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Feb 9, 2017 - builds along the polymer chain under force.3 In recent decades, many kinds of mechanophores, such as spiropyran (SP),4,5 spirothiopyran ...
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An Investigation of the Selective Chain Scission at Centered Diels− Alder Mechanophore under Ultrasonication Han-Yi Duan, Yu-Xiang Wang, Li-Jun Wang, Yu-Qin Min, Xing-Hong Zhang,* and Bin-Yang Du* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: It is a challenging topic to disconnect a linear polymer selectively at the mechanophore site by an external force in a “cold” fashion. In this work, the effect of the power output of ultrasonication on the selective cleavage at the centered urfuryl-maleimide Diels−Alder (DA) mechanophore of poly(methyl acrylate)s (DA-PMA-a and DA-PMA-b) were quantitatively investigated by comparative study on experimental and simulated chain scission kinetics as well as high-resolution 1H NMR spectroscopy (600 MHz). At low power output of the ultrasonication (2.10 W), DA-PMA-a with Mn of ca. 2Mlim (Mlim, below which no further chain scission was observed) presented a DI (degradation index)−t (sonication time) plot with a turnover point at ca. 1.0 and no clear variation of the molecular weight after the turnover, which met well with the calculated center cleavage mode. At 5.52 W, DA-PMA-a and a poly(methyl acrylate) that contained two centered ester bonds (ester-PMA) presented similar DI−t plots with turnover points less than 1.0 within same sonication times, while poly(methyl acrylate) with fully carbon−carbon chain (PMA) had a turnover at DI value of ca. 0.5. By way of contrast, high power output of the ultrasonication (5.52 W) caused a possible cleavage of ester bonds of DA-PMA-a, which would mask the selective cleavage at the DA site. High-resolution 1H NMR result of DA-PMA-b (115.8 kDa, Mn was slightly higher than 2Mlim) showed that DA conversions were up to 55% under 2.10 W and 38% under 5.52 W. The kinetics from GPC traces and 1H NMR results of DA-PMA-b as well as 1H NMR results of DA-PMA-c (68.4 kDa, Mn was slightly higher than Mlim) under sonication confirmed the observation that low power output favored selective chain scission at DA site. The turnover point in the DI−t plot might be used as characteristic parameter to gauge the selective chain scission at mechanophore site for single mechanophore-centered polymers.

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INTRODUCTION Modern polymer mechanochemistry is a field that centralizes on the mechanical-force-driven reactions in polymeric materials and solution.1,2 Mechanophore, a molecular unit that possesses mechanically labile bonds, is the basic element of polymer mechanochemistry that can be selectively activated when stress builds along the polymer chain under force.3 In recent decades, many kinds of mechanophores, such as spiropyran (SP),4,5 spirothiopyran,6 Diels−Alder (DA) adduct,7−9 gem-dichlorocyclopropanes (gDCC),10,11 1,2-dioxetane,12 and silver− carbene moiety,13 were discovered to undergo various chemical reactions upon force. Thereof, when the bond of DA adduct in a polymer in dilute solution is cleaved by the irradiation of the ultrasound, the polymer chain will be disconnected.8 This is different than the newly reported mechanically induced depolymerization that polymer was totally decomposed under force.14 The selectivity of such chain scission at the desired bonds of the mechanophore site is a key factor to force-induced position-oriented polymer degradation,9 which will have a wide application such as controlled release and delivery of drugs.15,16 For a linear polymer, the selectivity of the chain scission at the mechanophore can be defined as the molar ratio of the © XXXX American Chemical Society

cleaved target bonds in mechanophores to all cleaved chemical bonds in the polymer chain under ultrasonication, as shown in Figure 1. Such chain scission selectivity at the mechanophore site of a linear polymer is primarily determined by the chain structure, including the mechanophore structure, the chemical bonds around the mechanophore, molecular weight (MW) and polydispersity index (PDI), and chain conformation.1,17−19 Thereof, some reported mechanophores, such as azo linkage (24−30 kcal/mol),20 peroxide bond (ca. 35 kcal/mol),12,21 disulfide bond (60 kcal/mol),22 and DA bond (ca. 23 kcal/ mol),23 which have lower bond dissociation energes than that of the carbon−carbon bond (88 kcal/mol), were reported to cleave selectively. Generally, the seletive chain scission at the mechanophore site was investigated by the apparent kinetics determined by GPC (gel permeation chromatography) test and simulation of GPC curves based on Gaussian cleavage model.24 In addition, the determination of the residual groups of the cleaved polymers was often interfered by other signals from the Received: November 2, 2016 Revised: January 26, 2017

