The research in my group involves the synthesis and characterization of vinyl polymers. Listed below are some of our current projects.
Synthesis of Fluorene End-labeled Polymers. The polymerization of vinyl-aromatic monomers off of chromophore-containing alkyl halide initiators using "controlled/living" radical polymerization techniques is a major thrust of our research. Atom transfer radical polymerization (ATRP), as an example, is used in our labs as an alternative, synthetically less demanding procedure than traditional anionic polymerization. By employing 9-bromoanthracene (a) and its derivatives as ATRP initiators, we have successfully produced polymers of a low polydispersity index (PDI) and possessing near quantitative flourene labeling (c).
Current projects include employing derivatives of 9-bromofluorene, such as 2,7,9-tribromofluorene, as ATRP initiators to allow for aryl halide coupling of the fluorene component, leading to polymer-grafted poly(fluorene). Research is underway to prevent radical-radical coupling of the initiating fluorenyl radical (b), by replacing the 9-H with a bulky alkyl group. Number average molecular weight of the chromophore labeled polymers using solely UV/Vis spectroscopy is also being studied.
b>Anthracene Photodimers as Initiators in ATRP. ATRP generally requires an alkyl halide initiator, with a chlorine or bromine atom bound to an sp3 hybridized carbon. Thus, to use a specific chromophore as an initiator, synthetic modification must be made to place an appropriate alkyl halide group as a substituent on the chromophore. Because anthracene and its derivatives are known to photodimerize through the 9-,10- positions, as shown below, 9-haloanthracenes are effectively converted from aryl halides into alkyl halides.
Photodimerization of 9-bromoanthracene
If 9-bromo or 9-chloroanthracene photodimers are used as ATRP initiators, polymers with a central anthracene photodimer are expected to be produced. Cleavage of this polymer-bound photodimer with light or heat will generate two equivalents of anthracene-bound polymer, as shown below.
ATRP Using Photodimers of 9-bromoanthracene as Initiators
However, inadvertant destruction of the anthracene photodimer, both prior to and during the polymerization, has prevented us from controlling the system. Current work involves temperature, light, and solvent effects to the polymerization and the anthracene content of the resulting polymers. Kinetic analysis allows to us to monitor the monomer conversion and polymer growth along with the regeneration of the anthracene chromophore.
Lewis Acid-Induced N-methyleneamines as Cationic Initiators. Trisubstituted hexahydro-1,3,5-triazines (1), in the presence of strong Lewis Acids, such as TiCl4, have been reported to form N-methyleneamines (existing as 2 or 3), as shown in below. These reactive electrophiles (which are unstable and difficult to isolate) are short lived intermediates susceptible to nucleophilic attack, and thus valuable in the synthesis of various amines.
Formation of N-methyleneamines
We predicted these may be suitable initators in the cationic polymerization of vinyl monomers. The schemes shown below summarize the predicted mechanism leading to the products observed thus far in our lab. By choosing a triazine lacking an aromatic substituent (Scheme 3, 7), we are able to prevent intramolecular electrophilic aromatic substitution and grow linear polymers (8). This project is in the initial stages and needs further work to fully characterize the system.
Eric S. Tillman