Transparency of Hydrophobic Vinyl Polymers Containing Small Amounts of Inorganic Chloride Salts

Termination of n-BuLi-initiated anionic polymerizations of vinyl monomers (i.e. styrene or butadiene) by chlorotrimethylsilae (CTMS) or methoxytrimethylsilane (MTMS) resulted in an opaque or transparent polymer product. Small amounts of salt (i.e. LiCl from CTMS- or Li(OCH₃) from MTMS-terminated polymerization) generated during termination stage was presumably the cause of the transparency difference. To verify this, turbid solutions were prepared by mixing transparent polymers (prepared from a H₂O-terminated anionic polymerization) with controlled amounts of LiCl in mixed tetrahydrofuran/cyclohexane (THF/CHE) solvents. Reduced viscosity (_red) and particle size (determined from light scattering) of these turbid solutions were found to increase with polymer and LiCl content. On the contrary, both _red and particle size decrease with increasing THF content of the mixed solvent. These results suggest that interactions between the LiCl/THF complex and the polymer chains are responsible for particle formation and hence turbid appearance of the solutions. Solution ⁷Li NMR spectra of the salt/polymer solutions indicated that polystyrene (PSt) has stronger interactions with LiCl as compared to polybutadiene (PBu), which resulted in a more homogeneously salt distribution in PSt than PBu as evaluated from the TEM micrographs of the corresponding bulk samples after solvent removal. A model with the included LiCl/THF complexes surrounded by the polymer coils was proposed to account for the resulting turbidity in solutions and in the bulk state.