Multi Sided Vs Smooth Sided Cents

Before the 12 sided coin was introduced in 1982 the mint had tried reducing the diameter and thickness of the cent to offset the rising price of copper.The new design was larger,thicker,and weighed less than it's 1981 counterpart yet still retained .980 copper content.Round cents returned in 1997 when the plated cents replaced the bronze.

12-sided coins first appeared in Canada in late 1942, when they were introduced to nickels because of the change from nickel to tombac (brass) to prevent confusion with pennies with tombacs that tarnished over time.

I wasn't sure why they did it to pennies, but the explanation given by nickel nubber makes sense,

I know they removed the 12 sides from pennies in 1997 because they had difficulties copper plating steel blanks that were 12 sided.