Passing Vars To An In Statment

i am going mad. when i hardcode my usernames into my in statment i get the results i expect...

Code------------------------------
Select u.UserID from dbo.[User] u where u.UserName in ('iresponsejamie.seaton','iresponsephil.smith)


Result---------------------------
3509
3506




but when i use a variable with more than one user in it and pass that into my in clause i get nothing returned when i should have the same result set.

Code------------------------------
DECLARE @CollectorList VarChar(100)

Set @CollectorList= '''iresponsejamie.seaton'',''iresponsephil.smith'''

Select u.UserID from dbo.[User] u where u.UserName in (@CollectorList)

Result---------------------------
nothing


as i said if i use - Set @CollectorList= 'iresponsejamie.seaton' - it works fine but when i include another user - Set @CollectorList= '''iresponsejamie.seaton'',''iresponsephil.smith''' - i get nothing back.

any ideas guys