Electronic Structure Calculations and Experimental Studies on the Thermal Initiation of the Twin Polymerization Process

Presented here is a combined computational and experimental study on the thermal initiation process of the twin polymerization. Although thermally initiated twin polymerization offers a versatile scheme for obtaining hybrid organic/inorganic nanocomposite materials, the mechanism for its initiation is very different from the proton‐initiated twin polymerization. In this study, the basic mechanism of the early steps of the polymerization process of 4 H,4 H′‐2,2′‐spirobi[benzo[d][1,3,2]dioxasiline] was investigated by using electronic structure calculations in conjunction with experimental differential scanning calorimetry studies. This way, the influences on the thermally initiated twin polymerization process could be analyzed in detail. The previous mechanistic hypotheses are systematically assessed herein to show that, based on the results, a new hypothesis for an initiation mechanism can be formulated that is in agreement with all experimental observations. These results suggest that, before the formation of the polymer networks, the thermal initiation starts with the formation of low‐molecular‐weight fragments that react to yield acidic groups. If a sufficient amount of these form, the reaction is ultimately funneled into a mechanism similar to that of proton‐initiated twin polymerization

Spirocyclic tin salicyl alcoholates – a combined experimental and theoretical study on their structures, ¹¹⁹Sn NMR chemical shifts and reactivity in thermally induced twin polymerization

The spirocyclic tin salicyl alcoholate, 4H,4′H-2,2′-spirobi[benzo[d][1,3,2]dioxastannine] (1), and its 6,6′-dimethoxy (2) and 8,8′-di-tert-butyl-6,6′-dimethyl derivative (3) were synthesized and thermally induced twin polymerization of precursor 2 was performed to give a SnO2-containing hybrid material. Studies on the molecular structures of 1–3 were carried out using 119Sn{1H} CP MAS NMR spectroscopy and DFT calculations. Crystallization of compound 3 from dimethyl sulfoxide solution provided the Lewis acid–base adduct 3(dmso)2 exhibiting a hexacoordinated tin atom in the solid state, in agreement with the results of the spectroscopic and DFT calculation data. 119Sn NMR spectroscopy of the compounds 1–3 and 3(dmso)2 revealed equilibria among the diverse oligomers in solution phase pointing at hexacoordinated tin atoms

The systematics of a small spineless Desmodesmus species, D. Costato-Granulatus (Sphaeropleales, Chlorophyceae) based on ITS2 rDNA sequence analyses and cell wall morphology

Desmodesmus species taxonomy is one of the most long-standing issues in green microalgal systematics due to problems associated with phenotypic plasticity. Whereas more recent species descriptions and identifications are mainly based on cell wall structures and the use of cultures, comparisons with molecular phylogenies are largely lacking. In this study, the phylogenetic relationships between 22 clones identified as Desmodesmus costato-granulatus (Skuja) E. H. Hegew. were assessed using ITS2 rDNA sequence data in combination with cell wall morphology. The unrooted ITS2 phylogeny showed that the clones cluster into five groups, which also differ in their cell wall structures. Therefore, the taxon is split into five species: D. costato-granulatus, D. elegans, D. fennicus, D. regularis, and D. ultrasquamatus. Compared with other Desmodesmus species, intraspecific sequence variation is extensive and may contain additional (pseudo)cryptic diversity. Compensatory base changes were near-absent within the species and varied from one to 11 between species. Relationships among the species were unresolved. Despite this, they clustered together with the two other Desmodesmus species having a combination of small and large warts in a well-supported lineage. Remarkably, ITS2 sequence variation in this lineage is as high as between all other included Desmodesmus species, even though the morphology of its members is rather uniform

Nitrogen stable isotope ratios of lake macrophytes in relation to growth form and nutrient-limitation

Values are presented of natural abundance nitrogen stable isotope ratios (15N ‰) for macrophyte samples from 30 U.K. upland tarns. Values for individual macrophyte species at the different sites varied between –9.9‰ and 10.6‰, a range of 20.5‰. The average value of a particular macrophyte species was slightly less variable with a range of 14.5‰ and the average range per site was 10.4‰. To explore the possible causes of this very high variation, macrophytes were allocated into three categories according to their growth form and hence whether they probably obtained their nitrogen i) from both water and sediment (such as elodeids and isoetids), ii) mainly from the sediment (such as rooted emergent or floating-leaved plants) and iii) mainly from the water (such as filamentous algae, bryophytes and rootless vascular plants). An analysis of variance showed that δ15N differed according to presumed nitrogen source: average values were 0.3, 1.6 and 3.0‰ for macrophytes that obtain their nitrogen respectively from the water, from both sources and from the sediment. Hence part of the overall variation may be related to the lower mean site δ15N of the total nitrogen in water (1.6‰) compared to the sediment (3.0‰). However, much of the observed wide variation in macrophyte δ15N could be related to differences in nutrient limitation. Macrophytes restricted to the water for their nitrogen source showed statistically significant positive correlations between δ15N and nitrogen-limitation at the site and negative correlations with phosphorus-limitation at the site. The results suggest that the large variation in the δ15N value of freshwater macrophytes may be caused by a combination of differences in the δ15N value of the main nitrogen source used and differential isotopic discrimination caused by variable nitrogen-availability relative to demand. Thus analysis of the δ15N value of macrophytes that obtain their nitrogen from the water, such as bryophytes and filamentous algae, might offer a convenient and effective way of assessing nitrogen-limitation at a site