Variable In DDL Statement In Procedure

Create table tbl(title nvarchar(40))

Create procedure df_bppr @de nvarchar(30)
As
Begin
Declare @sstr nvarchar(500)
Set @sstr = N'Alter Table tbl Add Constraint df_title Default '+ @de + ' For title'
Exec sp_executesql @sstr, @de
End

Execute df_bppr @de = 'NoTitle'

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Msg 102, Level 15, State 1, Line 1
Incorrect syntax near 'NoTitle'.
Msg 128, Level 15, State 1, Line 1
The name "NoTitle" is not permitted in this context. Valid expressions are constants, constant expressions, and (in some contexts) variables. Column names are not permitted.

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I created the procedure without any error. But when i execute the procedure it shows the above error

I want to create a proc which dynamically change the default value for more than one column with same default value.

Vijai