Mechanochemical Degradation of Denpols: Synthesis and Ultrasound

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Mechanochemical Degradation of Denpols: Synthesis and UltrasoundInduced Chain Scission of Polyphenylene-Based Dendronized Polymers Gregory I. Peterson, Ki-Taek Bang, and Tae-Lim Choi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05110 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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Journal of the American Chemical Society

Mechanochemical Degradation of Denpols: Synthesis and Ultrasound-Induced Chain Scission of Polyphenylene-Based Dendronized Polymers Gregory I. Peterson,† Ki-Taek Bang,† and Tae-Lim Choi* Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea ABSTRACT: New polyphenylene-based dendronized polymers (denpols), exhibiting extended and rigid conformations, were prepared using ring-opening metathesis polymerization (ROMP). Their mechanochemical degradation was explored in ultrasoundinduced elongational flow fields. Degradation rate constants were obtained for polyphenylene-based denpols, of varying generation, across a degree of polymerization (DP) range of ca. 100 to 600. In general, it was found that larger side chains led to increased degradation rates and that the rate enhancement was proportional to the natural log of persistence length (Ln(lp)) or the square root of monomer molecular weight (Mmon0.5). These relationships led to the generation of "master curves" in which the rate constant trends for each polymer series converged, enabling accurate prediction of degradation rate constants for related polymers bearing long alkyl chains or ester-type dendrons. Furthermore, we observed evidence for, and used computational modeling to support, polymer chains undergoing multiple scissions during a single elongation event, leading to faster degradation of daughter fragments that come from parent polymers with large side chains.

Introduction One of the defining features of polymer mechanochemistry is the use of polymers as scaffolds for directing a mechanical input into a chemical output.1 While the simplest chemical output is polymer backbone bond homolysis, with the introduction of specific force-responsive moieties, termed mechanophores,2 into the polymer backbone, the chemical output can be tailored for specific applications such as force sensors, mechano-catalysts, and self-healing materials.3 In addition to the development of new mechanophores,4 recently much attention has been given to identifying how the regioand stereochemistry of mechanophores,5 and linkers attached to mechanophores,6 can influence mechanochemical reactivity. Often the polymer chains themselves are treated simply as "generic handles" for applying force to mechanophores.7 While several studies have shed light on the importance of the polymer backbone to mechanochemical reactions,8 fundamental questions about the role of the polymer backbone remain. One question that we were intrigued by was, how does the size of side chains on a polymer influence its mechanochemical reactivity. Conflicting results have been obtained in regards to the role of side chain size and its effect on degradation kinetics. In solution, mechanical forces are commonly applied to polymers via ultrasound-induced elongational flow fields.1g This is a method that is particularly conducive to mechanistic and kinetics studies due to its use of small sample quantities, high

reproducibility, and compatibility with common analytical techniques. For linear polymers, ultrasonic degradation rates increase with increasing molecular weight (M) or degree of polymerization (DP), and degradation does not occur below a limiting M or DP. One of the earliest studies that probed the role of side chains size was by Thomas,9 who compared the ultrasonic degradation rates of poly(methyl methacrylate) (PMMA) and poly(dodecyl methacrylate) (PDDMA) and observed 3.5 times faster degradation for PDDMA compared to PMMA of the same DP. Malhotra,10 and Daraboina and Madras,11 also compared the ultrasonic degradation of poly(alkyl methacrylates). In both studies, their data suggested that polymers with larger side chains degrade faster, but exact rate constant trends were not identified.12 Contrary to these works, Moore and coworkers systematically studied the degradation of poly(acrylates) and found that the size of the side chain (comparing methyl, ethyl, and butyl isomers) did not influence degradation rate trends when comparing polymers based on their DP.7 Simon and coworkers also observed that the side chain size was inconsequential to degradation for polystyrenes (PS) (comparing PS to brominated PS) and polynorbornenes (PNB) (comparing PNBs with and without a silyl protecting group on the side chain) when comparing polymers with equivalent DP.13 The conclusions of the earlier studies (Thomas, Malhotra, and Madras) are harder to defend as they each have shortcomings (such as lack of comparisons between polymers with consistent DP or M, limited number of comparisons, use of

