Development of a Library of N-Substituted Maleimides for the Local Functionalization of Linear Polymer Chains

A novel kinetic process was investigated for functionalizing on-demand local regions of well-defined linear polystyrene chains. This concept relies on the atom transfer radical copolymerization (ATRP) of functional N-substituted maleimides with styrene. This copolymerization is a controlled radical process, which combines two unique kinetic features: i) all the polymers chains are growing simultaneously and ii) the cross-propagation of the comonomers is highly-favored as compared to homopolymerization. Thus, discrete amounts of N-substituted maleimides (e.g., 1 equiv as compared to initiator) are consumed extremely fast in the copolymerization process and are therefore locally incorporated in narrow regions of the growing polystyrene chains. MALDI-TOF analysis of model copolymers indicated that this kinetic concept is efficient. Although a sequence distribution is observed, well-defined polymer chains having only one or two functional maleimide units per chain were found to be the most abundant species. Furthermore, the position of the functional groups in the polystyrene chains can be kinetically-controlled by adding the N-substituted maleimides at desired times during the course of the polymerization. This method is very versatile and can be applied to a wide variety of N-substituted maleimides. Herein, a library of 20 different maleimides bearing various functional groups (e.g., aromatic moieties, fluorinated groups, hydroxy functions, protected esters, protected amines, light-responsive moieties, fluorophores and biorelevant functions such as short poly(ethylene glycol) segments or biotin moieties) was investigated. In most cases, the functional N-substituted maleimides could be efficiently incorporated in the polystyrene chains.