The Structure of Poly(carbonsuboxide) on the Atomic Scale: A Solid-State NMR Study

• Published in: Chemistry : a European journal 2005-07, Vol.11 (15), p.4429-4440
• Main Authors: Schmedt auf der Günne, Jörn, Beck, Johannes, Hoffbauer, Wilfried, Krieger-Beck, Petra
• Format: Article
• Language: English
• Subjects: amorphous materials; density functional calculations; NMR spectroscopy; solid-state structures
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200401133

In this contribution we present a study of the structure of amorphous poly(carbonsuboxide) (C3O2)x by 13C solid‐state NMR spectroscopy supported by infrared spectroscopy and chemical analysis. Poly(carbonsuboxide) was obtained by polymerization of carbonsuboxide C3O2, which in turn was synthesized from malonic acid bis(trimethylsilylester). Two different 13C labeling schemes were applied to probe inter‐ and intramonomeric bonds in the polymer by dipolar solid‐state NMR methods and also to allow quantitative 13C MAS NMR spectra. Four types of carbon environments can be distinguished in the NMR spectra. Double‐quantum and triple‐quantum 2D correlation experiments were used to assign the observed peaks using the through‐space and through‐bond dipolar coupling. In order to obtain distance constraints for the intermonomeric bonds, double‐quantum constant‐time experiments were performed. In these experiments an additional filter step was applied to suppress contributions from not directly bonded 13C,13C spin pairs. The 13C NMR intensities, chemical shifts, connectivities and distances gave constraints for both the polymerization mechanism and the short‐range order of the polymer. The experimental results were complemented by bond lengths predicted by density functional theory methods for several previously suggested models. Based on the presented evidence we can unambiguously exclude models based on γ‐pyronic units and support models based on α‐pyronic units. The possibility of planar ladder‐ and bracelet‐like α‐pyronic structures is discussed. 99 Years of Poly(carbonsuboxide): The structure of amorphous poly(carbonsuboxide) has been debated for almost a century. It is formed in a spontaneous reaction from the highly reactive monomer C3O2. The amorphous polymer has been isotopically labelled, so that modern 13C solid‐state NMR experiments can be used to determine a detailed picture of its local structure. By using NMR distances, correlations and intensities, long discussed structural issues could be resolved.