PolymerChemistry

COMMUNICATION

Cite this: Polym. Chem., 2014, 5,6893

Received 14th July 2014,Accepted 24th August 2014

DOI: 10.1039/c4py00964a

www.rsc.org/polymers

Mechanical activation of a dithioesterderivative-based retro RAFT-HDA reaction†

Zhijian Wang, Zhiyong Ma, Zhenyu Zhang, Feng Wu, Hong Jiang and Xinru Jia*

A new mechanophore was synthesized from a dithioester deriva-

tive 1 and an open-chain diene 2 by the HDA reaction. It was

embedded in the polymer chain by initiating the polymerization of

methyl acrylate (MA). It was found that the PMA with Mn > 30 kDa

could undergo a mechanically induced retro RAFT-HDA reaction

rapidly at ambient temperature. The released species was evi-

denced chemically and spectroscopically, and the liberated

dithioester derivative 1 maintained the activity to react with cyclo-

pentadiene (Cp) or to copolymerize with styrene (St).

Polymer mechanochemistry, which means to induce a chemi-cal transformation with mechanical force by using a polymeras an actuator, has attracted increasing interest currently.1 Theapplication of force for chemical systems is believed to havethe advantages of minimizing undesirable reactivity and acti-vating otherwise kinetically inaccessible processes. Theexamples include the mechanically activated gem-dihalocyclo-propanes,2 benzocyclobutene,3 latent catalysts,4 force inducedelectrocyclic ring opening,5 isomerations6 cycloreversion reac-tions,7 etc. Moore et al. reported a series of mechanoresponsivepolymers with regard to force-induced activation of covalentbonds.3,5a,b,f,g,8 Their excellent work revealed how to harnessthe mechanical force for regulating chemical transformations.In 2011, Bielawski et al. reported a novel example that uncov-ered the first use of mechanical force to overcome prohibitivelyhigh thermal barriers related to retro [4 + 2] cycloadditionreactions.7b

We previously reported the mechanochromic properties ofspirolactam that suffered from a force-induced ring-openingreaction with colour change.9 Recently, we also paid attentionto the search for new mechanophores embedded in polymerchains, by which we aimed to further understand mechano-

chemical transformations. Herein, we report a mechanicallyactivated retro RAFT-HDA reaction with a dithioester derivativeas a mechanophore. Our work is inspired by the report ofBarner-Kowollik et al., who elegantly disclosed many variantsof reversible addition–fragmentation chain-transfer (RAFT)-hetero Diels–Alder (HDA) reactions by modifying the mole-cular structures in order to tune the activation temperature.10

The superiority of our system is that (1) the retro RAFT-HDAreaction can be quickly achieved at ambient temperature; (2)the reaction progress can be monitored by UV-Vis spectroscopyand observed by naked eyes because of the yellow colour fromthe absorption of CvS groups; and (3) the released moieties ofdithioester derivative 1 maintains the activity to further reactwith cyclopentadiene (Cp) or copolymerize with styrene (St) viaRAFT polymerization. To the best of our knowledge, thedithioester-based RAFT agent as a mechanophore is a rarelyreported example in polymer mechanochemistry.

The structure of the mechanophore, named DAPy-2Br, isshown in Scheme 1. It was synthesized via a [4 + 2] HDA cyclo-addition reaction from a pyridinyl dithioester derivative 1 andan open-chain diene 2. Although DAPy-2Br is a mixture ofisomers, only one structure is considered in this study forbrevity.10e 1H NMR, 13C NMR and HR-MS measurements wereused to verify the structure and purity of DAPy-2Br. Thedetailed synthetic procedures and characterization aredescribed in the ESI.†

After confirming the structure of DAPy-2Br, it was used asan initiator to produce mechanophore-linked polymers(DAPyPn) designed with a mechanophore in each polymerchain via single-electron-transfer living radical polymerization(SET-LRP). The polymerization was conducted at room temp-erature in dimethyl sulfoxide (DMSO) with Cu(0) as the catalystand hexamethylated tris(2-aminoethyl) amine (Me6(TREN)) asthe ligand. For example, the polymer with a number averagemolecular weight (Mn) of 80 kDa and a polydispersity (PDI) of1.4, measured by gel permeation chromatography (GPC), wasobtained after DAPy-2Br and MA (feed ratio = 1/1100) werekept at room temperature for 24 h (DAPyP6). After treating thecrude products by precipitation in methanol, DAPyP6 was

†Electronic supplementary information (ESI) available: Synthetic procedures,computational and experimental details. See DOI: 10.1039/c4py00964a

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer

Chemistry and Physics of the Ministry of Education, College of Chemistry and

Molecular Engineering, Peking University, Beijing 100871, China.

