Ruthenium tris(bipyridine)-centered linear and star-shaped polystyrenes: Making atom transfer radical polymerization and metal complex initiators compatible

The ligand derivative, 4,4'-bis(chloromethyl)-2,2'-bipyridine (bpy(CH2Cl)(2 )), and Ru(II) complexes with 2, 4, or 6 pendant halomethyl groups were emp loyed as initiators in the atom transfer radical polymerization (ATRP) of s tyrene to produce linear and star polymers with ligands and chromophores at discrete positions in the polymer architectures. With the metalloinitiator s, [Ru(bpy)(n)-{bpy(CH2Cl)(2)}(3-n)](PF6)(2) (n = 0, 1, 2), styrene polymer izations were run in bulk monomer, as well as in the presence of small amou nts of anisole (14% v/v vs styrene), employing either CuCl/2bpy(C13H27)(2) or CuBr/1,1,4,7,10,10-hexamethyltriethylenetetraamine (HMTETA) as the ATRP catalyst. Kinetics experiments were performed to determine the level of mol ecular weight control that is attainable in these polymerizations. With the former catalyst and when anisole is added, reactions exhibited increased c ontrol for the metalloinitiators and ligand initiators. Since the dicationi c metalloinitiators exhibited limited solubility, which correlated with poo r initiation, attempts were made to improve the compatibility of metallorea gents in the nonpolar ATRP medium. Di- and tetrafunctional metalloinitiator s modified with alkyl chains, [Ru{bpy(C13H27)(2)}(n){bPY(CH2Cl)(2)}(3-n)](P F6)(2) (n = 1, 2), displayed improved initiation and molecular weights clos er to targeted values. However, attempts to improve the solubility of the h omoleptic complex, [Ru{bpy(CH2Cl)(2)}(3)](PF6)(2) by substituting a BAr4'- counterion for PF6- did not enhance molecular. weight control. The use of D MF, a more polar solvent, in place of anisole did increase solubility of th e hexafunctional initiator; at low monomer conversion, polydispersities wer e lower in DMF vs anisole. Polymers were characterized by gel permeation ch romatography (GPC) with refractive index (RI) and multiangle laser light sc attering (MALLS) detection, by UV/vis spectroscopy to confirm the covalent attachment of Ru(II) chromophores to polystyrene chains, and by modulated d ifferential scanning calorimetry (MDSC).