  Dynamic Nuclear Polarization (DNP) is to date the most effective technique to
increase the nuclear polarization up to a factor $100,000$ opening disruptive
perspectives for medical applications. In DNP, the nuclear spins are driven to
an - out of equilibrium - hyperpolarized state by microwave saturation of the
electron spins in interaction with them. Here we show that the electron dipolar
interactions compete with the local magnetic fields resulting in two distinct
dynamical phases: for strong interactions the electron spins equilibrate to an
extremely low effective temperature that boosts DNP efficiency. For weak
interaction this spin temperature is not defined and the polarization profile
has an 'hole burning' shape characteristic of the non interacting case. The
study of the many-body eigenstates reveals that these two phases are intimately
related to the problem of thermalization in closed quantum systems where
breaking of ergodicity is expected varying the strength of the interactions.
