ABSTRACT
Functional polymers are important among the several primary representatives of modern day materials. Polymers are similar to small organic and inorganic molecules in certain ways and yet possess novel and unusual characteristics such as molecular entanglement, viscosity, viscoelasticity, glass transition, and few other specific properties. Polymers are primarily synthesized by the chain growth or step growth kinetics. The most common method of synthesizing commercial polymers in very large volume or bulk is the conventional free radical polymerization (CFRP), which is one form of chain growth polymerization. The others include cationic, anionic, and organometallic based polymerizations. The major concern associated with CFRP is that the polymers synthesized exhibit broad molecular weight distribution (MWD) and therefore distribution of properties. The physical properties of polymers such as mechanical strength, melt viscosity, and diffusion coefficient in solution depend critically on the molecular weight and MWD. In addition, they invariably cannot be used for the preparation of polymers with varying molecular architecture. For example, chain extension using other monomers to prepare block copolymers requires an active chain end. This led to the development of new approaches called “controlled/ living radical polymerizations” (CRP) that were able to minimize the limitations of CFRP. The IUPAC nomenclature for CRP is reversible-deactivation radical polymerization (RDRP). In RDRP, the lifetime of the growing chains is relatively higher (>1 h) than that in CFRP (~1 s).1 The RDRP relies on a peculiar kinetic behavior called persistent radical effect (PRE)2,3 whereas in CFRP, a steady state concentration of radicals (~10-8 M) is achieved due to similar rates of initiation and termination steps. In CFRP, at the end of polymerization, all the chain ends are dead (not radical in nature) while in RDRP, the chain ends in the polymers can be reactivated. These active chain ends enable further functionalization of the polymer chains. In CFRP, bimolecular termination occurs between two growing chains with a free radical chain end, leading to the formation of dead polymer chains. On the other hand, in RDRP, the termination rate decreases with time due to PRE. The consequence of these kinetics steps is that in CFRP the molecular weight of the polymer increases dramatically at very low monomer conversion and remains the same almost until the end of the conversion of all the monomers. In the case of RDRP, in view of the control achieved over the chain propagation step (and with minimum termination) the chain continues to grow with monomer conversion resulting in a linear variation of molecular weight
with monomer conversion. In simpler words, polymers with controllable molecular weight, desired architecture, and narrow MWD can be successfully synthesized using RDRP.