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Figure 1. Possible chain scission site around the mechanophore of a linear polymer in a dilute solution (using DA linkage-centered polymer as an example).

Scheme 1. Syntheses of DA-PMAs, Ester-PMA, and PMA from Different Initiatorsa

a The syntheses of initiator 1, 2-bromo-2-methyl-propionic acid 1-(2-bromo-2-methyl-propionyloxymethyl)-3,5-dioxo-10-oxa-4-azatricyclo[5.2.1.02,6]dec-8-en-4-ylmethyl ester, 2, 2-bromo-2-methylpropionic acid 4-[2-(2-bromo-2-methylpropionyloxy)ethyl]-3,5-dioxo-10-oxa-4azatricyclo[5.2.1.02,6]dec-8-en-1-ylmethyl ester, and 3, 2-bromo-2-methylpropionic acid 2-(2-bromo-2-methylpropionyloxy)ethyl ester are described in the Supporting Information (Figures S1−S9).

impurities because these signals were rather weak in 1H NMR spectra, and the signals from nonselective chain scission at other bonds were often indistinguishable. Therefore, a comparative investigation of kinetic analysis and high-resolution 1 H NMR characterization is required for evaluating the selective cleavage at the mechanophore site.24,25 To study the chain scission kinetic, the ultrasonication conditions should be judiciously selected and strictly fixed for the testing. The polymer mass concentration is preferably ≤3.0 mg/mL that could approximately avoid the chain entanglement.10,26 The power output of the ultrasonication is an important factor for selective chain scission because the vibration amplitude of the ultrasonic probe depends on the power output applied and the diameters of the probe. Higher vibration amplitude leads to bigger cavitation. Previously, the ultrasonic probe with a sectional diameter of ca. 13.0 mm was exclusively applied (Table S1), and the applied power intensity of the ultrasound (the ratio of the power output to the sectional area of the probe) was 7.9−14.6 W/cm2 for 10−12 mL polymer solution; therefore, the power output of the ultrasonication was ca. 10.2−18.6 W.4,24,26−28 In general, as power intensity increases, the rate of chain scission increases due to more cavitation events per unit of volume.29 However,

the effects of low power output of the ultrasonication on the chain scission of high MW polymers are rarely explored. In this work, the effect of various low power outputs (2.10− 5.52 W) of the ultrasonication for DA-centered poly(methyl acrylate)s were investigated via the comparative studies on the experimental and simulated chain scission kinetics as well as the precise determination of the end groups of the polymer. We observed that a lower power output of the ultrasonication favored the selective cleavage at predesigned DA site for a single DA-centered polymer with high MW.

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RESULTS AND DISCUSSION Syntheses of Polymers. To investigate the selective chain scission at the predesigned mechanophore site of a polymer under ultrasonication, DA-centered poly(methyl acrylate)s (DA-PMAs) were synthesized by using single-electron-transfer living-radical polymerization (SET-LRP) (Scheme 1).30 The DA moiety was chosen as the model mechanophore because it had low activation barrier energy (ca. 23 kcal/mol) and could be cleaved via retro-DA (r-DA) reaction upon ultrasonication at low temperature (ca. 0−20 °C).7,8,15 DA-PMA-a and DA-PMAb with MWs of 105.0 and 115.8 kDa, respectively, were synthesized by using initiators 1 and 2 (cf. Scheme 1 and Figure S10). The molecular weights of DA-PMA-a and DA-PMA-b B

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Figure 2. Simulated GPC curves (from determined GPC curve, black curve in each chart) calculated from different cleavage models. (A) Center cleavage model. (B) Random cleavage model. (C) Gaussian cleavage model. The insets sketch the probability of the cleavage of different sites in polymer chain.

