Synonyms for Separatist (same or very similar meaning)
WordNet sense 1 (an advocate of secession or separation from a larger group (such as an established church or a national union)):
separationist
WordNet sense 2 (having separated or advocating separation from another entity or policy or attitude):
breakaway, fissiparous
WordNet sense 3 (free from external control and constraint):
independent
From the ODE community, based on WordNet
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