  The low degradability of common polymers composed of light elements, results
in a serious impact on the environment, which has become an urgent problem to
be solved. As the reverse process of monomer polymerization, what deviates
degradation from the idealized sequential depolymerization process, thereby
bringing strange degradation products or even hindering further degradation?
This is a key issue at the atomic level that must be addressed. Herein, we
reveal that hydrogen atom transfer (HAT) during degradation, which is usually
attributed to the thermal effect, unexpectedly exhibits a strong
high-temperature tunnelling effect. This gives a possible answer to the above
question. High-precision first-principles calculations show that, in various
possible HAT pathways, lower energy barrier and stronger tunnelling effect make
the HAT reaction related to the active end of the polymer occur more easily. In
particular, although the energy barrier of the HAT reaction is only of 0.01
magnitude different from depolymerization, the tunnelling probability of the
former can be 14~32 orders of magnitude greater than that of the latter.
Furthermore, chain scission following HAT will lead to a variety of products
other than monomers. Our work highlights that quantum tunnelling may be an
important source of uncertainty in degradation and will provide a direction for
regulating the polymer degradation process.
