Dial in Your Dispersity Etical Molecular Weight,” Recalls Inspired by the Perfect Monodisper- Synthetic Linear Polymers Typi- Anastasaki

RESEARCH HIGHLIGHTS Nature Reviews Chemistry | https://doi.org/10.1038/s41570-020-0198-8 | Published online 27 May 2020 POLYMER CHEMISTRY at the cost of slow polymerization, dead polymer chains or a mismatch between experimental and theor- Dial in your dispersity etical molecular weight,” recalls Inspired by the perfect monodisper- Synthetic linear polymers typi- Anastasaki. One such ‘bad’ agent sity of proteins, polymer research cally exist as mixtures of species for methacrylates is the alkyl We needed has long favoured methodologies that can vary in their degree of dithiocarbamate [1-(4-chloro- them to be that afford products with narrow polymerization and the nature 3,5-dimethylpyrazolyl)]C(S) molecular weight distributions. of their end groups. The former is S(2-cyano-2-butyl), which is ‘bad’ enough However, high dispersity polymers described by dispersity (Đ), the slower to transfer chains and to give us high are ostensibly more processable quotient of the weight average to gives Đ = 1.65 under the same dispersity and are widely used in industry. the number average molecular conditions. We surprisingly do not have good weight and a value that greatly affects With ‘good’ and ‘bad’ agents routes to these materials because physical properties. Historically, in hand, how do we arrive at an of the misconception that high high Đ polymers have had limited intermediate dispersity? Anastasaki, dispersity must come at the expense livingness because not all chains Junkers and colleagues show that of ‘livingness’ — the propensity of have the requisite active end groups simply mixing two different agents polymer chains to undergo further at which to uniformly grow another affords Đ between the two extremes, reactions. A team led by Athina polymer domain or attach the poly- depending on the ratio of agents Anastasaki and Tanja Junkers put mer to a surface. One route to vinyl used. Size-exclusion chromatography paid to this with an unconventional polymers with high end-group reveals a monomodal product distri- controlled radical polymerization fidelity is reversible addition– bution, consistent with the growing method that exploits multiple chain- fragmentation chain transfer chains spending time on both agents. transfer agents, as described in Chem, (RAFT) polymerization, whereby Overall, the team’s seven agents to tune dispersity of a living polymer a growing polymer radical Pn reversi- enabled them to tune the dispersity over a wide range. bly adds to a chain-transfer agent of polymethacrylates, polyacrylates, ZC(S)SR (Z is an aryl/heteroatomic polyacrylamides, polystyrene, group) to give the C-centred poly(vinyl ketone) and poly(vinyl radical intermediate ZC(SPn)SR. acetate). This product then liberates either Although convenient and the S-bound Pn or R radical, so amenable to a broad substrate scope, one chain continues to grow by the mixed-agent method does reacting with monomer molecules have limitations. “The brutal truth while the other chain takes a break is that we can control dispersity on the agent. very well but not the shape of the RAFT is geared towards low Đ molecular weight distribution, which polymers and if we want to increase also affects properties,” laments this value we could simply initiate Anastasaki. Mass spectrometry chain growth at different times. shows that they get a mixture of 1 Yet, realizing this with flow or light living polymers, Z C(S)SPn and 2 is non-trivial, so Anastasaki, Junkers Z C(S)SPn, that feature the two and colleagues considered the different agents as end groups. chemistry of chain-transfer agents, This mixture can be converted into beginning with alkyl dithiobenzoate diblock copolymers but without PhC(S)S(2-cyano-2-propyl), which perfect control of overall dispersity. enables rapid chain-transfer via However, if we could shape product a benzylic radical intermediate. distributions and have uniform end This archetypical agent gave groups, the method may well enable poly(methyl methacrylate) with unprecedented control over polymers a number average molecular and their remarkable properties. weight of 25,300 Da and a low Đ David Schilter of 1.13, so the team sought worse ORIGINAL Article Whitfield, R. et al. Tailoring agents to realize higher Đ. “We polymer dispersity by RAFT polymerization: a needed them to be ‘bad’ enough versatile approach. Chem https://doi.org/10.1016/ j.chempr.2020.04.020 (2020) Credit: David Schilter/Springer Nature Limited to give us high dispersity but not NATURE REVIEWS | CHEMISTRY VOLUME 4 | JULY 2020 | 331.

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