Anionic Synthesis of Block Copolymers for Photonics Applications

Anionic synthesis of well-defined polystyrene-block-polyvinylpyridine copolymers required the use of special conditions including lithium chloride and 1,1-diphenylethylene as additives, low temperature of reaction (-78 °C), highly diluted monomer at -78 °C and efficient stirring (Morton-type, creased reactor). Low molecular weight polystyrene-block-poly(2-vinylpyridine) copolymers (M_n = 6000 g/mol) were synthesized with average-molecular weights in agreement with the theoretically calculated M_ns and narrow M_w/M_ns (≤ 1.1). Polystyrene-block-polyvinylpyridine copolymers were selected for the fabrication of uniformly dispersed metal oxide nanoparticles (cobalt and iron oxides) due to the coordinating ligand character of the vinylpyridine units. The incorporation of the inorganic salts (1 mol-eq of inorg. salt per mol of vinylpyridine units) was 57 wt% when polystyrene-block-poly(2-vinylpyridine-co-4-vinylpyridine) (M_n= 59,000 g/mol, M_w/M_n = 1.09, f_{v PVP} = 0.19) was used and 18 wt% when polystyrene-block-poly(2-vinylpyridine) (M_n= 39,000 g/mol, M_w/M_n = 1.07, f_{v PVP} = 0.14) was used.

The end-capping reaction of polymeric chain-ends with 1,1-diphenylethylene (DPE) was studied using 2D NMR spectroscopic and MALDI-TOF mass spectrometric analyses. Oligomerization of DPE was observed using a 15-fold excess of DPE in the end-capping of poly(butadienyl)lithium (M_n = 2,200 g/mol, M_w/M_n = 1.06) but not in the case of poly(styryl)lithium (M_n = 2,000 g/mol, M_w/M_n = 1.02). Although oligomerization of DPE has been previously reported in the synthesis of 1,1-diphenylhexyllithium (6-11% oligomer with 5.4-fold excess of DPE), there are no studies showing the presence of DPE oligomer in the end-capping reaction of polymeric living carbanions.

Additionally, the synthesis of poly(para-phenylene) has been studied using different precursor polymers [poly(1,3-cyclohexadienes) (M_n = 1,600 and 3,100 g/mol, M_w/M_n = 1.1 and 1.03) and poly(2-phenyl-1,3-cyclohexadiene) (M_n = 10,000 g/mol, M_w/M_n = 1.27) homopolymers] and dehydrogenating agents [2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil)] (4 mol-eq of benzoquinone per mol of cyclohexadiene unit). The results indicate that the efficiency of the dehydrogenation reaction was limited by precipitation of the polymer in the case of poly(1,3-cyclohexadiene). The advantage of using poly(2-phenyl-1,3-cyclohexadiene) lies in the formation of a soluble poly(2-phenyl-para-phenylene) with thermal stability and mechanical properties similar to those of the insoluble poly(para-phenylene) (TGA _res [1000 °C] = 40.2%).