Diphenylethylene Monomer Synthesis

Commercially, the best way to prepare 1,1-DPE is probably to react styrene and benzene with one another and then to dehydrogenate the resulting 1,1-diphenylethane to 1,1-diphenylethylene. This has been developed to the pilot plant stage in BASF [7]. 

On a laboratory scale, 1,1-diphenylethylene may be prepared using the Grignard reaction between phenylmagnesium bromide and acetophenone [6]. 

Owing to the stability of the DPE radical, S/DPE polymers cannot be prepared using free radical polymerization, but they can be easily produced using the anionic polymerization technique. The copolymers are, however, limited to a maximum DPE content of 50 mol% because two consecutive DPE units are not possible in the polymer chain for steric reasons. This leads to a reactivity ratio rDPEC dd/ ds) = 0. 

The polymer can be produced in either cyclohexane or ethylbenzene using butyllithium, preferably sec-butyllithium. Since the addition of a styrene molecule to a DPE chain end is slow (decrease of resonance stability) and the addition of a DPE monomer to a growing styryl chain is fast (increased resonance stability), the polymerization rate decreases with increasing DPE content in the polymerizing monomer mixture. The reactivity ratio rs(K s/Ksd) in cyclohexane was found to be between 0.44 (50 °C) and 0.72 (70 °C). 

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Anionic initiator, ec-Butyllithium 1.3 M solution in cyclohexane (Aldrich) was used as obtained. Free-radical initiator, 1,1 -Azobiso(cyclohexane carbonitrile)(ACHN, Aldrich) was purified by recrystallization in methanol. l,r-Diphenylethylene (DPE, Aldrich) was purified by two consecutive distillations over CaH2 and n-butyl-lithium. The solvent, tetrahydrofuran (THF, Aldrich) used in anionic polymerization was dried by distillation over sodium wire in the presence of benzoquinone until a blue color developed and remained, and by consecutive vacuum distillations over LiAlH4 and n-butyllithium. Toluene used for fi e-radical polymerization was purified by distillation over CaH2 in a vacuum apparatus. All other chemicals used in monomer synthesis were used as purchased. 

This strategy, based on 1,1-diphenylethylenes, which are non-homopolymeri-zable monomers, has been developed mainly by the Quirk [174] and Hi-rao [178] groups. DPEs continue to find applications for the synthesis of the //-stars. A few recent examples are given below. 

In a process related to RAFT, BASF workers have shown that 1,1-diphenylethylene will control the molecular weight of PMMA and polystyrene, and permit block polymer synthesis [92]. They propose that radical chain ends add to the diphenylethylene to form a stable diphenylalkyl radical that does not add more monomer but can reverse to diphenylethylene and the same radical chain end for addition of more monomer. The diphenylalkyl radical cap has the additional possibility of forming a reversible dimer (Scheme 35). 

To overcome the difficulty in the crossover step a general methodology has been developed in our laboratory for the synthesis of block copolymers when the second monomer is more reactive than the first one. It involves the intermediate capping reaction with non-(homo)polymerizable monomers such as i) 1,1-diphenylethylene (DPE) and its derivatives and ii) 2-substitut-ed furans. 

Simpler procedures are of course available for the preparation and characterisation of carbenium ions in solution, particularly for the more stable ones. Concentrated sulphuric acid was extensively used as protogenic medium before the superacid mixtures were shown to be superior, but many of the spectroscopic assignements in those earlier studies were later proved erroneous, particularly in the case of such reactive entities as the 1-phenylethylium ion Model monomers which cannot polymerise because of steric hindrance can generate fairly stable carbenium ions by interacting with Lewis or Br nsted acids in normal cationic polymerisation conditions. Thus, 1,1-diphenylethylene and its dimer, and 1,1-diphenylpropene give rise to typical visible absorption bands from which the concentration of the corresponding diphenyl-methylium ions can be accurately calculated. As for carbenium ions capable of forming stable salts, their synthesis and characterisation is obviously easy. 

Living polymerization of azo monomers is one of the most effective ways to prepare well-defined azo BCs. Generally, a monodispersed macroinitiator should be prepared first. It is then used as an initiator for the subsequent polymerization of azo monomers. Finkelmann and Bohnert (1994) first reported the synthesis of LC-side chain AB azo BCs by direct anionic polymerization of an azo monomer. As shown in Scheme 12.1, the polymerization of polystyrene (PS)-based diblock copolymers was carried out from a PS-lithium capped with 1,1-diphenylethylene (DPE), whereas the poly(methyl methacrylate) (PMMA)-based diblock copolymers were prepared by addition of methyl methacrylate (MMA) monomers to the living azo polyanion, obtained by reaction of l,l-diphenyl-3-methylpentylithium (DPPL) with the azo monomer in tetrahydrofuran (THF) at lower temperature. By this method, a series of well-defined azo BCs were obtained with controlled molecular weights and narrow polydispersities (Lehmann et al., 2000). 

The synthesis of the poly[styrene-6-(hydroxystyrene-g-ethylene oxide)-6-styrene] (25), P[S-6-(HS-g-EO)-6-PS], has been reported. The backbone was a triblock copolymer, poly(styrene-6-t-butoxystyrene-6-styrene) (8), prepared by anionic polymerization by sequential addition of monomers. The protected t-butyl group was removed by treatment with HBr leading to the formation of P(S-6-HS-b-S) triblocks (9) (eq. 11). The metallation of the hydroxyl groups was performed in THF using either cumyl potassium or diphenylethylene potassium (eq. 12). The addition of EO generated the block graft copolymers (eq. 13). 

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