were ca. 2-fold of the threshold molecular weight (Mlim, ca. 50.0 kDa), below which no further chain scission was observed under ultrasonication.31,32 Ideally, both were supposed to cleave one time during ultrasonication and hence suitable for the kinetic study. In addition, DA-PMA-c, with a Mn of 68.4 kDa which was higher than Mlim and synthesized from initiator 1, ester-PMA which had two alkoxy ester bonds in the midpoint of the chain (Mn: 97.9 kDa, PDI: 1.23, ca. 2Mlim), and a poly(methyl acrylate) with fully carbon−carbon chain (denoted as PMA, Mn: 85.4 kDa, PDI: 1.25, ca. 2Mlim) (Figure S10) were obtained by the same method as controls for comparing the contribution of DA mechanophore and ester bonds to the chain scission. Simulation of the Chain Cleavage Mode. Modeling of polymer chain length distribution upon both selective and Gaussian distribution was conducted using the procedure specified by Glynn and co-workers33 and used as references for the following experimental studies. The chain cleavage kinetic mode was simulated by using the GPC curve of DA-PMA-a as the initial data. The primary assumptions for the modeling include (i) each polymer chain is independent and has no effect on the degradation of other polymer chains in the system, (ii) only one chemical bond will break when a chain is chosen to be degraded, (iii) the molecular weight of initiator is not taken into consideration, and (iv) mechanical force is the only external stimulus leading to the chain cleavage. The detail of the simulation is given in the Supporting Information (Figure S13). Three typical evolution situations were simulated, and the corresponding GPC curves are presented in Figure 2. To exclude the effects of reaction kinetics which depends on temperature, polymer concentration, and sonication intensity, we followed Glynn’s method and used degradation index (DI) as a general indicator of extent of degradation. DI is defined as the ratio of the chain breaking events to the original number of

polymer chains. In the real scenario, DI could also be calculated by using eq 1

DI =

M n,0 M n, t

−1 (1)

where Mn,0 is the initial Mn and Mn,t is the molecular weight when chain breaking happens t times. Therefore, the experimental DI value can be used to simulate GPC curves, and the difference between the simulation and experimental results would be found. As shown in Figure 2A, polymer chains follow a center cleavage pattern where chemical bonds at the center of polymer chains have dramatically greater chance to be broken compared with those bonds at other places. The highMW portion of simulated GPC decreased gradually, and a portion of lower molecular weight increased slowly as the DI value increased. A shoulder peak emerged and grew gradually and a platform with double peaks can be observed when DI was 0.4. Figure 2B shows a random cleavage model where every site of the polymer chain has the same probability of bond cleavage. The molecular weight decreased gradually, and there was not shoulder peak or platform as DI evolved. The decreasing height and broadening width of GPC traces with random cleavage model contrast significantly with those of GPC traces with the center cleavage pattern (Figure 2A). A typical Gaussian cleavage model can be seen in Figure 2C, which is an intermediate pattern between center-selective scission and random cleavage. Like the random cleavage scenario, Figure 2C exhibits no signs of shoulder peaks and platform curves with double peaks. However, the height of these traces first declined then increased again, which resembles the pattern of centerselective cleavage. Further kinetic analysis is needed for differentiating the center and Gaussian cleavage models. C

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Figure 3. GPC traces showing the effects of ultrasound with different output energies (1.0 s on, 2.0 s off; irradiation time refers to total time of on) on the cleavage of polymers. (A) DA-PMA-a, 2.10 W; (B) DA-PMA-a, 5.52 W; (C) ester-PMA, 2.10 W; (D) ester-PMA, 5.52 W; (E) PMA, 2.10 W; (F) PMA, 5.52 W.

