Poly(phenylene ethynylene) Conjugated Polyelectrolytes Synthesized via Chain-Growth Polymerization

This study examines the use of Pd(0)-catalyzed chain-growth Sonogashira coupling to prepare a series of poly(phenylene ethynylene) (PPE)-type conjugated polyelectrolytes that feature alkyl sulfonate (-R-SO3-)-solubilizing groups. The polymerization used an AB-type monomer, I-Ar-C equivalent to ECH, with pendant sulfonate ester units (-O-CH2CH2CH2 - SO3R), wherein the ester serves as a protecting group for the sulfonate groups. The conjugated polyelectrolytes were produced by hydrolysis of the ester-protecting groups post polymerization. As a point of reference, PPE-type polymers with nominally the same structures were prepared by step-growth polymerization of the AB monomer under conventional Sonagashira conditions, as well as via AA + BB polymerization of disubstituted monomers I-Ar-I and HC equivalent to C-Ar-C equivalent to CH. The ester-protected polymers were characterized by gel permeation chromatography in tetrahydrofuran (THF) solution. Samples prepared by chain growth have M-n values ranging from 4.6 to 7.5 kDa and have comparatively low polydispersity indices, (D) over bar similar to 1.1-1.2. The step-growth have M-n values ranging from 4.6 to 7.5 kDa and have comparatively low polydispersity indices, (D) over bar similar to 1.1-1.2. The step-growth polymers have M-n ranging from 11.3 to 13.5 kD, with higher dispersity, (D) over bar similar to 1.5-2.3. The photophysical properties of the samples were compared, as both the ester-protected forms (in THF solution) and the polyelectrolyte forms (in MeOH and water). In general, the polymers prepared by chain growth have higher fluorescence quantum yields and better-resolved spectra, suggesting that the chains are comparatively defect-free and do not aggregate in solution.