Catalyst-Free Photoinduced End-Group Removal of Thiocarbonylthio

Letter pubs.acs.org/macroletters

Catalyst-Free Photoinduced End-Group Removal of Thiocarbonylthio Functionality R. Nicholas Carmean, C. Adrian Figg, Georg M. Scheutz, Tomohiro Kubo, and Brent S. Sumerlin* George and Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science and Engineering, Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: An initiator- and catalyst-free method for polymer end-group modification has been designed. Under long-wave ultraviolet irradiation, polymers with thiocarbonylthio end groups undergo photolytic cleavage to reveal an active macroradical capable of irreversible termination with a suitable hydrogen source. This straightforward method was successfully demonstrated by the removal of a range of end groups that commonly result from reversible addition−fragmentation chain transfer or photoiniferter polymerizations, including trithiocarbonate, dithiobenzoate, xanthate, and dithiocarbamate mediating agents. This strategy proved efficient for polymers derived from acrylamido, acrylic, methacrylic, styrenic, and vinylpyrrolidone monomers.

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thiocarbonylthio moiety between dormant and active chains. Many photoiniferter polymerizations also result in thiocarbonylthio end groups.7 While RAFT and photoiniferter polymerizations are tolerant to a wide range of reaction conditions and functional monomers, the CTA chain end is often unstable at high temperatures 8−10 and reactive with a variety of nucleophiles, including water.8 Postpolymerization modification of well-defined macromolecules prepared through these methods often requires conversion of the reactive omegaterminus to a more inert group prior to pendent group functionalization.11 Furthermore, since most thiocarbonylthio moieties have an absorbance in the visible region, materials prepared from RAFT polymerization are often colored, which could present challenges for applications that require optical clarity. To alleviate the problems that may arise from reactive thiocarbonylthio chain ends, many successful end-group removal strategies have been developed.8,12−16 One common method involves reduction of the thiocarbonylthio group to a thiol that can subsequently be reacted with transition metals, other thiols via disulfide formation, or Michael-acceptors to produce a thioether.17−22 Alternatively, radical-induced reduction pathways enable complete removal of sulfur from the chain end and can also facilitate end-group functionalization.15,23 This approach typically relies on decomposition of an external initiator at high temperatures and subsequent addition of the initiator-derived radical to the thiocarbonyl end group to

eversible-deactivation radical polymerization (RDRP) techniques lead to macromolecules with controlled molecular weights,1 molecular weight distributions,2 and architectures.3−5 These polymerization methods rely on reversible deactivation of the propagating chain end with a mediating agent. As a result, successful RDRP often results in high chain-end retention of the mediating group.6 In the case of reversible addition−fragmentation chain transfer (RAFT) polymerization, a chain transfer agent (CTA) facilitates controlled chain growth by degenerative transfer of a

Figure 1. Photoinduced end-group removal (PEGR) process and substrate scope. PEGR occurs upon excitiation and photolytic cleavage of a thiocarbonylthio chain end under mild UV-light in the presence of a hydrogen donor. © XXXX American Chemical Society

Received: January 19, 2017 Accepted: February 3, 2017

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DOI: 10.1021/acsmacrolett.7b00038 ACS Macro Lett. 2017, 6, 185−189

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ACS Macro Letters

Figure 2. Photoinduced end-group removal (PEGR) of trithiocarbonate (TTC) chain ends from poly(N,N-dimethylacrylamide) (PDMA) was complete in 24 h with N-ethylpiperidine hypophosphite as the hydrogen source. (a) The size exclusion chromatogram shows a similar molecular weight distribution for the original and final polymer, and no coupling was evident. (b) Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry of PDMA-H confirmed full conversion of the trithiocarbonate mediating agent to a hydrogen. (c) 1H NMR spectroscopy was used to monitor the conversion of the trithiocarbonate end group to a hydrogen by observing the area of the terminal methine proton (5.25 ppm) adjacent to the trithiocarbonate relative to the terminal methyl protons of the CTA (0.91 ppm). (d) The near-linear semilogarithmic plot indicated PEGR operates via pseudo-first order kinetics.

presented a route to end-group replacement by the hydrogen abstraction approach, employing a low-temperature, visiblelight-mediated end-group removal technique facilitated by a thiazine photocatalyst.14 Photochemically induced cleavage of the trithiocarbonate and halogen end groups from RAFT- or atom transfer radical polymerization (ATRP)-generated chains in the presence of a hydrogen donor led to hydrogenterminated chains. The approach we report here operates without addition of a photocatalyst or initiator, has been applied to trithiocarbonate, dithiobenzoate, xanthate, and dithiocarbamate chain ends, and has proven to be applicable to polymers derived from acrylamido, acrylic, methacrylic, styrenic, and vinylpyrrolidone monomers (Figure 1). In all cases, this strategy leads to unreactive and colorless hydrogenterminated macromolecules and, unlike previous reports, relies only on direct activation of the thiocarbonylthio functionality under mild UV light.

