Rexall was the trading name of the United Drug Company, a franchise of chemist/drug retail stores in Boston, USA. In 1920 they approached Sir Jesse Boot of the Boots Pure Drug Company, Nottingham and eventually purchased the company, giving Sir Jesse Boot sufficient funds to permit him to donate money to create a Chair of Chemistry for research in the University College. Boots was bought back into British ownership in 1933 by a consortium which included Sir Jesse Boot's son. 

Wikipedia has a more detailed history of the enterprise.