Elsevier

Polymer

Volume 39, Issue 2, 1998, Pages 405-412
Polymer

The chemistry of novolac resins - VI. Reactions between benzoxazine intermediates and model phenols

https://doi.org/10.1016/S0032-3861(97)00288-7Get rights and content

Abstract

The reaction between 3-(,5-dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3-benzoxazine and either 2,4-xylenol or 2,6-xylenol was studied by 13C n.m.r. techniques, to model the reactions between benzoxazine intermediates and free ortho- and para-phenolic sites in the curing of novolac resins with hexamethylenetetramine (HMTA). The results indicate that 2,4-xylenol can directly react with benzoxazine intermediate at low temperature via several pathways to form methylene linkages between phenolic rings. 2,2′-Methylene-4,4′,6,6′-tetramethyldiphenol is the dominant product after heating benzoxazine and 2,4-xylenol to 205°C. However, no reaction occurs between the benzoxazine and 2,6-xylenol before the decomposition of the benzoxazine structure. At higher temperatures, 2,6-xylenol reacts with the decomposition species of the benzoxazine to form para-para and ortho-para methylene linkages between phenolic rings, together with ortho-ortho methylene linkages generated from the decomposition of the benzoxazine. Minor amounts of nitrogen-containing structures, such as amines, amides and imines, were also formed. The results when applied to novolac/HMTA systems provide possible reaction mechanisms and pathways from the benzoxazine intermediates and vacant ortho- and para-phenolic reactive sites leading to the final cross-linking network. © 1997 Elsevier Science Ltd.

Section snippets

INTRODUCTION

Substituted benzoxazines and benzylamines are the major first-formed intermediates produced in the curing of novolac resins with hexamethylenetetramine (HMTA), and further reaction of these intermediates leads to a highly cross-linked network1, 2, 3, 4, 5, 6. The thermal decomposition of 3-(3,5-dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,-4-dihydro-(2H)-1,3-benzoxazine, as a model of benzoxazine intermediates, has been reported in the previous paper[7], providing reaction pathways from benzoxazine

Samples

3-(3,5-Dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3-benzoxazine was prepared by the method reported previously4, 6. The compound was heated with 2,4- or 2,6-xylenol, respectively, in a ratio of 1:1 by wt% in a Eurotherm 902 oven under the same conditions used to cure novolac resins with HMTA[6]and the benzoxazine model system[7]. The samples were heated at 90°C for 6 h, then the temperature was increased at a rate of 3.7°C h−1 until 135°C, thereafter, 12°C h−1 until 205°C, and

Reactions between benzoxazine and 2,4-xylenol

13C spectra of 3-(3,5-dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3-benzoxazine (1) heated with 2,4-xylenol to various temperatures are shown in Fig. 1Fig. 2. In the 13C spectra of low chemical shift range (10–100 ppm), the peaks with open circles are either CH or CH3, while the rest are CH2 carbons. In the high chemical shift range (100–200 ppm) the peaks marked by open circles are due to CH and the rest are quaternary carbons, as detected by the DEPT technique. The three strong

CONCLUSION

The 13C n.m.r. study provides direct evidence about the formation of methylene linkages between phenol rings from reactions of 3-(3,5-dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3-benzoxazine and 2,4- or 2,6-xylenols. Various amines, amides and imines were also obtained during the reaction process, but the diphenylmethanes (dimers) are the predominant products in both systems. However, the reaction pathways of the two systems are different. The benzoxazine can react with

Acknowledgements

The work was supported by the Australian Industry Research and Development Board (Grant no. 15068), Australian Research Council, and Comalco Aluminium Limited. We also thank Dr. M.G. Looney and Mr. A.C. Potter for their helpful discussion.

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