Light Leads to Ultra-Long Polymer Chains in Water

Abstract

The synthesis of controlled ultra-high-molecular-weight polymers has been challenging in living radical polymerization. In this issue of Chem, Sumerlin and coworkers report that mild UV light irradiation activates thiocarbonylthio compounds in water to form those over 1 × 106 Da. The synthesis of controlled ultra-high-molecular-weight polymers has been challenging in living radical polymerization. In this issue of Chem, Sumerlin and coworkers report that mild UV light irradiation activates thiocarbonylthio compounds in water to form those over 1 × 106 Da. The formation of ultra-high-molecular-weight (UHMW) polymers, which are typically more than 106 Da, requires highly effective or selective propagation reactions with minimal side reactions, such as irreversible termination and chain-transfer reactions. For simplicity, the synthesis of a polymer larger than 1 × 106 Da from a monomer with a molar mass of 100 requires that the selectivity be more than four nines (99.99%) because it continues to propagate at least 10,000 times. In addition, the synthesis of polymers with nearly the same chain lengths or narrow molecular-weight distributions requires that the initiation occur nearly simultaneously for all polymer chains and be faster than the propagation. Therefore, four elementary reactions (i.e., initiation, propagation, termination, and chain transfer) must be highly controlled. Controlled or “living” radical polymerization is a rapidly growing research area about which more than 2,000 related papers have been published in the past year.1Matyjaszewski K. Tsarevsky N.V. J. Am. Chem. Soc. 2014; 136: 6513-6533Crossref Scopus (912) Google Scholar, 2Ouchi M. Terashima T. Sawamoto M. Chem. Rev. 2009; 109: 4963-5050Crossref Scopus (1081) Google Scholar, 3Moad G. Rizzardo E. Thang S.H. Acc. Chem. Res. 2008; 41: 1133-1142Crossref Scopus (600) Google Scholar Most of the currently developed “living” radical polymerizations rely on the reversible deactivation process. In this process, the propagating radical species is reversibly deactivated into a covalent dormant species. These processes have been named reversible deactivation radical polymerizations (RDRPs) by the International Union of Pure and Applied Chemistry4Jenkins A.D. Jones R.G. Moad G. Pure Appl. Chem. 2010; 82: 483-491Google Scholar and can be classified into at least three mechanisms (i.e., dissociation-combination, atom-transfer, and degenerative chain-transfer mechanisms).5Goto A. Fukuda T. Prog. Polym. Sci. 2004; 29: 329-385Crossref Scopus (812) Google Scholar The reversible deactivation process is able to diminish the termination reaction between the neutral growing radical species and affords nearly equal opportunity for chain growth to all polymer chains. This enables control of the molecular weights and chain end groups as well as the precisely controlled synthesis of various polymer architectures, such as block, graft, brush, star, and hyperbranched polymers, which can be applicable to a wide range of materials. However, one of the most important and unresolved issues in RDRPs is the synthesis of controlled UHMW polymers under normal conditions, which imposes a limitation on RDRPs for material design. This is because the propagating species in the RDRP is still a radical, which can undergo intrinsic side reactions in the end. Increasing the selectivity could solve the problem, but this has yet to be achieved. “Photo” is now one of the most active keywords in both organic reactions and polymerizations because photoexcitation leads to cleavage of stable covalent bonds even under mild conditions, which results in active species and/or intermediates and unprecedented or highly selective reactions. This situation is also observed in the field of RDRPs, where various efficient photoinduced systems have been developed to improve the efficiency and selectivity in living propagation.6Chen M. Zhong M. Johnson J.A. Chem. Rev. 2016; 116: 10167-10211Crossref Scopus (709) Google Scholar, 7Dadashi-Silab S. Doran S. Yagci Y. Chem. Rev. 2016; 116: 10212-10275Crossref Scopus (558) Google Scholar, 8McKenzie T.G. Fu Q. Uchiyama M. Satoh K. Xu J. Boyer C. Kamigaito M. Qiao G.G. Adv. Sci. 2016; 3: 1500394Crossref Scopus (195) Google Scholar In this issue of Chem, Sumerlin and coworkers have succeeded in the synthesis of UHMW polymers that are more than 8 × 106 Da and have narrow molecular-weight distributions (Mw/Mn = 1.2) by employing a mild UV light source or sunshine for thiocarbonylthio compounds (R–SC(S)Z) without any additional radical sources in the radical polymerization of dimethylacrylamide (DMA) in water at 35°C (Figure 1),9Carmean R.N. Becker T.E. Sims M.B. Sumerlin B.S. Chem. 2017; 2: 93-101Scopus (153) Google Scholar which could be the ambient outside temperature on a hot summer day in Florida. In general, thiocarbonylthio compounds are used as reversible chain-transfer agents for controlling radical polymerization initiated by a radical initiator, such as AIBN (4,4’-azobisisobutyronitrile), via a reversible addition-fragmentation chain-transfer (RAFT) process, which involves