Urushiol polymerization

A number of applications of commercial lacs and of separated urushiol have been referred to (ref. 2). As with the phenolic lipids of Anacardium occidentale a great deal of work has been carried out particularly in Japan and China to diversify the uses of lacs from Rhus vemicifera. It is widely employed in artistic decoration, building materials, textile equipment and furniture. The industrial utilisation of polyketide natural products including the phenolic lipid urushiol has been reviewed (ref. 314). The great number of uses largely comprise polymerisation reactions and some non-polymeric processes, some of both of which are described in the next sections. 

X.Z. Zheng, J.B. Weng, Q.M. Huang, B.H. Hu, T. Qiao, and P. Deng, Fabrication of a stable poly(vinylpyrrolidone)/poly(urushiol) multilayer ultratbin film tbrougb layer-by-layer assembly and photo-induced polymerization. Colloid Surf. A, 337, 15 20 (2009). 

C atalytic site of peroxidase is a heme, which is rapidly oxidized in its free form to hematin. p-Ethylphenol was polymerized using hematin as catalyst in an aqueous DMF (325). Iron-)VA -ethylenebis(salicyhdeneamine) (Fe-salen) also can be regarded as model complex of peroxidase. Fe-salen catalyzed an oxidative polymerization of various phenols such as 2,6-dimethylphenol, bisphenol A, cardanol, and urushiol analogues (251,293,326-329). The polymerization of 2,6-difluorophenol by Fe-salen produced a crystalline fluorinated PPO derivative (330). 

Fig. 10 shows the presence of grains in the sap film in the process of drying. These support the previous assumption that the sap film is composed of polymerized urushiol and plant gum domains. 

Considering these findings we here propose a cell structure for the durable Japanese lacquer film the lacquer film is composed of cells or grains of 0.1 ym size packed densely in the film. The cells have plant gum wall with polymerized urushiol inside, and are firmly... 

Scheme 2. Mechanism of Laccase-catalyzed polymerization of urushiol.

In this cell structure model, the plant gum provides a protective barrier towards diffusion of oxygen into the polymerized urushiol inside, resulting in and explaining the high durability of Japanese lacquer films. Since the plant gum is hygroscopic, absorption of humidity would break down the barrier characteristics, and thereby causes degradation of the polymerized urushiol in the lacquer film, but a combination with hydrohobic polymerized urushiol results in a cell structure wall of low humidity absorption. 

Polymerization of Urushlol 25 Since urushiol is a major component of the sap or lacquer, participation of urushiol, in the lacquermaking process from the sap or in the film formation process from the sap or lacquer is significant. 

By a series of studies of the enzymatic and non-enzymatic polymerization mechanism of urushiol. Scheme 2 has been established. Urushiol (1) is oxidized into the corresponding quinone (2)26 d formed quinone (2) undergoes C-C and C-0 coupling reaction with catechol nucleuse or the triene side chain urushiol, a major component of urushiol, giving dimeric urushiol (3), (4) and (5) 27-30 ... 

The catechol nucleus of these dimeric urushiol undergoes enzymatic oxidation into the corresponding quinones following the same type C-C and C-0 coupling reaction with urushiol or with each other. Thus urushiol grows in its polymerization up to of 20,000-30,000 in the lacquer-making process from sap. 

In addition,considering that the super surable properties of Japanese lacquer is dependent on the cell structure made up of urushiol ( a phenolic compound) and plant gum (polysaccharides), based on this model, this cell structure in theory, generally speaking, could be expanded to improve the durability of all synthetic polymeric materials in a combination with polysaccharides which are most aboundant, being made by cultivable plants utilizing the endless resources of sun rays, water and carbon dioxide. Much research is contemplated. 

Furthermore, new crosslinkable pol)T)henols based on model urushi were designed and synthesized by the oxidative polymerization of other urushiol analogues using Fe-salen, a model catalyst of peroxidase [22-24]. Such polyphenols were readily cmed to give crosslinked polymeric films with... 

The urushi lacquer has been used for more than 5000 years in China " and it is known as a highly durable material. Polymerization of urushiol, the major component of the lacquer, involves laccase-catalyzed dimerization and aerobic oxidative polymerization, " and the drying process takes a very long time. Several studies on shortening of this time have been carried out UV curing " " and hybridizing with other reactive polymers or monomers. " " Cardanol has a similar structure to urushiol, and the enzymatic oxidative polymerization of cardanol were reported by three research groups. " The development of the polymerization process leads to artificial urushi . 

The oriental lacquer is prepared from the sap of the varnish tree" Rhus verni-ciflua, which is an emulsion of an aqueous phase and an organic phase called urushi-ol. The urushi fraction consists of a mixture of catechol derivatives substituted in 3-po-sition with aliphatic C- 5 or C- 7 side chains. About 60% of these side chains are trie-nes. The structure of the trienes is similar to that in linseed or tung oil and is essential for the effectiveness of the oxidative curing of oriental lacquer. We have utilized new techniques and used combinations of modern techniques to analyze the urushiol mixtures and to characterize and identify each individual compound. We have also developed ultraviolet stabilizers for oriental lacquer, stabilizers that could be incorporated into the polymerizing mixture during the curing process to result in ultraviolet stabilized oriental lacquer. 

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