E-mail: xrjia@pku.edu.cn; Fax: +86-010-62751708; Tel: +86-10-62752102

This journal is © The Royal Society of Chemistry 2014 Polym. Chem., 2014, 5, 6893–6897 | 6893

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dissolved in CHCl3 (20 mg mL−1) and subjected to sonicationin an ice bath. Aliquots of the solution were withdrawn atdifferent times and measured by GPC. Fig. 1 shows the GPCtraces of DAPyP6 at different ultrasonic irradiation times. Thepeak broadened and a new shoulder peak appeared upon soni-cation for 0.5 h. Further increasing the sonication time to 3 h,the Mn of the polymer reduced to 43 kDa and PDI decreased to1.3. That corresponded to the cleavage of the polymer near thehalf of the chain.

As reported in the literature, the initial molecular weight isan important factor influencing the scission of polymers.7b Ingeneral, longer polymer chains attached to mechanophoresfacilitate scission reactions. To test this point, a series of poly-mers with different Mn were synthesized by controlling thefeed ratio of MA and DAPy-2Br. The obtained polymers werepurified and sonicated for different times to monitor the dis-sociation process. As a result, we found that the mechanicallyactivated retro RAFT-HDA reaction is also molecular weightdependent. This implies that the chain cleavage process is notinduced thermally.7b,11 The short chain polymer DAPyP1 (Mn =10 kDa, PDI = 1.8) showed no reduction in the molecularweight as compared to the original sample, while for the poly-mers with longer chains (Mn > 30 kDa), we observed thereduction of Mn (Table 1). These results were consistent withthe report by Bielawski, who concluded that the polymers with

Mn > 40 kDa facilitated the ultrasonic-induced dissociation ofthe [4 + 2] cycloaddition adduct from maleimide and furan.7b

To clarify that the polymeric chain scission was indeed dueto the mechanically facilitated retro RAFT-HDA cycloadditionreaction, a control experiment was performed using thesamples prepared from two PMA chains linked with a benzenering in the center, named as HQPn (ESI†). No chain scissionwas observed for the control samples with Mn 43 kDa aftersonication for 3 h, while for the sample of Mn 90 kDa, wefound a slight decrease in Mn (Fig. S2†), which was consistentwith the report by Boydston et al. who compared the chain scis-sion rate constant of PMA homopolymers and mechanophore-containing PMAs. They found that the rate constant of PMAhomopolymer chain degradation was much slower than thescission of polymer chain with a mechanophore under soni-cation.12 These results confirmed that the aforementionedpolymer chain cleavage was from a sonochemically inducedtransformation.

To evidence the ultrasound-induced chain dissociation,UV-Vis and NMR measurements were performed to identifythe released species of dithioester derivative 1 and open-chain

Scheme 1 (a) Synthesis of DAPy-Pn; and (b) cleavage of DAPy-Pn by ultrasound irradiation.

Fig. 1 GPC traces of DAPyP6 before sonication (black), sonication for0.5 h (red) and 3 h (blue).

Table 1 Molecular weight data of the polymers before and aftersonication

Presonication Postsonicationa

Polymer Mnb [kDa] PDIc Mn

b [kDa] PDIc

DAPyP1 10 1.8 10 1.9DAPyP2 21 1.5 18 1.6DAPyP3 30 1.4 27 1.5DAPyP4 38 1.5 25 1.3DAPyP5 49 1.4 30 1.3DAPyP6 80 1.4 43 1.3DAPyP7 82 1.5 42 1.2

a The polymers were dissolved in CHCl3 (20 mg mL−1) and subjected tosonication for 3 h. b Mn refers to the number average molecular weightof the polymers. c PDI refers to the polydispersity index which iscalculated by the equation: PDI = Mw/Mn, where Mw represents theweight average molecular weight. Mn and Mw were determined by GPCwith THF as the eluent and polystyrene as the standards.

Communication Polymer Chemistry

6894 | Polym. Chem., 2014, 5, 6893–6897 This journal is © The Royal Society of Chemistry 2014

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diene 2 upon sonication of DAPyP6 in CHCl3. As known, thedithioesters are usually coloured due to the prohibited π–πtransition of CvS moieties (Fig. S1†).10e To prove this, weobserved the colour change of DAPyP6 solution before andafter sonication. It showed obvious colour transition, from col-ourless to pale yellow after sonication at ambient temperature(Fig. 2a), which gave us a visual evidence of the liberation of 1and directly supported the occurrence of an ultrasound-induced retro RAFT-HDA cycloaddition reaction in our system.As a comparison, the control sample HQP2 showed no colourvariation after sonication for 3 h under the same conditions(Fig. S3†). The released 1 was monitored by UV-Vis spec-troscopy (Fig. 2b), in which the absorption peak at 360 nmcorresponding to the dithioester derivative 1 appeared and itsintensity increased gradually over time. The absorbanceshowed a linear relationship with sonication time, indicatingthat the cleavage reaction rate followed the first-order kinetics.

