The research in my group involves the synthesis and characterization of vinyl polymers. Listed below are some of our current projects.

Synthesis of Polymers Labeled with Aromatic Chromophores.

Fluorene and Fluorene Derivatives. The polymerization of vinyl-aromatic monomers off of chromophore-containing 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-bromofluorene as an ATRP initiator, we have successfully produced polymers of a low polydispersity index (PDI) and possessing near quantitative flourene labeling. The system, leading to fluorene-labeled polymers (boxed), is summarized below.

Scheme 1.

We are ultimately interested in applying this system to produce 2,7-dibromofluorene-labeled polymers for employment as macromonomers in polyfluorene synthesis. To this end, we have developed a variation of reverse ATRP (RATRP) that leads to polymers with high amounts of end-labeling. The radical initiator is decomposed and incubated in the presence of a chromophore label containing one or more radically transferable hydrogen atoms, prior to the addition of monomer. 

Anthracene Photodimers as Radical Initiators. Photodimers of 9-bromoanthracene are being studied as alkyl halide initiators in atom transfer radical polymerizations (ATRP) leading to anthracene-labeled polymers. Converting the aryl bromide on the anthracene moiety into an alkyl bromide via a [4 + 4] cycloaddition reaction effectively generates the photodimer with two alkyl halide sites, capable of initiating styrene in ATRP reactions. Heat-induced cleavage of the photodimer is occurring during the polymerization, with the resulting polymers showing absorbance bands indicative of an anthracene-bound polymer. Plots of monomer conversion as a function of Mn displayed non-linearity, consistent with slow initiation from the bridgehead halide on the anthracene photodimer and cleavage of the centrally-bound chromophore during the reaction. We have thoroughly studied the mechanistic details of the reaction scheme, shown below, in hopes of gaining better control over the system. Current research involves employing monomers with increased rates of polymerization, allowing the reaction to be performed at lower temperatures. New areas to be explored include other aromatic systems capable of photodimerization, such as coumarin or tetracene.

Tillman Group publications in these areas:

Tillman, E.S.; Miller, D.J.; Roof, A.C. "Photodimers of 9-Haloanthracenes as Initiators in Atom Transfer Radical Polymerization: Effect of the Bridgehead Halogen" Polymer Bulletin 2007, 58, 881-891.

Roof, A.C.; Tillman, E.S.; Malik, R.E.; Roland, A.M.; Miller, D.J.; Sarry, L.R. "Mechanistic Investigation of 9-Bromoanthracene Photodimers as Initiators in Atom Transfer Radical Polymerization" Polymer 2006, 24, 3325-3335.

Tillman, E.S.; Roof, A.C.; Palmer, S.M.; Zarko, B.A.; Goodman, C.C.; Roland, A.M. "Synthesis of Chromophore-labeled Polymers and Their Molecular Weight Determination Using UV/Vis Spectroscopy" J. Chem. Educ. 2006, 83, 1215-1218.

Ludwig, B.; Tillman, E.S. "Polystyrene End-labeled with 2,7-Dibromofluorene Synthesized Using an Adaptation of Reverse Atom Transfer Radical Polymerization" 2005, Macromol. Chem. Phys., 206(21), 2143-2152.

Ludwig, B.; Tillman, E.S. "Synthesis of 2,7-Dibromofluorene-labeled Macromonomers Utilizing a Variation of Reverse Atom Transfer Radical Polymerization" ACS Div. Polym. Sci., Polym. Prepr. 2005, 46(2), 1047-1048.

Goodman, C.C.; Roof, A.C.; Tillman, E.S.; Ludwig, B.; Chon, D.; Weigley, M.I. "Synthesis and Characterization of Fluorene End-Labeled Polymers Using Atom Transfer Radical Polymerization" J. Polym. Sci., Part A, Polym. Chem. 2005, 43(12), 2657-2665..

Goodman, C.C.; Chon, D. Tillman, E.S. "9-Bromofluorene as an Initiator in Controlled Radical Polymerizations" ACS Div. Polym. Sci, Polym. Prepr.  2004, 45(1), 1012.

Roof, A.C.; Bayne, L.J.; Tillman, E.S. "Use of 9-Bromoanthracene Photodimers in the Atom Transfer Radical Polymerization of Styrene" ACS Div. Polym. Sci, Polym. Prepr.  2004, 45(2), 675-676.

Lewis Acid-Induced N-methyleneamines as Cationic Initiators.

Trisubstituted hexahydro-1,3,5-triazines, in the presence of strong Lewis Acids, such as TiCl4, have been reported to form N-methyleneamines. 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.

We predicted these would be suitable initiators in the cationic polymerization of vinyl monomers. By choosing a triazine lacking an aromatic substituent (R = alkyl), we are able to prevent intramolecular electrophilic aromatic substitution and grow linear polymers. Shown below is an example of such a system, which is currently being utilized in our lab (R = methyl, R' = aryl, alkoxy). We are currently studying ways to quantitate the amine end-group using fluorescent labeling, in addition to studying the initiation system further.

Tillman Group publications in this area:

Cohen, N.A.; Tillman, E.S. "Controlling Polymerizations Initiated by N-methyleneamines: Effect of Solvent, Temperature, and Additive" Polymer Preprints 2007, 43(1), 379-380.

McKnight, J.N.; Tillman, E.S.; Sarry, L.R."Lewis Acid Induced N-methyleneamines as Cationic Initiators Leading to Amine-Terminated Polymers" Macromolecular Rapid Communications 2006, 27, 1578-1583.

Silane Radical Atom Abstraction.

The synthesis of bromine-terminated polymers is easily accomplished using ATRP methods. Synthetic methodology that activates these chain termini for further reactions (chain elongation, coupling, synthesis of block copolymers, etc...) but does not rely on metal catalysis is of great interest. To this end, we have developed silane radical atom abstraction (SRAA), which exploits the bromophilicity of silane radicals towards bromine. This creates polymer radicals that are capable of coupling or adding new monomer (chain elongation) when coupled with nitroxide mediated polymerization. The reaction scheme is outlined below.

Tillman Group publications in this area:

Thakur, S.; Tillman, E.S. "Efficient Metal-free Coupling of Polystyrene Chains Using Silane Radical Atom Abstraction" Journal of Polymer Science, Part A: Polymer Chemistry 2007, 45, 3488-3493.