Dehydrogenative polymerization

According to a widely accepted concept, lignin [8068-00-6] may be defined as an amorphous, polyphenoHc material arising from enzymatic dehydrogenative polymerization of three phenylpropanoid monomers, namely, coniferyl alcohol [485-35-5] (2), sinapyl alcohol [537-35-7] (3), and /)-coumaryl alcohol (1). 

The discussion about the mechanism of the dehydrogenative polymerization reaction has not yet been completed. However, the reaction mechanism seems to be strongly influenced by the specific random conditions that apply for each particular system. Presumably with late transition metals a silylene mechanism is more appropriate. It may be a matter of the steric constraints of the system to shift the reaction towards a-bond metathesis. 

Tanahashi M. Takeuchi H. Higuchi T. Dehydrogenative polymerization of 3,5-disubstituted p-coumaryl alcohols. Wood Res. 1976, 61, 44—53. 

Sasaki, S. Nishida, T. Tsutsumi, Y. Kondo, R. Lignin dehydrogenative polymerization mechanism a poplar cell wall peroxidase directly oxidizes polymer lignin and produces in vitro dehydrogenative polymer rich in P-O-4 linkage. FEBS Lett. 2004, 562, 197-201. 

Figure 1. Removal of 3H at position 5 of the guaiacyl ring of coniferyl alcohol (I) by formation of ring substituted structures (V, VI, VII) during dehydrogenative polymerization.

Figure 2. Dehydrogenative polymerization of a mixture of p-coumaryl alcohol-[ring-2-3H] and coniferyl alcohol-[U-14C], and nitrobenzene oxidation of the DHP to give p-hydroxybenzaldehyde-[ring-2-3H] and vanillin-[formyl-14C].

The double-labeling technique is also useful for in vitro studies on the mechanism of dehydrogenative polymerization of monolignols. 

At this point, it must be emphasized that, based on previous structural analysis of isolated lignins (27), appropriate lignin model compounds and synthetic dehydrogenatively polymerized (DHP) lignin preparations (28-... 

Figure 1. Formation of guaiacyl lignin and lignin-carbohydrate complexes (LCC) via dehydrogenative polymerization of coniferyl alcohol.

Dehydrogenative Coupling. Transition-metal catalyzed polymerization of silanes appears to hold promise as a viable route to polysilanes. A number of transition-metal complexes have been investigated, with titanium and zirconium complexes being the most promising (105—108). Only primary silanes are active toward polymerization, and molecular weights are rather low. The dehydrogenative polymerization is depicted in reaction 11, where Cp = cyclopentadienyl ... 

Besides the cr-bond metathesis mechanism proposed by Tilley23 for the dehydrogenative coupling of silanes, a Zr(II) pathway25 and a silylene mechanism26 have been proposed based on the nature of the products. The dehydrogenative polymerization of 1,2,3-trimethyltrisilane or of a mixture of diastereomers of 1,2,3,4-tetramethyltetrasilane showed evidence that, besides Tilley s mechanism, a further mechanism is present. The product formation can be explained by a silylene mechanism where the silylenes are formed by a-elimination from the silyl complexes by a new type of /J-elimination which involves Si—Si bond cleavage (/F-bond elimination) as described in Scheme 727. 

Guan, S. Y., Mylnar, J., and Sarkanen, S., 1997, Dehydrogenative polymerization of coniferyl alcohol on macromolecular lignin templates,... 

Tilley, T. D. Woo, H.-G. Catalytic Dehydrogenative Polymerization of Silanes to Polysilanes by Zirconocene and Hafnocene Catalysts. A New polymerization Mechanism, (H. F. Harrod, R. M. Laine, Eds.) Inorganic and Organometallic Oligomers and Polymers, Kluwer Academic Publishers, Netherlands, 1991, p. 3. 

Research on the optimization of reaction conditions and the elucidation of reaction mechanisms for the dehydrogenative polymerization of secondary stannanes is rather limited. In this context, Tilley and coworkers proposed a chain-grow mechanism82 (Scheme 17), which is similar to that proposed for the dehydropolymerization of organosilanes62,63,105. 

Yamaguchi H, Maeda Y, Sakata I (1992) Applications of phenol dehydrogenative polymerization by laccase to bonding among woody-fibers. Mokuzai Gakkaishi 38 931-937... 

Yamaguchi H, Nagamori N, Sakata I (1991) Application of the dehydrogenative polymerization of vanillic acid to bonding of woody fibers. Mokuzai Gakkaishi 37 220-226... 

The sequence of events in coke formation was studies in the model reaction [70] of H-Y zeolite with propene at 723 K. Under these drastic conditions the soluble white coke formed rapidly within 20 min and was converted into insoluble coke within 6h under inert gas without loosing carbon atoms in the deposit. Due to the larger pores in the Y-zcolites compared to the ZSM type zeolites used in the other studies mentioned so far, the structure of the aromatic molecules is somewhat different. The soluble coke in this system consisted of alkyl cyclopentapyrenes (C H2 -26. Type A) as the hydrogen-rich primary product and of alkyl benzoperylenes (C H2n- 2. Type B) and alkyl coro-nenes (CrtH2 -36. Type C) as matured components. The temporal evolution of the various products is presented in Fig. 14. It clearly emerges that the soluble coke fractions are precursors for the insoluble coke and that within the soluble coke fraction the final steps of dehydrogenation-polymerization are very slow compared to the initial formation of smaller aromatic molecules from propene. The sequential formation of precursors with decreasing C H ratio follows from the shift of the maximum in the abundance of each fraction on the time axis. 

II), In the carbonization of anthracene (III), molecular size increases by dehydrogenative polymerization without molecular rearrangements to form dibenzoperylene (IV) (46, 47),... 

Heat Treatment Temperature and Soak Time. A study of Mochida and Marsh (53) indicates, unlike classical kinetics, that time and temperature for mesophase formation are not interdependent. The reason for this is the controlling influence of viscosity (not found for reactions in the gas or solution phase). Maximum size of optical texture and coalescence results if the mesophase is formed under conditions which provide a minimum viscosity as quickly as possible. Probably, rate controlling processes for mesophase growth are not the dehydrogenated polymerization reactions. Therefore, the attainment, relatively quickly, of temperature of 400°C has provided the necessary size of molecule and consequently the resultant mesophase sbbws minimum viscosity because it is at a high temperature ( 400°C). Mesophase formed at lower (relative) temperatures can have a higher viscosity and coalescence behaviour can be restricted. 

Polynuclear hydrocarbons, e.g, anthracene, form mesophase by dehydrogenative polymerization reactions. Functional groups and heteroatoms hinder mesophase growth. 

This product may be derived from a glyceraldehyde-2-aryl ether unit formed via a displacement reaction in the dehydrogenative polymerization of coniferyl alcohol (Lundquist et al 1967)... 

Variable-temperature Raman spectroscopy has been used to explore the structure of polygermanes." Ultraviolet photoelectron spectroscopy has been employed to characterize the Ge-Ge backbone bonding." " Additional, Ge NMR spectra have been taken of key precursors for dehydrogenative polymerization, ArGeH3, Ar2GeH2, and Ar3GeH." ... 

The dehydrogenating polymerization of hydrosilanes was found to be catalyzed by Group 4 metallocene alkyl and sdyl derivatives (equation 26). For zirconocene and hafhocene catalysts, the reaction was found to proceed via a-bond metathesis steps. ... 

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