Experimental Chain Scission Kinetics. Bearing the above simulation results in mind, comparative degradation kinetic (DI vs sonication time t) studies were carried out under strict ultrasonication conditions, using the equipment shown in Figure S14. The power output of the ultrasound (5.52 and 2.10 W) was strictly calibrated by the reported method4 (Figure S15 and Table S2). The volume of the solution (10.0 mL) and the distance of the probe to the bottom of the cell (10.0 mm) were fixed for ultrasonication. DA-PMA-a (ester-PMA, PMA)/THF concentration was 3.0 mg/mL and irradiated by ultrasound with a sonication probe with a diameter of 3.24 mm in dried THF under an argon atmosphere at 6−9 °C for 120 min. The chain scission kinetics of all polymers were monitored by GPC tests under the identical conditions (Figure 3). The variation of the GPC traces of DA-PMA-a with increasing time under 2.10 and 5.52 W (Figure 3A,B) was nearly identical to the prediction of Figure 2A, which indicated that the evolution of GPC curves of DA-PMA-a with time under 2.10 and 5.52 W could be considered to obey a center cleavage model. Furthermore, the determined DI values of DA-PMA-a under 2.10 W were used to simulate the GPC data based on center cleavage model (Figure 2A); the calculated GPC curves agreed well with those of real ones at each DI value with accepted deviations (Figure S17). On the other hand, GPC traces of ester-PMA and PMA under 2.10 and 5.52 W (Figure 3C−F) were mostly similar to

the case of Figure 2C, which is a Gaussian cleavage model. It was observed that Mns of the daughter polymers were nearly half of the parent polymers regardless of the different power outputs employed, as summarized in Table 1. Clearly, only Table 1. Molecular Weight Data of DA-PMA-a, DA-PMA-b, DA-PMA-c, Ester-PMA, and PMA before and after Ultrasonication (120 min) 2.10 W

5.52 W

samples

Mn (kDa)

PDI

Mn (kDa)

PDI

Mn (kDa)

PDI

DA-PMA-a DA-PMA-b DA-PMA-c ester-PMA PMA

105.0 115.8 68.4 97.9 85.4

1.30 1.09 1.25 1.23 1.25

51.7 56.0 42.1 45.5 40.0

1.25 1.28 1.26 1.27 1.26

50.5 60.2 52.8 49.6 48.8

1.26 1.26 1.21 1.27 1.27

depending on the comparison of the initial and final GPC data as well as the evolution of GPC curves, it is hard to deduce the selective cleavage at the DA site of DA-PMA-a. Therefore, a more detail chain scission kinetic should be further explored for the estimation of the selective cleavage at the DA site for DAPMA-a. From the GPC data in Figure 3, eq 2, which was developed by Malhotra and co-workers based on the Gaussian cleavage D

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Figure 4. Plots of DI vs sonication time of DA-PMA-a, ester-PMA, and PMA under ultrasonication with power outputs of 2.10 W (A) and 5.52 W (B), respectively. DI = Mn,0/Mn,t − 1.

model,34 could be used to calculate the rate constant of the chain scission for various polymers: 1 1 k = + t M n, t M n,0 M0

Table 2. Degradation Rate Constant of DA-PMA-a, DAPMA-b, Ester-PMA, and PMA under the Irradiation of Ultrasonication (Slope = DPk, 105k/min−1)a power output

(2) 2.10 W

where Mn,0 is the initial molecular weight, M0 is the molecular weight of the monomer unit, and Mn,t is the number-average molecular weight obtained at the time t under ultrasonication. Since DI was defined as eq 1, we have DI =

M n,0 M0

kt = DPkt

DA-PMA-a DA-PMA-b ester-PMA PMA

(3)

5.52 W

k1

k2

k1

k2

1.52 1.07 0.87b

0 0.17 0.17b

1.65 1.02 0.60b 1.74 1.23

0.29 0.15 0.11b 0.23 0.31

0.79 0.92

a

DP of DA-PMA-a, DA-PMA-b, ester-PMA, and PMA were estimated to be 1220, 1337, 1137, and 992, respectively, based on their Mns from GPC data.k1 and k2 are the rate constants before and after turning point during ultrasonication. bEstimated by the 1H NMR spectrum.

