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Living Polymerization and Molecular Weight

Can two polymer chains be the same length? They can if you use living polymerization.
Can two vines grow be the same length? Can two polymer chains be the same length? They can if you use living polymerization.

Living polymerization doesn’t mean alive in the biological sense. As defined by the International Union of Pure and Applied Chemistry (IUPAC) definition, living polymerization is a chain polymerization from which chain transfer and chain termination are absent. Living polymerization is used to produce materials with narrow molecular weight distribution, an important property for many polymer applications.

Polymerization Process

There are four general processes occurring simultaneously during the polymerization process: chain initiation, chain propagation, chain transfer, and chain termination. In simple terms, each polymer chain starts to grow, propagates, and terminates at a certain time, and synchronizing these processes results in chains of similar length, or molecular weight, which is desirable. If the chain initiation rate is slower or comparable to propagation rate, some chains are being initiated while others are rapidly growing, resulting in longer and shorter chains. On the other hand, if initiation is much faster than the propagation, polymer chains start growing simultaneously, and grow uniformly. Now, if the chains are not terminated by any additional mechanism, the only factor defining their growth is the presence of a monomer. Once the monomer is depleted, the growth is complete, with resulting polymer chains of the same length.

Living polymerization, which has been studied for more than 70 years, can follow anionic, cationic, and radical polymerization mechanisms. Popular atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) are examples of living radical polymerization. Living polymerization allows you to obtain precisely controlled molecular weight and narrow molecular weight distribution, as well as complex polymer architectures.

Practical Applications

One of the exciting practical applications of living polymerization is nucleic acid-based therapeutics (or gene therapy), which is being actively studied to treat hereditary and infectious diseases. Nucleic acid-based drug candidates include DNA to repair nonfunctioning genes, and small interfering RNA (siRNA) to silence harmful genes. Polymers are used for nanoparticle delivery of nucleic acids inside the cell. An article in Accounts of Chemical Research explains:

Because of their self-assembly with nucleic acids into virus-sized nanoparticles and high transfection efficiency in vitro, cationic polymers have been extensively studied for nucleic acid delivery applications, but toxicity and particle stability have limited the clinical applications of these systems. The advent of living free radical polymerization has improved the quality, control, and reproducibility of these synthesized materials. This process yields well-defined, narrowly disperse materials with designed architectures and molecular weights. As a result, researchers can study the effects of polymer architecture and molecular weight on transfection efficiency and cytotoxicity.

Supramolecular Design

The idea of living polymerization has been taken one step further by scientists from National Institute for Materials Science (Japan), whose recent publication in Nature Chemistry describes the first application of living polymerization to the supramolecular domain, which mimics nature in the ways it assembles macromolecular structures. The scientists designed and studied living supramolecular polymerization of the porphyrin-based monomers into nanoparticles and nanofibers:

Despite the fact that the polymerization is non-covalent, the reaction kinetics are analogous to that of conventional chain growth polymerization, and the supramolecular polymers were synthesized with controlled length and narrow polydispersity.

Dispersity (formerly referred to as polydispersity index) is a measure of the heterogeneity of sizes of molecules in a material. When applying rational design to polymers to obtain materials with desired properties, we always aim to obtain monodisperse (uniform) plastic materials, so that they can behave in a uniform, predicted fashion. Living polymerization, among other controlled polymerization techniques, is a good way to get there.

Image by smithore/123RF.
Source: “Living Supramolecular Polymerization Realized Through a Biomimetic Approach,” by S. Ogi, K. Sugiyasu, S. Manna, S. Samitsu, and M. Takeuchi, Nature Chemistry 6, 188–195 (2014) doi:10.1038/nchem.1849.
Source: “Application of Living Free Radical Polymerization for Nucleic Acid Delivery,” by D.S. Chu, J.G. Schellinger, J. Shi, A.J. Convertine, P.S. Stayton, and S.H. Pun, Accounts of Chemical Research, July 17, 2012; 45(7):1089-99. doi: 10.1021/ar200242z.
Source: “Optimization of Brush-like Cationic Copolymers for Nonviral Gene Delivery,” by H. Wei, J.A. Pahang, and S.H. Pun, Biomacromolecules, January 14, 2013;14(1):275-84. doi: 10.1021/bm301747r.
Source: Non Living Polymerization Animation,
Source: Living Polymerization Animation,