Characterization of phenol‐terminated polystyrenes by means of 13C‐NMR spectroscopy

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AbstractIn the presence of many alkylphenols, cationic polymerization of styrene by aluminium chloride leads to low‐molecular‐weight polystyrenes that contain end groups derived from the phenols. The fraction of phenolic end groups in the polymer is estimated by ultraviolet (UV) measurements to be 40–70% dependent on phenol and the reaction conditions. Phenol is incorporated into the polymer over the whole range of molecular weights (up to 7000). At high phenol concentrations, a significant proportion of the product consists of 1:1 and 1:2 phenol‐styrene adducts. The position of attack of the growing carbenium ion on the phenol can be determined by 13C‐NMR spectroscopy. With 2,6‐dialkylphenols, such as 2,6‐di‐tert‐butylphenol, the high field aromatic resonance near 119.7 ppm is shifted downfield by about 16 ppm when the phenol is incorporated into the polystyrene as an end group. This is interpreted as an exclusive attack of the growing carbenium ion on the 4‐position of the phenol. With 2,4‐dialkylphenols, such as 2‐tert‐butyl‐4‐methylphenol, a corresponding downfield shift shows that reaction occurs only at the 6‐position. The preferred site of attack for phenols such as 2‐alkylphenols which lack both ortho‐ and parasubstituents, is the 4‐position. With such phenols attack at the 6‐position is not excluded. Low‐molecular‐weight adducts contain 1‐methylbenzyl end groups and 13C‐spectra are consistent with their presence in the higher‐molecular‐weight polystyrenes.