where k is the rate constant of the chain scission and DP is the degree of the polymerization of the initial polymer. Clearly, the mechanically degradation rate is closely related to the degree of the polymerization of the initial polymer.28,29,35 According to eq 3, the plot of DI−t will be linear with a slope of DPk if a Gaussian cleavage model was employed. For the center cleavage of a polymer with a Mn of ca. 2Mlim, a turnover point close to 1.0 would be observed within a certain time, followed a DI−t plot with k value close to zero, which is the kinetic characteristic of a highly selective and effective chain scission at the center of the polymer. The DI−t plots of DA-PMA-a, ester-PMA, and PMA under ultrasonication with power outputs of 2.10 and 5.52 W are shown in Figure 4. When the power output of the ultrasonication was 2.10 W, DA-PMA-a showed a clear turnover point close to 1.0 at ca. 50 min, after which the rate constant went to nearly zero, which meant that DA-PMA-a was cleaved rapidly until a critical time. Such DI−t plot of DAPMA-a was well in line with the prediction of the eq 3 under 2.10 W. Ester-PMA and PMA were also cleaved at 2.10 W but presented a linear DI−t plots within 120 min, which were well consistent with the Gaussian cleavage model. The degradation rate constants of ester-PMA and PMA were 0.79 × 10−5 and 0.92 × 10−5 min−1, respectively, which were much lower than that of DA-PMA-a before 50 min (1.52 × 10−5 min−1, Table 2). By way of contrast, the chain scission of DA-PMA-a was more selective at the DA site. It should be noted that the chain scission of ester-PMA and PMA did occur under 2.10 W of power output. Therefore, it is better to find a threshold (or minimum) power output, below which no chain scission of ester-PMA and PMA happens, might be more suitable for selective chain scission at DA site. In general, the power output of the ultrasonication could be turned by using sonication probes with different diameters and changing power amplitude.

When the power output of the ultrasonication was 5.52 W, DA-PMA-a and ester-PMA showed similar two-stage DI−t plots with clear turnover points at DI values of 0.88 and 0.75, respectively, at ca. 40 min (Figure 4B). Although ester-PMA did not contain DA bonds, the rate constant of ester-PMA at the first stage was 1.74 × 10−5 min−1, which is comparable to that of DA-PMA-a (1.65 × 10−5 min−1). Therefore, it is hard to judge the selective chain scission at the DA site for DA-PMA-a under 5.52 W through macroscopic degradation kinetics. Moreover, the DI−t plot of PMA without DA and ester bonds was also cleaved in a relative small rate constant of 1.23 × 10−5 min−1 and presented a turnover point at DI value of ca. 0.5. By comparing the three plots in Figure 4B, it was more possible that the ester bonds in DA-PMA-a and ester-PMA could be effectively cleaved under 5.52 W. That is for ester bond-linked DA-centered polymers, higher power output would cause more nonselective chain scission. Previously, Xia et al. reported that the centered ester bonds in a polymer with a disulfide mechanophore were broken upon ultrasound irradiation.36 Our recent work also suggested that the ester bond linked to spiropyran unit of an amphiphilic block copolymer in THF/water mixed solvent was also cleaved upon ultrasonication.37 In contrast to the carbon−carbon bond with small dipole moment, the ester bond, which has strong dipole moment, might be easier to cleave under high power output of ultrasonication. Once the ester bond of DA-PMA-a cleaved, MW of the resultant polymer was also halved, while DA linkage was retained, which led to the decrease of the selectivity of rDA reaction of DA-PMA-a under 5.52 W. These phenomena could not be distinguished by kinetic comparison under high E

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Figure 5. 1H NMR spectra of DA-PMA-b that sonicated for 20, 40, and 80 min under power output of (A) 2.10 W and (B) 5.52 W (600 MHz, 1500 scanning times, d6-DMSO). No sampling operation during each sonication experiments.

Figure 6. Plots of DI (DA conversion) vs sonication time of DA-PMA-b under ultrasonication with power outputs of 2.10 W (A) and 5.52 W (B). DI = Mn,0/Mn,t − 1. DA conversion is the integral ratio of A7.67/(A7.67 + A5.14) in Figure 5.

