✍ Scribed by M. Suchopárek; J. Dybal; J. Spěváček
No coin nor oath required. For personal study only.
The conformational structure of t-butyl propionate and oligomers (the dimer and trimer) and polymers of t-butyl acrylate (tBuA) was characterized using a combination of i.r. spectroscopy, NMR spectroscopy (~H-and ~3C-) and computational molecular mechanics methods, MM2. The i.r. spectra and the MM2 method indicated that t-butyl propionate prefers the trans form in rotation around the CHz--CO bond. The i.r. spectra of the tBuA oligomers and polymers were found to contain a pair of conformation-sensitive bands in the region around 930 and 910cm t, corresponding to the pair of conformers formed by rotation around the bonds of the main chain. It was found by both the MM2 computation method and analysis of the temperature dependence of the ~Jcn interaction constant of the ~ carbonyl carbon and the 2-methylene protons that, in rotation around the
bond in the tBuA dimer, the gauche form is energetically more favourable than the trans. This conclusion is supported for the tBuA trimer by the determination of scalar interaction of one of the 2-methylene protons with one of the methyls bonded to the 1-carbon. Analysis of the temperature dependences of 3JHH and 3Jca interaction constants as well as the calculations of the rotation about the CH2--CH 2 bond in the tBuA dimer unambiguously indicate that the trans form is energetically favourable. It follows from the results of the MM2 computational method that the tt chain conformation is preferred for the meso diad of poly(t-butyl acrylate) (with sa substituent arrangement) and that the tg (ss, sa) and t~ (as) conformations are present in somewhat smaller amounts. In the racemic diad, the tt conformation with antiparallel substituents predominates.
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