Thrust 1 - Creation of Well-defined Macromolecules
The IRG has three major thrusts; the first is the creation of well-defined macromolecules. As the control of structure and function on the nanometer level requires well-defined macromolecules, the ability to produce narrowly dispersed products of controlled molecular weight and defined functionality together with stereochemical monomer sequence control is the enabling science that drives the study of new materials. The second thrust, cooperative assembly of functional materials, focuses on the design and synthesis of novel self-assembling materials and their molecular recognition properties as a function of secondary structure and chirality. We aim to understand how non-covalent interactions at the bio-nano interface can drive new intra-and inter- chain interactions and how chain architecture affects supramolecular assembly of composite systems on multiple length scales and dimensions. The third thrust, templating of ordered nanostructures for applications in microelectronics, focuses on promising applications of functionalized and architecturally defined macromolecules in the templating of ordered nanostructures, either in bulk or as thin films. Such nanostructures have enormous potential in a variety of different applications.
Organocatalytic living polymerization. Waymouth, Wade, Swope, and Hedrick have pioneered the development of organocatalytic strategies for the synthesis of well-defined macromolecules. Among the successful organocatalysts discovered in our laboratories are tertiary amines such as 4-dimethylaminopyridine (DMAP), phosphines, and N-heterocyclic carbenes (NHCs). For these potent organocatalysts we have proposed monomer-activated mechanisms. Recently, we have developed an alternative pathway for the ring-opening polymerization (ROP) of cyclic esters through bifunctional organocatalysis using thiourea-tertiary amines. The carbonyl group of a lactide monomer is activated towards electrophilic attack by the thiourea via hydrogen bonding, and the initiating/propagating alcohols are activated as nucleophiles by the tertiary amine.
We have developed another general class of organic catalyst based on phosphazene bases, such as 2-tert-butylimino-2-diethylamino-1,3-dimethyl-perhydro-1,3,2-diazaphosphorine (BEMP) and N’-tert-butyl-N,N,N’,N’,N”,N”-hexamethylphosphorimidic triamide (P1-t-Bu), that are active for the living ring-opening polymerization (ROP) of cyclic esters (Scheme 1). Polylactide and polyvalerolactone were prepared through this organocatalytic route at mild experimental conditions. The prepared polyesters possess predictable molecular weights, narrow polydispersities, and high end-group fidelity. The concentration of BEMP catalyst for ROP affects the reaction rate, the final conversion of the monomer, and the polydispersities of the polyesters. Polymerization of rac-lactide yields isotactic-enriched poly(lactide) with the probability of isotactic propagation (Pi) equal to 0.70. ROP of caprolactone using BEMP needs to be carried out at an elevated temperature (80°C). Several chain-extended block copolymerizations were also successfully carried out with hydroxyl-functional macroinitiators, rac-lactide monomer, and BEMP catalyst. Mechanistic studies suggest that the intermolecular hydrogen bonding of the alcohol initiator to phosphazene bases activates the alcohol for ROP of cyclic esters.
Scheme 1. Living ring-opening polymerization of cyclic esters.