Gilch route

The polymerization of l,4-bis(halomethyl)benzenes to PPVs in the presence of a large excess of potassium f-butoxide is referred as the Gilch route [81]. The method was first described for the synthesis of unsubstituted PPV 60, but -unfortunately - this route produces the PPV as an intractable, insoluble powder. However, the adaptation of the Gilch route to the polymerization of l,4-bis(halo-methyl)benzenes possessing solubilizing side groups gives access to soluble PPV materials. 

SCHEME 2.4 General synthetic route or Gilch route to solution-processable PPV derivatives (From Gilch, H.G. and Wheelwright, W.L., J. Polym. Sci. Part A, 4, 1337, 1966.)... 

The substitution of the aromatic backbone ring by alkylphenyl groups is helpful to get soluble PPX types. The polymer synthesis using these monomers can proceed in a THF, dioxane, or benzene solution. The polymer is formed by a dehydrochlorination reaction using tert-butyl oxide. The synthesis is shown in Figure 2.4. The dehydrohalogenation reaction is also referred to as the Gilch route. 

Poly(p-xylylene) polymer and copolymers will become thermally crosslinkable, if they contain acyclohexenyl moiety [50]. Suchpolymers can be prepared via the Gilch route [51] as shown in Figure 2.7. 

PPV by the Gilch route has been discussed. Here, in situ-generated p-quinodimethanes are shown to be the active monomers. 

Other Routes. Substituted PPV derivatives have also been prepared by several other routes. In cases where the resulting polymers are soluble, these methods are often successful in preparing high molecular weight material. The Gilch route has been used to prepare phenyl and crown ether substituted polymers... 

Fig. 13.5 Typical monomers used in the Gilch route to PPV derivatives. R is a solubilizing group, and X is Cl or Br.

A potentially very useful variant of the Gilch route to PPV uses a sulfone or sulfoxide group (i.e., X2. Scheme 13) in place of one of the halide atoms [110]. The resulting precursor polymers are soluble in organic solvents such as chloroform and can be thermally converted at low temperatures. For instance, the phenylsul-fenyl-substituted precursor (X2 = SOPh) apparently undergoes conversion at temperatures of 100°C, although higher temperatures may be needed to volatilize the eliminated sulfur compounds from the film. 

Figure 24 Molecular weight distributions of MEH-PPV prepared by microwave-assisted ROMP (3) and that prepared by the Gilch route (5) using calculated Mark-Houwink parameters. Reproduced with permission from Spring, A. M. Yu, C.-Y. Horie, M. Turner, M. L. Chem. Commun. 2009, 2676. ...

A common feature of these three methods is the polymerization of 1,4-xylylene intermediates to 1,4-phenyleneethylenes which are then subjected to thermal 1,2-elimination reactions of SR2/HCI [29], HCl [30, 32] or RSOH [31], The Gilch route proved to be very valuable for technical processes. 

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