promote fragmentation of the polymer chain end. Coupling of the resulting active chain end with an initiator fragment has been shown to be a highly effective method for functionalization and end-group removal.23 We reasoned that an alternative route to facile end-group removal lies in exploiting the photochemical properties of the thiocarbonylthio moiety. Under ultraviolet (UV) light, the thiocarbonylthio group undergoes homolytic cleavage between the carbon−sulfur bond most susceptible to fragmentation to reveal a carbon- and sulfur-centered radical.4,24−26 This photolytic activation mechanism can be used for controlling chain growth27−29 and has recently been employed by our group to prepare the highest molecular weight polymers ever reported for a controlled radical polymerization process.30 We envisioned using this approach of chain-end activation in the presence of a hydrogen donor to rapidly and efficiently remove thiocarbonylthio end groups under mild conditions. Indeed, during the course of this work, Hawker and co-workers 186

DOI: 10.1021/acsmacrolett.7b00038 ACS Macro Lett. 2017, 6, 185−189

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ACS Macro Letters

Figure 3. Pseudo-first order kinetic plot of photoinduced end group removal of trithiocarbonate (TTC) and xanthate (XAN) chain ends from poly(N,N-dimethlyacrylamide) (PDMA). The n-to-π* photolysis pathway of PDMA-XAN under mild UV light (emission maximum, λmax near 365 nm) resulted in rapid end group transformation.

Our investigation into photoinduced end-group removal (PEGR) began with the transformation of trithiocarbonate (TTC)-terminated polymers to hydrogen-terminated chains, using N-ethylpiperidine hypophosphite (EPHP) as a hydrogen atom donor. Hypophosphite salts have proven to be effective hydrogen sources that are often employed in radical-induced reduction and allow facile purification of the resulting polymer products.15 With an excess of EPHP (15 molar equiv) and under mild UV irradiation (365 nm, 7.0 mW/cm 2 ), trithiocarbonate photolysis liberates the polymer chain end to yield a macroradical that quickly and irreversibly abstracts a hydrogen from the phosphorus−hydrogen bond of the hypophosphite salt.31 This process was first demonstrated with poly(N,N-dimethylacrylamide) (PDMA) prepared by RAFT polymerization with a trithiocarbonate CTA (PDMATTC). Under constant irradiation, the end group transformation was monitored via 1H NMR spectroscopy by observing the disappearance of the terminal methine proton (5.25 ppm) adjacent to the trithiocarbonate relative to the terminal methyl protons of the CTA (0.91 ppm, Figure 2c). The resulting colorless polymer had a nearly identical molecular weight distribution to that of the original polymer, as determined by size exclusion chromatography (SEC), and full end-group removal was observed within 24 h, as confirmed with matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-ToF MS, Figures 2 and S1). The kinetic data obtained from 1H NMR spectroscopy revealed that this photoinduced end-group removal process occurs through pseudo-first order kinetics, as evidenced by the near-linear semilogarithmic plot (Figure 2d). An increase in the EPHP stoichiometry to 30 molar equiv resulted in a similar reaction rate, suggesting CTA photolysis was the ratedetermining step under these conditions. This process, however, could be readily accelerated with the addition of a tertiary amine, similar to photoinduced electron-transfer RAFT (PET-RAFT) polymerization recently developed by Boyer and

Figure 4. Photoinduced end-group removal (PEGR) of a dithiobenzoate (DTB) chain end from poly(methyl methacrylate) (PMMA) was complete in 24 h with N-ethylpiperidine hypophosphite as the hydrogen source. (a) The size exclusion chromatogram shows a similar molecular weight distribution for the original and final polymer, and no coupling was evident. (b) Matrix-assisted laser desorptionionization time-of-flight mass spectrometry of PMMA-H confirmed full conversion of the dithiobenzoate mediating agent to a hydrogen. (c) The near-linear semilogarithmic plot indicates PEGR operates via pseudo-first order kinetics under these conditions.

co-workers (Figure S2).32−34 In general, the data presented in Figure 2d also provide insight into the rate of chain-end activation which is of particular interest for the related process 187