a degenerative chain-transfer mechanism. However, new polymer chains are continuously supplied from the radical source during the polymerization. Although this behavior does not substantially affect the resulting molecular weight of the polymer when the target molecular weight is low, it becomes gradually more important as the target molecular weight increases. Because the authors use the term “iniferter” for trithiocarbonate and xanthate in this paper, a similar thiocarbonylthio compound (i.e., dithiocarbamate) was originally used for controlling radical polymerization in the absence of any other radical initiators under strong UV light.10Otsu T. J. Polym. Sci. A Polym. Chem. 2000; 38: 2121-2136Crossref Scopus (535) Google Scholar The word “iniferter” was coined by the developer (Otsu) in the 1980s because it was supposed to play three roles as an initiator, a chain-transfer agent, and a terminator. As an initiator, the iniferter does generate the initiating radical species via cleavage of the thioester bond under UV light irradiation. Although the iniferter system is recognized as one of the first “living” radical polymerizations, the molecular-weight control was not as good as that provided by recently developed RDRPs for several reasons. Here, the authors first used a trithiocarbonate as the iniferter to more precisely control the molecular weight via a faster reversible deactivation process under mild UV light to reduce other undesirable bond-breaking reactions. To target the synthesis of UHMW polymers, the authors selected DMA as the monomer and water as the solvent because high propagation and low termination rate constants are typically obtained in radical polymerization of acrylamides in water even at ambient temperature. In addition, the low temperature is favorable for suppressing undesirable side reactions. Therefore, using a very simple system, the authors cleverly determined the optimal conditions to overcome the challenges of radical polymerization. According to their results, the polymerization proceeded nearly quantitatively according to near-linear pseudo-order first-order kinetics to afford well-controlled UHMW polymers with Mn > 5 × 106 and narrow molecular-weight distributions (Mw/Mn = 1.1–1.4) in 10 hr. The chain-extension reaction, which confirms the chain-end fidelity (i.e., “living” nature of the polymerization), was successfully performed even for the UHMW polymers (1 × 106). The polymerization quickly responded to the light’s being turned off and on while maintaining the living nature. Furthermore, xanthate was employed rather than trithiocarbonate because more efficient photoexcitation was expected as a result of the blue shift of the n-to-π* transition to the wavelength of the UV light (365 nm). Indeed, a faster polymerization occurred with xanthate, and a monomer conversion of 93% was reached in 30 min, resulting in similar UHMW polymers (1 × 106). With the fast system in hand, the authors focused on synthesizing super UHMW polymers and ultimately succeeded in the synthesis of polymers with Mn = 8.57 × 106 and Mw/Mn = 1.17 in 2 hr. These results provide additional insight into the mechanism of iniferter polymerization. Although the iniferter concept was postulated more than 30 years ago, the iniferter can now be regarded as a mixed system involving both dissociation-combination and degenerative chain-transfer mechanisms. Namely, its roles as an initiator and a terminator are for the former, and its role as a chain-transfer agent is for the latter. According to the authors, both mechanisms might be involved in the reported polymerization, and their contributions depend on the conditions. In the early stage of polymerization at a low viscosity, the latter mechanism is predominant. However, at an UHMW, the former contribution must be important because of the low mobility of the long polymer chain. In addition, the contribution of the dissociation-combination mechanism is suggested by the good results obtained with xanthate under UV irradiation because xanthate is not effective for conjugated monomers, such as DMA under typical RAFT conditions. Although it is unclear how the thiocarbonylthio radical is stable (similarly to the persistent radical), elucidation of the mechanism of iniferter polymerization will contribute to further development of RDRPs. The versatility of this iniferter system for other vinyl monomers should also be clarified for its application. On the occasion of the 60th anniversary of living anionic polymerization, which was discovered by Szwarc in 1956, this paper could open a new era for truly living radical polymerization. Ultra-High Molecular Weights via Aqueous Reversible-Deactivation Radical PolymerizationCarmean et al.ChemJanuary 12, 2017In BriefAlthough progress in living polymerization methods has enabled access to polymers with controlled molecular weights and architectures, avenues to new ultra-high-molecular-weight (UHMW) materials are limited. Sumerlin and coworkers reveal a simple, catalyst-free route to well-defined UHMW polymers and block copolymers with degrees of polymerization above 85,000. Full-Text PDF Open Archive