A new analytical method based on the 2D HSQC NMR sequence is presented, which can be applied for quantitative structural determination of complicated polymers. The influence of T1 and T2 relaxations, off-resonance effects, coupling constants and homonuclear couplings are discussed. It was found that the T2 values measured on polymeric samples with the conventional HSQC-CPMG sequence could not be used to correct the errors caused by T2 relaxations during the polarization transfer delay. A unique way of selecting the proper internal standard reference signal(s) is therefore proposed to eliminate the major errors caused by T2 relaxations, resonance offsets, coupling constant deviations and homonuclear couplings. Two polymer samples, a cellulose triacetate and an acetylated lignin, have been used to illustrate the principles. The methodology developed in this work is robust to instrument miss-setting and it can find wide-spread applications in areas where a quantitative analysis of structurally complicated polymers is necessary.

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A spiro-dienone structure (b-1/a-O-a) has been observed as one of the important structures present in spruce and aspen lignins, with abundance as high as 3% in spruce lignin.Lignin macromolecules consist of phenylpropanoid units that are linked to each other by various types of ether and carboncarbon bonds. Opinions on the occurrence and frequency of the b-1 linkage in lignin have been quite divergent among wood chemists. The abundance of the b-1 structure has been estimated to range from 1% to 15% in spruce lignin, 1-3 depending on the analytical method applied. The b-1 structure has been observed to be one of the major type of lignin inter-unit linkages by studies using mild hydrolysis 4,5 and thioacidolysis. 6 On the other hand, efforts to detect the b-1 structure in milled wood lignin (MWL) by using 1 H NMR, 1 2D HMQC NMR 7,8 or ozonation 2 have not been very successful, suggesting that this structure might only be a minor constituent in milled wood lignin.To explain the discrepancies in the applied analytical methods, it has been suggested that a precursor of the b-1 structure, a dienone structure (I in Scheme 1) might be present in native lignin. 9 On acid hydrolysis, the dienone structure would be converted to the b-1 structure (II in Scheme 1).There have been several reports in the literature showing supporting evidence for the presence of such a precursor in lignin. We have observed previously that mild acid hydrolysis of spruce wood meal, pre-methylated with diazomethane, resulted in the release of lignin b-1 dimers predominantly carrying a free phenolic hydroxy group on ring B. 6 This observation suggested that the ring B originally was present in the form of a dienone structure. More evidence supporting the existence of the dienone structures in lignin includes the recent isolation of the sesquineolignan woorenol, 10 the synthesis of spirocyclohexadienone structure by radical coupling of ligninlike model compounds 11 and the isolation of an isochroman structure among the lignin degradation products after a DFRC treatment 12 as well as some 3D NMR evidence. 13 In the present communication, we present direct observations of the spirodienone structures of type I in spruce and aspen MWLs by NMR techniques.Two spruce MWL samples and one aspen MWL were prepared. One of the spruce lignin samples (MWL1) was prepared from freshly cut spruce wood by milling with steelballs for 3 days. 14 The other spruce lignin sample (MWL2) was prepared from an aged spruce wood log by milling with porcelain balls for 14 days. 15 The spiro-dienone structure was † Electronic supplementary information (ESI) available: 13 C, QUAT, HMBC and HSQC NMR spectra. See http://www.rsc.org/suppdata/cc/b1/ b108285j/ Scheme 1 Fig. 1 Signals for the spiro-dienone structure (b-1/a-O-a) were observed and assigned in the HSQC spectrum of an acetylated spruce milled wood lignin (MWL1 acetate in acetone-d 6 ). This journal is

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