Can the Hock Process Be Used to Produce Phenol from Polystyrene?

Polystyrene (PS) is a widely used thermoplastic polymer,
but its
very low recycling rate has motivated consideration of chemical conversion
strategies to convert waste PS into value-added products. Oxidation
methods have been widely studied, but they typically generate benzoic
acid, a product with a relatively low market demand. Phenol is a higher
volume chemical that would be an appealing target, but no methods
currently exist for the conversion of PS into phenol. The repeat unit
in PS closely resembles cumene, the primary feedstock used to produce
phenol through the Hock process. Here, we investigate prospects for
adapting the Hock process to PS, generating hydroperoxides through
the autoxidation of benzylic C–H bonds followed by the acid-promoted
rearrangement of the hydroperoxides to afford phenol and a partially
oxygenated polymer. Experimental and computational studies of dimeric
and trimeric PS model compounds show that neighboring phenyl rings
impose conformational constraints that raise the barrier to hydrogen-atom
transfer from the tertiary benzylic C–H bond. These effects
are also evident with PS and contribute to lower yields of phenol
when PS is subjected to Hock process conditions. These results provide
valuable insights that have important implications for other efforts
that seek to adapt small-molecule reactivity to polymeric feedstocks.