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45427-AC7
Water as the Preferred Solvent for Lewis Acid-Catalyzed Free Radical Polymerization and Initiation of Spontaneous 'Charge-Transfer' Polymerization
Lewis acid-catalyzed free radical copolymerization LAFR of vinyl monomers, was been studied intensively during the 1960's and 1970's, as described in numerous review articles. Typically an electron-poor monomer such as acrylonitrile is complexed to a Lewis acid and an electron-rich comonomer such as styrene is added with stirring. A free radical initiator is often added and the mixture is heated to begin polymerization. The copolymers produced by this process generally are alternating 1:1 copolymers formed in moderate yields
Although much success was achieved in this area during the 1960's and 1970's, research languished in recent years. Limitations include the fact that the electron-rich comonomer must be a hydrocarbon-styrene, butadiene, isobutene. This is presumably because, if an electron-donating nitrogen- or oxygen-containing substituent were present in the electron-rich comonomer, it would complex preferentially to Lewis acid and become deactivated. Another limitation of the existing procedures is the lack of any reaction solvent. The reactions are carried out in bulk, leading to extremely viscous reaction mixtures which cannot be stirred after a certain point. Inhomogeneity and heat buildup are caused under these circumstances. A polar inert solvent would be highly desirable in overcoming the cited difficulties and reviving the field.
We have investigated two highly polar solvents for carrying out these copolymerizations: water and tetramethylenesulfone! Consider water first.
Highly concentrated solutions of zinc halides in water were prepared, for example 84 wt% ZnBr2, and acrylonitrile was added. Remarkably, introduction of equivalent quantities of even quite oleophilic comonomers such as styrene or vinyl benzoate still gave optically clear solutions.
The dissolution in sulfolane of zinc bromide was aided when acrylonitrile was added to form the complex. Lithium salts, such as the perchlorate, which we have used in earlier work, were also used.
Prior complexation of zinc bromide to acrylonitrile and chilling both the resulting solution of its complex as well as the electron-rich comonomer to about 0 ˚C averted adventitious cationic homo¬poly¬merization. Irgacure 819 photoinitiator was added, argon was bubbled through the solution to remove oxygen, and ultraviolet irradiation was begun.
The copolymerization results in these two very different solvents were remarkably similar. Very good results, namely high yields, high molecular weights, and rather broad monomodal MWD's were obtained with all styrenes tried except 4-t-butyl and 4-t-butoxystyrenes. Alternating copolymers were obtained. Interestingly, p-methoxystyrene, p-ethoxystyrene, and p-acetoxystyrene all copolymerized well with acrylonitrile, from which we conclude that competitive complexation of the substituents with the Lewis acid to deactivate them did not occur. Similarly good results were obtained for vinyl esters, namely the acetate, pivalate, and benzoate.
Turning to nitrogen-containing comonomers, again very good results were obtained with N-vinylamides, including -pyrrolidone, -formamide, -acetamide, and -N-methylacetamide. N-Vinylimidazole also copolymerized well, as did 2- and 4-vinyl¬pyrid¬ines.
In contrast, vinyl ethers did not give satisfactory copolymerization in either solvent. Perhaps the oxygen atom complexes preferentially to the Lewis acid, deactivating the vinyl ether.
In sulfolane, electrophilic lithium salts, particularly the perchlorate, were also effective in some of the monomer combinations we investigated. In water lithium perchlorate proved to be ineffective as the Lewis acid. Perhaps due to a strong interaction of lithium ion (hard acid) with water (hard base), so that it cannot complex the vinyl monomer.
NMR Complexation Studies. – Successful competitive complexation, of Lewis acid to the electron-poor monomer rather than the electron-rich monomer is crucial to the success of these copolymerizations. We have used the NMR shifts as an indication of the relative strength of complexation, without unwanted oligomerization. Deuterated sulfolane is not readily available, so deuteronitromethane was used in its place as a polar non-complexing solvent. We were also able to use zinc bromide and lithium perchlorate in D2O solution, without unwanted oligomerization.
Conclusions- Accordingly we have now found two highly polar, inert solvents useful for carrying out Lewis acid catalyzed free radical copolymerization of a wide variety of readily available monomers, and have overcome the difficulties listed by Sherrington.