(DA-PMA-b, Mn: 115.8 kDa; PDI: 1.09) was characterized by high-resolution 600 MHz 1H NMR spectroscopy at different sonication times under 2.10 and 5.52 W, respectively. The chemical shifts of the remained DA group of DA-PMA-b and the free furan ring could be respectively identified to be 5.14 and at 7.67 ppm without interference of other impurities and thus could be used to calculate the DA conversion quantitatively after each ultrasonication (Figure 5). DA conversion vs sonication time of DA-PMA-b under 2.10 and 5.52 W is plotted in Figure 6. DA conversions of DA-PMA-b under 2.10 W were calculated to be 35, 45, and 55% for 20, 40, and 80 min ultrasonication, respectively, and higher than those under 5.52 W (22, 32, and 38% for 20, 40, and 80 min, respectively). This result strongly confirmed that low power output of ultrasonication favored the selective chain scission at DA site and was well consistent with the kinetic result of DA-PMA-a. The plot of DA conversion vs sonication time under 2.10 W showed a higher k1 of 0.87 × 10−5 min−1 than that under 5.52 W (0.60 × 10−5 min−1, Table 2). Note that the DI−t plot of DA-PMA-b from the GPC traces showed higher chain scission rate constants than those estimated from NMR results, as shown in Figure 6 (and Table 2). Clearly, the nonselective chain scission at other site of the polymer chain occurred. Based on 1H NMR results, the turnover point of DI−t plot of DA-PMA-b approached to 1.0 within 80 min under 2.10 W. While in the case of 5.52 W, the turnover point was ca. 0.78. Therefore, the turnover point of the DI−t plot could be regarded as an effect parameter to gauge the chain scission selectivity at DA site. When the turnover point of DI−t plot was more close to 1.0, the chain scission was more selective at the DA site, which was also proved the

power output. In addition, k2 values of three samples in Figure 4B were in the range of (0.29−0.31) × 10−5 min−1 (after 40 min ultrasonication), indicating that small portion of high-MW polymers were retained due to less selective chain scission. The following 1H NMR results were performed, and the results confirmed the above deductions. Nuclear Magnetic Resonance (NMR) Characterization. The precise characterization of the end group of the polymers could be helpful for directly evaluating the selective cleavage. Because of quite low concentration of the residual groups, trapping or isotope labeling the residual groups was often applied to detect the expected chain scission qualitatively.1,2,27,38 However, the premise of this method is (1) the expected end groups could be produced without any side reaction during ultrasonication, (2) the end groups could react with the exogenous agents highly efficiently and irreversibly, and (3) the excessive unreacted labeling agents could be removed completely, which do not apply to certain practical situation.26 The NMR spectrum is a powerful method for precisely characterizing the end groups of the daughter polymers after ultrasonication.24,25,39 Very recently, Moore and co-workers reported a direct characterization of the cycloelimination of a β-lactam mechanophore by using 500 MHz 1H NMR spectra.24 Craig and co-workers reported the mechanochemical cleavage of a coumarin dimer in the chain center by using 400 and 500 MHz 1H NMR spectrometer.25 Sijbesma and co-workers reported the mechanical cleavage of DA adducts of π-extended anthracene and maleimide by using 400 MHz 1H NMR.39 Our group succeeded in using the direct 1 H NMR characterization for a DA-rich thermoset polymer in 2015.9 Herein, the chain scission kinetic of a DA-centered PMA F

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thinner probe with a low power output (2.10 W) presented high selective chain scission at DA site. By comparative study on the GPC and NMR results, this work decoupled the selective scission at DA mechanophore site and random scission at the ester bond (and C−C bond) at high power output for a polymer with a single DA mechanophore. Very recently, Craig and co-workers clarified the selective gDCC mechanophore activation relative to the C−C scission for a polymer with multiple mechanophores;28 the power of ultrasound shows an effect on the conversion of gDCC for given MW. The present work clarifies the effect of low power output (2.10−5.52 W for 10 mL solution, using a thinner probe) on the selective chain scission at single DA mechanophore at the center of a polymer.

