Polymerization of β-Propiolactone

On treatment with catalytic base (RO^(–)), β-propiolactone (structure on the right) polymerizes to a colorless flexible solid. Write a plausible structure for this polymer, indicating the chain residues at either end by asterisks. (e.g. *-ABAB-*)
To draw a formula use the drawing window on the left below. Do not try to draw hydrogen atoms. Draw two, and only two, repeating units of the polymer, and designate the terminal (chain extension) bonds by asterisks. To enter an asterisk open the Extended Table (the button below the periodic table on the right toolbar) and choose A (any atom). When you are finished, check your answer by pressing the Check Answer button. A help screen is available for the drawing window.