Mechanistic investigation of 9-bromoanthracene photodimers as initiators in atom transfer radical polymerization

Copolyacrylates with phenylalanine and anthracene entities prepared by ATRP and microwave irradiation

“…Evidence for the photochemical dimerization of 9-substituted anthracene from the above copolymer necessitated a subsequent investigation, in which irradiation of polymer solution up to the disappearance of the anthracene bands (after 70 min) was followed of its heating at 85 1C for 48 h. By this simple test, it seems that the photodimer suffers a cleavage reaction under temperature action and returns to the initially anthracene structure (about 65%), confirming, thus, the reversibility of such process (thermally induced or irradiated at 260 nm) in agreement with the literature data [58]. However, the photochemical process can be expressed by the first-order rate equation lnA 0 /A t ¼kt, where A 0 and A t are values of the absorbance at time t 0 and t, respectively, and k is the rate constant.…”

Effect of the Ligand in Atom Transfer Radical Polymerization Reactions Initiated by Photodimers of 9‐Bromoanthracene

“…The most effective system thus far has employed 9-BA photodimers as initiators, PMDETA as the ligand, and Cu(I)Cl as the catalyst; however, the M n values of polymers produced were approximately three times greater than anticipated, with PDI values typically between 1.6 and 1.8. [17] In all cases, cleavage of the photodimer occurred during the polymerization and quantitative anthracene-labeling resulted, as determined by UV-Vis spectroscopy.…”

“…[17,18] 9-BA (2.0 g, 7.78 mmol) was dissolved in 65 mL of benzene in a flask that was equipped with a condensing column and drying tube. The flask was irradiated with longwave UV light (Blak-Ray; Model B100 AP, l ¼ 365 nm), and the photodimerization was monitored using UV-Vis spectroscopy and was considered complete when the absorption peaks due to anthracene (between 350 and 400 nm, max at 370 nm) completely disappeared (%48 h).…”

“…Control over the system has not yet been achieved, with polymerizations failing to exhibit characteristics indicative of a controlled radical polymerization when N,N,N 0 ,N 00 ,N 00 -pentamethylethyleneamine (PMDETA) was used as the ligand in copper-catalyzed ATRP reactions. [17,18] The generation of polymers with much higher-than-expected number average molecular weight (M n ) values (two to four-fold higher, based on initial monomer-to-initiator ratios) and broad molecular weight distributions (%1.6-1.8) were consistently observed, while monitored polymerizations displayed a nonlinear relationship between M n values and percent monomer conversion. UV-Vis analyses of the polymeric products indicated thermolysis of the photodimer occurred over the course of the polymerization, generating polymers that were quantitatively labeled with anthracene (E) rather than containing polymer-bound photodimers (D).…”

“…Altering the order of addition (by either the ligand or photodimer used to commence the polymerization), adding copper (II) halide to alter the k act /k deact equilibrium, and the other variations were unable to overcome this. [17,18] Conversely, abandoning ATRP altogether and relying on silane radicals to abstract bridgehead bromine atoms, followed by mediation of the polymerization using TEMPO, lead to controlled polymerizations (PDI's % 1.3) but only partial anthracene labeling (%25%). [4] In this paper, we report an attempt to overcome the inherently slow initiation from the bridgehead position of the photodimer initiator by the use of the highly active ligands tris [2-(dimethylamino)ethyl]amine (Me 6 TREN) and tris[(2-pyridyl)methyl]amine (TPMA) to create more initiating sites at the onset of the reaction, thus increasing R i .…”