Why is it easy to add bromine to styrene and why is it so difficult to add bromine to cinnamic acid?

Why is it easy to add bromine to styrene and why is it so difficult to add bromine to cinnamic acid?

Can someone show me the full reaction mechanism between cinnamic acid, pyridinium bromide perbromide, acetic acid and dichloromethane, including all enantiomeres (isomeres)? I can only figure out two.