Chemical Science View Article Online MINIREVIEW View Journal | View Issue Tailoring polymer dispersity and shape of molecular weight distributions: methods and Cite this: Chem. Sci.,2019,10,8724 All publication charges for this article applications have been paid for by the Royal Society of Chemistry Richard Whitfield, † Nghia P. Truong, † Daniel Messmer, Kostas Parkatzidis, Manon Rolland and Athina Anastasaki * The width and shape of molecular weight distributions can significantly affect the properties of polymeric materials and thus are key parameters to control. This mini-review aims to critically summarise recent Received 18th July 2019 approaches developed to tailor molecular weight distributions and highlights the strengths and Accepted 27th August 2019 limitations of each technique. Special emphasis will also be given to applications where tuning the DOI: 10.1039/c9sc03546j molecular weight distribution has been used as a strategy to not only enhance polymer properties but rsc.li/chemical-science also to increase the fundamental understanding behind complex mechanisms and phenomena. Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Đ ¼ can be calculated using the equation Mw/Mn, where Mw is Introduction 2 the weight average molecular weight (SNiMi /SNiMi) and Mn is S S Polymers are unlike small molecules in that they do not possess the number average molecular weight ( NiMi/ Ni). Ni and Mi are a unique molecular weight, instead they exhibit a molecular the number molecular weight of one polymeric specie within 2,3 weight distribution (MWD). Dispersity (Đ, formerly referred to the distribution. Mw and Mn can be determined experimen- as polydispersity index or PDI) is a measure of the width of tally utilising static light scattering and vapour pressure a MWD and describes the heterogeneity (or uniformity) of the osmometry, respectively. The most widely used technique to various chain molecular weights within a polymeric material.1 It measure the dispersity and shape of polymer MWDs however is This article is licensed under a size exclusion chromatography (SEC, formerly known as gel permeation chromatography, GPC), a chromatographic method Laboratory of Polymeric Materials, Department of Materials, ETH Zurich, in which polymer chains are separated based on their hydro- Vladimir-Prelog-Weg 5, Zurich 8093, Switzerland. E-mail: athina.anastasaki@mat. dynamic volumes. This allows the molecular weight of polymers Open Access Article. Published on 28 August 2019. Downloaded 9/23/2021 2:46:36 PM. ethz.ch to be determined relative to standards of known molecular † Both authors contributed equally to the work. Richard Whiteld was born in Nghia Truong is a senior scien- the town of Grantham in tist in the Department of Mate- England, and received his rials at ETH Zurich. He is also Master's degree from Durham an ARC DECRA research fellow University. He completed his and MIPS teaching fellow at PhD under the supervision of Monash University. He Prof. David Haddleton at the completed bachelor and master University of Warwick, which degrees from Vietnam National included 2 months as a visiting University – Ho Chi Minh City scholar at the University of Cal- and then received his PhD from ifornia Santa Barbara. He has the University of Queensland, subsequently undertaken an Australia. His current research Early Career Innovation Fellow- focuses on engineering smart ship in collaboration with the Institute of Advanced Study at the polymers and nanostructured materials for a better understanding University of Warwick and as of 2019 is the rst postdoctoral into the interface of nanomaterials with biological systems (at ETH researcher in the Laboratory of Polymeric Materials at ETH Zurich. Zurich) and translating the knowledge of the bio-nano interactions to pharmaceutical sciences and biotechnology (at Monash University). 8724 | Chem. Sci.,2019,10,8724–8734 This journal is © The Royal Society of Chemistry 2019 View Article Online Minireview Chemical Science weight, commonly poly(methyl methacrylate) or polystyrene.4,5 addition–fragmentation chain-transfer (RAFT) polymerisation13 While dispersity is a crude parameter for estimating the and nitroxide mediated polymerisation (NMP)14 have enabled uniformity of polymer chain lengths, the shape of MWDs the synthesis of well-dened macromolecules with controlled provide much more information about the range of chain molecular weight, architecture, end-group delity and dis- lengths. For example, two polymeric samples with the same persity.15–23 In fact, the high end-group delity and the dispersity can have symmetric or asymmetric MWDs, (skewed controlled nature of the polymerisation is typically conrmed towards either high or low molecular weight), which can by low dispersity