sequence order of turnover point of 1.0 (2.10 W) > 0.8 (5.52 W) for DA-PMA-a (Figure 4). The chain scission kinetic study for DA-PMA-b was also carried out by using the same continuous interval sampling method as DA-PMA-a, as shown in Figure S18. It was found that the DI values at 20, 40, and 80 min in Figure 6 were identical to those DI values in Figure S18, indicating the satisfied repeatability of the sonication tests. The turnover points of DI−t plots for DA-PMA-b were estimated to be 0.90 and 0.78 at 60 min for 2.10 and 5.52 W, respectively, which suggests again that selective chain scission at DA site corresponds closely to the kinetic character of a high DI value at turnover point of the DI−t plot. The chain scission rate constants of DA-PMA-b at 2.10 and 5.52 W were estimated to be 1.07 × 10−5 and 1.02 × 10−5 min−1, respectively, and nearly the same (Table 2), but lower than those of DA-PMA-a. Possibly, the differences of their DA structures (Scheme 1), higher Mn (>2Mlim), and narrower PDI of DA-PMA-b caused kinetic deviation between DA-PMA-a and DA-PMA-b. The chain scission of the daughter polymers of DA-PMA-b was observed (Figure S18) when prolonging the sonication time.28 Once the daughter polymers produced, the second chain scission occurred and resulted in the deviations of the apparent kinetics of DA-PMA-b from the ideal center cleavage model. We also examined a DA-centered PMA with a Mn of 68.4 kDa that was slightly higher than Mlim (DA-PMA-c) under various power outputs by using high-resolution 1H NMR spectroscopy. The DA conversion under ultrasonication was estimated to be 40, 30, and ca. 20% for 2.10, 4.31, and 5.52 W, respectively (Figure 7). Correspondingly, Mns of DA-PMA-c

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CONCLUSIONS In summary, the chain scission kinetic of DA-PMAs under various power outputs of ultrasonication was investigated by comparative mathematical simulation and kinetic studies as well as the direct characterization of the end groups of the daughter polymers by high-resolution 1H NMR spectroscopy. Compared with the chain scission behaviors of ester-PMA and PMA, the chain scission of DA-PMAs with ca. 2Mlim was more selective at DA site and underwent a center cleavage model at low power output of 2.10 W. High power output of 5.52 W caused a low selective scission at DA site due to the chain scission at the ester bond and C−C bond. The rate constants of selective (DA) and nonselective scissions (C−O and C−C) at high ultrasonication power were clearly decoupled. Therefore, to improve the selectivity of mechanophore cleavage at the center of a polymer, a polymer for testing should have a Mn of ca. 2Mlim that ensured all chains could be cleaved only one time. Moreover, a threshold power output should be first investigated because low-energy output will ensure minimal chain cleavage at other sites of the polymer chain. Since it is hard to use NMR technique to capture the weak signals from the chain scission at other sites, such as C−O and C−O bonds, the turnover point of the DI−t plot is proposed to be used as a parameter for gauging the selective chain scission at mechanophore site for polymers with single mechanophore in the center. The turnover approaches to 1.0 fast means highly selective chain scission at mechanophore site of a polymer. The result of this work could be used to judge force-induced site-specific chain scission at the mechanophore site of high-MW polymers.

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Figure 7. 1H NMR spectra of DA-PMA-c sonicated at power outputs of 2.10, 4.31, and 5.52 W (600 MHz, d6-DMSO).

EXPERIMENTAL SECTION

Synthesis of Poly(methyl acrylate)s. 5.2 mg of Cu(0) powder and 2.5 mL of dried DMSO were added into a vial, and the mixture was sonicated for 5 min. An aliquot of the 0.92 mL mixture (1.16 mg, 0.025 mmol, 2.5 equiv) was removed and added to a 10 mL Schlenk flash equipped with a Teflon stir bar. 1.7 mL of methyl acrylate (1.6 g, 18.78 mmol) as monomer and 8 μL of Me6TREN (0.030 mmol, 3 equiv) as the ligand were also added into the Schlenk flash. The Schlenk flash was then sealed, and three cycles of freeze−pump−thaw were conducted to remove the dissolved oxygen with the use of Schlenk line. At the last cycle, the initiator (0.010 mmol, 1 equiv) was added into the Schlenk flash when it is filled with nitrogen and soaked into the liquid nitrogen. Finishing the three freeze−pump−thaw cycles, the Schlenk flash was allowed to stir in a water bath after being backfilled with nitrogen. The polymerization was conducted in 25 °C for 5−10 min and then exposed to air, and 5 mL of THF was added to quench the polymerization. The liquid mixture was filtered through a short pad of silica gel to remove the metal catalyst. Solvent was removed by a vacuum, and the concentrated polymer solution was

