Chemical recycling of polybutadiene rubber with tunable thermal depolymerization enabled by microencapsulated metathesis catalysts

The effective management of plastic waste streams to prevent plastic land and water
pollution is a growing problem that is also one of the most important technological, political,
and economical challenges facing plastic materials today. To achieve this, it is desirable to
manufacture polymer materials that are both stable over their intended lifetime and easy to
cheaply recycle or repurpose them as desired. However, achieving both satisfactory stability
and recyclability is difficult for many commodity plastics. It is especially difficult to conceive this
with crosslinked polymers such as rubbers because they cannot be molten and reprocessed in
that manner. In this work, we explore the possibility for the in-situ depolymerization and
reprocessing of polybutadiene (PB) rubber using microencapsulated Grubbs’ catalysts. Second-
generation Hoveyda-Grubbs catalyst (HG2) contained within glassy thermoplastic microspheres
can be dispersed in PB rubber below the microsphere’s glass transition temperature (Tg)
without adverse depolymerization, evidenced by rubber with and without these microspheres
obtaining a somewhat similar shear storage modulus of ≈16 and ≈28 kPa respectively. The
thermoplastic’s Tg can be used to tune the depolymerization temperature via release of HG2
into the rubber matrix. For example, using poly(lactic acid) vs polysulfone results in a
depolymerization temperature of 85°C and 162°C respectively. Liquefaction of rubber to a
mixture of small molecules and oligomers is demonstrated using only a 0.01 mol% catalyst
loading using poly(lactic acid) as the encapsulant. At that same catalyst loading,
depolymerization occurs to a greater extent in comparison to two ex-situ approaches, including
a conventional solvent-assisted method, where it occurs at roughly twice the extent at each
given catalyst loading. In addition, depolymerization of the microsphere-loaded rubbers was
demonstrated for samples stored under nitrogen for 23 days. Lastly, we show that the
depolymerized products can be reprocessed back into solid rubber with similar mechanical
propertied. Henceforth, we envision that this approach could be used to recycle and reuse
crosslinked rubbers at the end of their product lifetime and divert it away from plastic waste
streams.