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potentially enhance the efficiency of PEGR of PS-DTB. Unlike previous attempts with EPHP as the hydrogen source, hydrogen abstraction by the styryl macroradical that resulted from photolysis was rapid, resulting in full conversion of PSDTB to PS-H with no evidence of coupling after 24 h (Figures S12 and S13). Dithiocarbamates (DTC) are often employed for a wide range of RDRP techniques and can be implemented as photoiniferters,7,38 (switchable)39 chain transfer agents,40 and pseudohalogens for ATRP41 to polymerize more- and lessactivated monomers, such as styrene and vinyl acetate, respectively.39 The versatility of these mediating agents confers increased significance on DTC-based PEGR, because successful end-group removal could be implemented in a variety of postRDRP modifications from a range of monomer classes. Complete PEGR of PS-DTC was observed in 24 h, demonstrating the organotin hydrogen source is also compatible with stable radicals derived from other thiocarbonylthio chain ends. Importantly, no coupling was observed by SEC, which indicated efficient hydrogen abstraction after photolysis (Figures S14 and S15). In many cases, end-group removal or modification is required prior to pendent group functionalization due to the inherent cross-reactivity of thiocarbonylthio moieties derived from RAFT or photoiniferter polymerization. While a number of successful removal strategies have been developed, many of these demand harsh reaction conditions for efficient end-group removal. However, with the approach described here, chain-end photolysis provides a low temperature, initiator- and catalystfree method to efficient end-group modification. The utility of PEGR was demonstrated on a broad range of polymers prepared by RDRP. Although this initial report outlines only the transformation of a thiocarbonylthio group to a colorless and unreactive hydrogen chain end, this straightforward photolysis process can, in principle, be extended to impart additional functionality onto polymeric chain ends via radical addition to nonpolymerizable CC bonds.

of photoiniferter polymerizations in which chain growth is initiated by thiocarbonylthio photolysis.30 The PEGR process was extended to TTC-terminated poly(methyl acrylate) (PMA-TTC) with EPHP as a hydrogen source. Similar to PDMA-TTC, PMA-TTC photolysis exhibited a similar rate, resulting in complete end-group removal in 24 h, as confirmed by MALDI-ToF MS (Figures S2−S4). The comparable radical stability of the polyacrylamide and polyacrylate carbon-centered radicals and similar steric constraint on the C−S bond between the terminal monomer unit and the TTC moiety likely contributed to the similar photolysis profile. Although similar in structure, xanthate mediating agents undergo photolysis through an activation that relies on an n-toπ* transition at 357 nm rather than the π-to-π* transition of trithiocarbonates at 309 nm under mild-UV light (emission maximum, λmax near 365 nm).35 Therefore, photolysis of xanthate-terminated chains is significantly faster than trithiocarbonate-terminated chains.30 The accelerated activation profile resulted in rapid end-group transformations of xanthateterminated polymers. Under identical reaction conditions as those employed for modifying PDMA-TTC, PDMA-XAN showed complete end-group removal in 8 h and exhibited a photolysis rate nearly six times more rapid than the PDMATTC analogue (Figures 3, S5, and S6). Although the higher rate of photolytic cleavage led to an increased radical concentration, no coupling was observed by SEC, and the molecular weight distribution closely matched the xanthateterminated starting material. Xanthates are typically employed to polymerize less activated monomers, such as N-vinylpyrrolidone (NVP), and although the pyrrolidone-centered radical is considerably less stable than that which would be derived from an acrylate or acrylamide chain end, the photoinduced end-group removal process was also efficient in removing the xanthate CTA from poly(N-vinylpyrrolidone), using EPHP as a hydrogen source (Figures S7 and S8). Dithiobenzoate mediating agents are often employed during the polymerization of highly activated monomers, as the stable lowest unoccupied molecular orbital of the CTA closely matches the energy of the singly occupied molecular orbital of the propagating radical.36 PEGR of dithiobenzoate (DTB)terminated poly(methyl methacrylate) (PMMA) was rapid and efficient. Although the tertiary macromolecular radical that results from end-group photolysis is slightly hindered and relatively stable, EPHP proved to be an efficient hydrogen source in this reaction as well (Figures 4, S9, and S10). Conversely, end-group removal from DTB-terminated polystyrene (PS-DTB) was unsuccessful, with only 90% chain-end conversion being observed in 48 h. Moreover, SEC suggested a significant amount of chain−chain coupling occurred, as evidenced by the high molecular weight shoulder present in the final polymer trace (Figure S11). The slow and incomplete end-group removal is attributed to the increased styryl radical stability, which facilitated irreversible macromolecular coupling and, potentially, reversible termination with the dithiobenzoatecentered radical, rather than hydrogen abstraction from the hypophosphite salt. Stannanes have also been employed in externally initiated, radical-induced reduction, resulting in efficient hydrogen abstraction and good end-group removal from polystyrene.37 The proficiency of organotin reagents as hydrogen sources, specifically tributyltin hydride, lies in the weak tin−hydrogen bond. We expected this more active hydrogen source would

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00038.

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Experimental section and additional characterization data (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu. ORCID

Brent S. Sumerlin: 0000-0001-5749-5444 Author Contributions

The manuscript was written through contributions of all authors. Funding

This material is based on work supported by the National Science Foundation (DMR-1606410). Notes

The authors declare no competing financial interest. 188

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DOI: 10.1021/acsmacrolett.7b00038 ACS Macro Lett. 2017, 6, 185−189