values and as such dispersities in the range of signicantly affect material properties. Đ must have a value Đ z 1.01–1.20 are routinely targeted.24–32 Conversely, broader equal to or greater than 1, but as the polymer chains approach molecular weight distributions (Đ > 1.4) are oen considered to uniform chain length, Đ approaches unity (1), which is exhibi- be a sign of uncontrolled or “failed” polymerisation and ted by some natural polymers such as DNA, peptides and necessitate additional optimisation to reduce the dispersity.33–38 proteins in which only one species is present. Achieving However, the dispersity is a key parameter that impacts the uniformity (monodispersity, Đ ¼ 1) is a great challenge in physical properties of polymers, since low and high dispersity polymer science and apart from very limited exceptions in polymers can exhibit complementary properties and func- synthesising oligomers, has yet to be achieved.6–8 However, the tions.39–41 Hence, being able to tune MWD dispersity and shape discoveries of anionic9 and cationic10 polymerisation as well as allows polymer chemists to control and advance material controlled radical polymerisation methodologies including properties for a multitude of applications. atom transfer radical polymerisation (ATRP),11,12 reversible Daniel Messmer obtained his Manon Rolland studied at CPE PhD in the ETH Zurich Depart- Lyon until 2017, where she Creative Commons Attribution-NonCommercial 3.0 Unported Licence. ment of Materials in 2018, graduated with a double working on the synthesis and Master's degree in Chemical characterization of very high Engineering and Innovative dendritic generation dendron- Materials. She then undertook ized polymers under the super- a research project at Monash vision of Prof. A. Dieter Schluter.¨ University, in Prof. Tom Davis's In early 2019, he joined the group, where she worked on research group of Prof. Athina controlled radical polymeriza- This article is licensed under a Anastasaki as a scienti c tion. She subsequently collaborator, working on the completed her second internship chemical modication of initia- at the University of California, tors and catalysts for (photo-)ATRP. His research interests revolve Santa Barbara, in the group of Professor Craig Hawker, which Open Access Article. Published on 28 August 2019. Downloaded 9/23/2021 2:46:36 PM. around the creation of complex structures using the tools of focused on photo-mediated polymerization. Aer graduation, she organic chemistry and polymer synthesis. worked at STMicroelectronics (Grenoble) as an etch process engi- neer. In 2019, she joined the group of Polymeric Materials at ETH Zurich where she is currently a PhD student. Kostas Parkatzidis studied Athina Anastasaki is an Assis- Material Science and Tech- tant Professor in the Materials nology at the University of Crete, Department of ETH Zurich and before obtaining his MSc degree the head of the Polymeric Mate- in Chemistry from the same rials Lab. She previously institution under the supervision completed her PhD studies with of Prof. Maria Vamvakaki. Professor Dave Haddleton at the During his Master's, he also University of Warwick for which joined the group of Dr Maria she won the Jon Weaver Award Farsari at the Institute of Elec- for the best PhD in Polymer tronic Structure and Laser at the Chemistry in the UK. She was Foundation for Research and then awarded an Elings Fellow- Technology in Hellas. In 2019, ship and a Global Marie Curie he started his doctoral studies in the group of Prof. Athina Anas- Fellowship to pursue post-doctoral work with Professor Craig tasaki, in the Department of Materials at ETH Zurich, working on Hawker at the University of California, Santa Barbara. Her group's controlled radical polymerization and block copolymer self- research currently focuses on controlled radical polymerization assembly. and polymeric self-assembly. This journal is © The Royal Society of Chemistry 2019 Chem. Sci.,2019,10,8724–8734 | 8725 View Article Online Chemical Science Minireview In this mini-review we rst present different methods that polymer blends have bimodal or multimodal MWDs which, have been developed to tailor MWDs. For each method, the depending on the targeted application, can be undesirable.47 A strengths and limitations are critically evaluated and discussed. recent approach to tune MWDs through the continuous mixing Finally, we highlight the various applications whereby varying of synthesised polymer fractions was developed by Boyer, Xu dispersity has been shown to be benecial. Remaining chal- and co-workers.48,49 The two groups elegantly employed ow lenges in the eld and future directions are also discussed. chemistry with photo-induced electron transfer (PET) RAFT polymerisation, generating