a polymer comprising the unit wherein r is selected form the group consisting of hydrogen , c 1 - c 18 alkyl , c 1 - c 18 aralkyl , c 1 - c 18 alkaryl , carboxylic acid , carboxy - c 1 - 6 alkyl , or any one of the c 1 - c 18 alkyl , c 1 - c 18 alkenyl , c 1 - c 18 aralkyl , c 1 - c 18 alkaryl substituted with a heteroatom within , or attached to , the carbon backbone ; r 1 is selected from the group consisting of hydrogen , c 1 - c 6 alkyl groups ; x is an acylating agent and wherein the polymer has a polydispersity of less than 1 . 4 , preferably less than 1 . 2 and a molecular weight of less than 100 , 000 , the polymer is preferably made by controlled radical polymerization and is useful in the production of polymer drug conjugates with desirable biological profiles .