sonicated at 2.10, 4.31, and 5.52 W were decreased to 42.1 (PDI: 1.26), 48.7 (PDI:1.27), and 52.8 kDa (PDI: 1.21), respectively. This result also suggests that 2.10 W favored the selective cleavage at DA site of DA-PMA under ultrasonication, which well agreed with the above kinetic results of DA-PMAs with Mns of ca. 2Mlim. In addition, a control ultrasonication test for DA-PMA-c was carried out by using a sonication probe of 12.7 mm under the power output of 5.23 W (the calibration is seen in Figure S16); the DA conversion was estimated to be 28% (curve 3 in Figure S19), which was higher than that of similar power output in the case of 3.24 mm sonication probe. Under the nearly same power output, the sonication probe with a diameter of 12.7 mm will generate small cavitation due to relatively small vibration amplitude. As a result, relatively more amounts of DA linkages could be cleaved. This explained that a G

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added dropwise into 300 mL of methanol. The resulting polymer was collected and dried under vacuum at 45 °C. DA-PMA-a: Mn: 105.0 kDa, PDI: 1.30; DA-PMA-b: Mn: 115.8 kDa, PDI: 1.09; DA-PMA-c: 68.4 kDa, PDI: 1.30; ester-PMA: Mn: 97.9 kDa, PDI: 1.23; PMA: Mn: 85.4 kDa, PDI: 1.25. Sonication Experiment. The output power of the sonicator under different amplitude was calibrated according to the literature procedure (see Supporting Information). 30 mg of polymers was dissolved into 10 mL of THF, and the solution was transferred into the two-necked Suslick cell. The diameter of the probe was about 3.24 mm, and the distance between the titanium tip and bottom of the Suslick cell was 1.0 cm. Before conducting the process of sonication, a cup of THF was placed in the sealed box where the sonication setup was installed in then a saturated vapor of THF can be obtained. The two-necked Suslick cell was sparged with argon for 20 min and cooled with an ice bath throughout the entire sonication process in order to maintain a constant temperature of 6−9 °C. Pulsed ultrasound (1 s on, 2 s off) was then applied to the system. As the sonication was started, aliquots of 200 μL solution were removed at 0, 5, 10, 20, 30, 40, 60, 80, 100, and 120 min (continuous sonication time) from the cell for GPC analysis; the residue of the polymer solution was used for NMR measurements. GPC and NMR Measurements. The polymer solution was filtered through a syringe filter (PTFE, 0.22 μm pore size) and analyzed by GPC. The number-averaged molecular weight (Mn) and polydispersity index (PDI) were determined by using a gel permeation chromatograph (PL-GPC 220) equipped with an HP 1100 pump from Agilent Technologies. The residue polymer solution after ultrasonication was transferred to a flask, and the solvent was quickly removed by vacuum at room temperature to obtain the resulting polymer. The polymer was collected and measured by 600 MHz NMR (Aligent DD2-600). Simulation. All simulations were done by using Matlab R2012a on the Asus Q302LA. The initial number of chains for calculation (N) were set as 1000. Detailed information on simulation is given in the Supporting Information.

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REFERENCES

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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/acs.macromol.6b02370. General methods and instrumentation, syntheses and spectral details of initiators and polymers, simulation process, sonication details, and other experimental data (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-H.Z.). *E-mail: [email protected] (B.-Y.D.). ORCID

Xing-Hong Zhang: 0000-0001-6543-0042 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support by the National Science Foundation of the People’s Republic of China (21604071 and 21322406). The authors also thank the reviewers for their constructive suggestions. H

DOI: 10.1021/acs.macromol.6b02370 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b02370 Macromolecules XXXX, XXX, XXX−XXX