We performed 1H NMR measurement to detect whetheropen-chain diene 2 was generated during the process. It wasfound that the signals from 5.0 ppm to 6.5 ppm became broadand split into four peaks corresponding to the double bondsof 2; and a new peak around 4.2 ppm correlated to the methylprotons of 2 increased obviously. These results suggested to usthe liberation of open-chain diene 2 from the retro RAFT-HDAreaction (Fig. S4†).

A labelled molecule (S3) with pyrene and cyclopentadiene(Cp) in the structure was synthesized in order to furtherconfirm the released species of 1 (Fig. S5†). S3 and trifluoro-acetic acid (TFA) were mixed with the sonicated solution ofDAPyP6. The resulting mixture was stirred at room temperaturefor 24 h, and then precipitated in petroleum ether. We found

that in the 1H NMR spectrum, a peak at 6.0 ppm emerged,which was related to the newly formed olefinic bond from thecycloaddition reaction. Besides, a new signal at 8.0 ppmassigned to the proton of the pyrene group was observed(Fig. S6†). Further analysis by fluorescence spectroscopyrevealed the excitation and emission band at λ = 342 nm and396 nm, respectively (Fig. S7†). Moreover, the solution oflabelled polymer showed a blue colour when irradiated by UVlight at the wavelength of 365 nm (Fig. S8†). These results evi-denced that 1 was liberated as a reactive moiety.

To gain additional support for the retro HDA reaction, thereleased 1 was used as a RAFT agent to polymerize with St. Wecollected the cleaved polymer after the sonication of DAPyP6and dissolved a portion in St. The mixture was kept under anitrogen atmosphere at 60 °C (Fig. S9†). At this temperature,the HDA adduct is stable and no retro HDA reaction wouldtake place.10b The resulting copolymer was subjected to GPCmeasurements after precipitation in methanol to removeunreacted monomers. We found that a new shoulder peakemerged, which was from the copolymer of DAPyPMA-b-PSt(Fig. S10†). Meanwhile, the peak consistent with the sonicatedsample was still retained because the cleaved polymer withdiene terminal groups yet existed in the system. These resultsimply that the polymerization indeed occurred. In contrast,radical polymerization of St alone under the same conditionsafforded PSt with the molecular weight much higher than theaforementioned polymer. 1H NMR spectra also confirmed thestructure of the DAPyPMA-b-PSt copolymer (Fig. S11†). We alsotried to couple the two parts of cleaved polymer chains, butfailed. That may be due to the low reactivity of open-chaindienes with dithioester derivative 1.

In order to further understand the force-induced chain scis-sion, we calculated the activation energy of the retroHDA-RAFT reaction in our system based on the Extended BellTheory (EBT) (ESI†).7c A RAFT-HDA analogue with a benzylring and a pyridine unit in the structure was employed as amodel for simplifying the calculation process (Fig. S12–14†).We selected two different atom pairs (C7–C20 and C17–C20)marked in blue and red, and applied a pulling force on them

Fig. 3 Computed changes in the activation energy for the cyclorever-sion of the RAFT-HDA adduct using different pulling points.

Fig. 2 (a) Ultrasound-induced colour change of DAPyP6 solutionbefore (left) and after (right) sonication, and (b) UV-Vis spectra ofDAPyP6 in CHCl3 (20 mg mL−1) upon sonication for different time; inset:the plots of absorbance of DAPyP6 at 360 nm vs. sonication time.

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(Fig. 3). The activation energy clearly reduced on increasingthe pulling force on red points (red line), while changing thepulling force on blue points showed no effect on the chainscission (blue line).

In summary, we have synthesized a new mechanophorecomposed of a dithioester derivative 1 and an open-chaindiene 2 via a HDA reaction. It was used to conduct SET-LRP foraffording PMAs with a mechanophore embedded in the mainchain. The stress-sensitive character of the polymers showedmolecular weight dependence. The polymers with a molecularweight over 30 kDa could suffer a fast retro RAFT-HDA reactionat ambient temperature upon ultrasound irradiation. Thereleased fragments were confirmed by UV-Vis, NMR spec-troscopy and by chemical reactions. Notably, the releaseddithioester derivative 1 was evidenced as an active species toreact with Cp or copolymerize with St by RAFT polymerization.The reported mechanophore may broaden the family ofpolymer mechanochemistry and give a new insight into thedevelopment of polymerization methods and new materials.

Acknowledgements

This work is financially supported by the National NaturalScience Foundation of China (21174005, 21274004) to X.-R. Jiaand partially supported by National Basic Research Program ofChina (973 program, no. 2011CB933300). We thank MrJunnian Wei and Prof. Xinghe Fan for the useful discussionsand help in the experiment.

Notes and references

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