The Collaborative International Dictionary of English v.0.48:

Unchain \Un*chain"\, v. t. [1st pref. un- + chain.]
   To free from chains or slavery; to let loose. --Prior.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unchain
    v 1: remove the chains from [ant: chain]
    2: make free

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "unchain":
   cast off, detach, discharge, disengage, doff, emancipate, free,
   liberate, loose, loosen, manumit, release, remove, take off, unbar,
   unbind, unbolt, unbridle, unbuckle, unbutton, uncage, unclasp,
   undo, unfasten, unfetter, ungag, unglue, unhandcuff, unhobble,
   unlace, unlash, unlatch, unleash, unlock, unloose, unloosen,
   unmanacle, unmuzzle, unpen, unpinion, unshackle, unstick, unstrap,
   untether, untie, untruss, unyoke, weigh anchor