The Design, Synthesis, and Controlled Polymerization of Cationic and Zwitterionic Norbornene Derivatives
Ring opening metathesis polymerization (ROMP) has been exploited for the controlled polymerization of cationic and zwitterionic norbornene-based monomers and employed in the preparation of homo- and block-copolymer systems in homogeneous organic media without the use of post polymerization modification or protecting group chemistries. Relying on previous knowledge of certain halogenated alcoholic organic solvents capable of solubilizing hydrophilic monomers, the first study, describes the synthesis and controlled polymerization of a series of new permanently cationic ammonium exo -7-oxanorbornene derivatives M31 via ROMP, with the first generation Grubbs catalyst 17 , in a novel solvent mixture comprised of 1:1 vol/vol 2,2,2-trifluoroethanol (TFE)/methylene chloride (CH2 Cl2 ). This cosolvent mixture was demonstrated to be a convenient reaction medium facilitating the polymerization of hydrophilic substrates by hydrophobic initiators under homogeneous conditions. Homo- and copolymerizations proceed rapidly yielding materials with controlled molecular masses, and narrow molecular mass distributions. It was demonstrated that this protocol is not limited to the use of TFE as a cosolvent and that additional halogenated alcohols, such as 2,2,2-trichloroethanol (TCE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), are also effective cosolvents for the controlled polymerization of such substrates. Finally, we demonstrate that the TFE/CH 2 Cl 2 mixture has no apparent detrimental effect on 17 . The second study describes results relating to the effect of halide counterion on the ROMP of a permanently cationic exo -7-oxanorbornene derivative whose synthesis we described recently in the presence of the 17 . Statistical copolymerizations of exo -benzyl-[2-(3,5-dioxo-10-oxa-4-aza-tricyclo[5.2.1.02,6 ]dec-8-en-4-yl)-ethyl]dimethyl ammonium bromide/chloride were conducted at molar ratios of 25:75, 50:50, and 75:25, and the polymerizations evaluated with respect to their kinetic features as well as their molecular mass profiles as a function of conversion and the ability to produce materials with narrow molecular mass distributions. Direct comparison of the statistical copolymerizations with the corresponding bromide/chloride homopolymerizations indicates that their polymerization characteristics are intermediate of that observed for the homopolymerizations. In all instances the copolymerizations appear controlled. The clearest effect is on the measured polydispersity index which in all instances coincides with that of the bromide homopolymerization and indicates a positive, beneficial effect even with only 25 mol% bromide comonomer. The polymerization characteristics are rationalized in terms of the in situ formation of the mixed Grubbs' derivative RuClBr(PCy 3 )2 CHPh and/or the dibromo analog RuBr2 (PCy3 )2 CHPh formed by halide exchange with the bromide counterions in exo -benzyl-[2-(3,5-dioxo-10-oxa-4-aza-tricyclo[126.96.36.199 2,6 ]dec-8-en-4-yl)-ethyl]dimethyl ammonium bromide MON -Bn-Cl. The third study describes the synthesis and controlled ROMP of highly functional zwitterionic sulfopropylbetaine- M32 and carboxyethylbetaine-exo -7-oxanorbornene derivatives M33 with the first generation Grubbs' initiator 17 in a TFE/CH 2 Cl2 solvent mixture. These are the first examples of such norbornene-based betaine substrates. Both species can be polymerized directly in a controlled manner in organic media as judged from the kinetic profiles and aqueous size exclusion chromatographic analysis. This represents the first time betaine monomers have been polymerized directly in a controlled fashion by a technique other than a controlled free radical polymerization process, and the first time it has have been achieved in organic, as opposed to aqueous, media. Finally, preliminary results demonstrate that water-soluble, salt-responsive AB diblock copolymers can be prepared and that such materials are able to undergo supramolecular self-assembly in aqueous media to yield nano-sized aggregates simply by controlling the aqueous electrolyte concentration.