B737-800
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
ATC Issue All Types; Inflight Event / Encounter Wake Vortex Encounter
Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification
Environment - Non Weather Related; Procedure
Encountered significant wake turbulence immediately after rotation behind A321.   Wake turbulence was constant so we informed Tower we would require offset from departure course (34R.Bangor9).  Reported A321 in sight and offset appx. 0.2mi left of course.   We were cleared for takeoff while the A321 was rotating and my first thought was to delay a bit.  I wish I had as I have never encountered wake turbulence so close to the ground nor do I wish to try it again.It is worth noting this is the second time recently I have encountered wake turbulence behind an Airbus 321 in Seattle and my First Officer was recently cleared for takeoff behind a Boeing 757 without standard separation in Seattle.  In the future; I will not accept a clearance to takeoff so close to another aircraft and I believe it would be wise to follow up with Seattle Tower to ensure standard departure separation is being used and they are not getting rushed with the high amount of departures they work these days.The Seattle Controllers do a remarkable job handling the heavy traffic load we have seen this summer but perhaps there is a trend here and they are getting a bit rushed.  As a pilot; I will ask for a bit of time before taking off behind another aircraft next time.
B737-800 Captain reported encountering wake turbulence departing SEA in trail of an A321; commenting that spacing appeared insufficient.