You'll find that it's seized on (with rust). First thing is to penetrate the rust
Try some rust buster, 50/50 Tranny fluid & Acetone mix (thx to ODK for that info)or buy some from an auto store or Bunnings...
Can try heating the sprocket and tapping it with a small sledgie Or combine the two methods BUT ya can't beat a good Puller (hehe)
I use this.. works well for me.
[Linked Image]

Regards,
Paul

Such is life.......
Nothing better than a "GreenField" just mowed..