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polymers with broad or unknown molecular weight dispersity, ÐM, or lack of replicate experiments), while the later studies may not have used monomers with side chains having large enough molecular weight differences to see an effect. Moore and coworkers speculated themselves whether their observed trends would hold with significantly longer alkyl chains.7 The ultrasonic degradation of polymers with very large side chains (e.g. graft or brush polymers) has been much less studied. Sheiko and coworkers explored the degradation of a series of poly(alkyl methacrylate-g-butylacrylate)s with constant backbone DP and increasing side chain DP.14 They found that as the side chain length increased, so did the overall degradation rate of the polymer (which was attributed to larger side chains having increased drag forces), but an exact relationship between the rate enhancement and the side chain length was not established. While the stability of graft polymers has been studied extensively using other elongational flow techniques, mostly contradictory results have been obtained in regards to the role of side chains.1e Overall, we concluded from our literature survey: small side chains may or may not increase the degradation rate, and large side chains do increase the degradation rate. However, the lack of a systematic study of polymers with consistent backbones, bearing small (e.g. short alkyl chains) up to very large (e.g. polymeric) sidechains, across a broad DP range (enabling generalized trends to be established), motivated us to pursue the topic ourselves. Determining the effect of side chain size alone on ultrasonic degradation rates requires comparing structurally consistent polymers. Ideally that means having identical backbones (so that the same bonds are breaking), fixed density of side chains, and constant side chain size for each side chain type being compared. We believed dendronized polymers (denpols) would be well-suited for meeting this criteria. Denpols are unique macromolecules containing dendronized side chains which enable precise control of the side chain structure and access to a wide range of molecular weights by tuning the dendron generation15. Unlike graft polymers, denpols do not have a dispersity associated with the side chain, which is important given that the effect of side chain dispersity on degradation rates is unclear. The structural versatility of denpols is an attractive feature that has led scientists to examine their behavior in a variety of fields, including photonics,16 electronics,17 biomaterials,18 self-assembly,19 drug delivery,20 waste treatment,21 energy storage,22 and rheology.23 However, the ultrasonic-degradation behavior of denpols has not been described. Polymer chemists have devoted much attention to the development of efficient synthetic methods for denpols. Early on, most denpols were synthesized by attaching dendrons at functional sites on the main chain in a convergent (graft-to)24 or divergent (graft-from)25 manner. These approaches, however, often unavoidably lead to defects in the dendron, especially for high generations, as well as inconsistent density

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of dendrons on the backbone. Directly polymerizing welldefined macromonomers (a graft-through approach)26 is an alternative method which enables the synthesis of dendrons without structural defects and polymers with dendrons on every repeat unit. In general, synthesis of denpols having high DPs is challenging due to steric repulsion between the bulky side chains during propagation. Gratifyingly, ring-opening metathesis polymerization (ROMP)27 of macromonomers with high-ring-strain cycloalkenes (such as norbornene) has been utilized to overcome this synthetic hurdle. Moreover, highly reactive fast-initiating Grubbs catalysts enable the synthesis of high-M denpols in a living manner, enabling molecular weight control and low dispersity,28 which are ideal for preparing polymers for a mechanochemical study. Recently, our group prepared PNBs containing up to fifth-generation ester-based dendrons.29 Moreover, ROMP of (endo-tricycle[4.2.2.0]deca3,9-diene) (TD) monomers afforded another series of poly(TD) (PTD) dendronized polymers with up to fourth generation dendrons with narrow dispersity (below 1.22).30 Scheme 1. General Synthetic Route for the Synthesis of Macromonomers.a

a) Reactions were conducted with the following general conditions: Diels-Alder reaction: o-xylene, 120 ºC or 150 ºC, 12 h; ester reduction: LiAlH4, tetrahydrofuran (THF), room temperature (RT), 1 h; bromination: CBr4, PPh3, THF or CH2Cl2, RT, 1 h; and substitution: triethylamine, N,N-dimethylformamide, 55 ºC, 12 h.

Scheme 2. Synthesis of Denpols with Müllen's Polyphenylene Dendrons and Other Polymers.a


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a) PTD-G0/G1 were obtained via ROMP using Grubbs 3rd-generation catalyst in CH2Cl2 at RT. PTD-G2 was obtained via ROMP using Grubbs 2nd-generation catalyst in dichloroethane at 50 ºC. PTD-EH/OD/eG3/eG4 were obtained via ROMP using Grubbs 3rd-generation catalyst in THF at RT.

For this report, we were inspired by the polyphenylene dendrimers developed by Müllen and coworkers,31 which have rarely been used in denpols.32 The bulky nature of these dendrons makes their polymerization difficult (i.e. obtaining high M with narrow dispersity), but we imagined ROMP would be capable of overcoming this challenge. Furthermore, we were intrigued by the novelty of these types of polymers and the potential to reach high Ms for side chains, even at low dendron generations. Herein, we describe the synthesis of a new class of polyphenylene-based denpols via living ROMP, and the comparison of their ultrasonic degradation to other PTDs with large side chains (overall, covering a range of side chain masses from ca. 100 Da up to ca. 3 kDa), to further study the role of side chain size on the kinetics of mechanochemical chain scission.

Results and Discussion Macromonomer and Polymer Synthesis. In order to synthesize denpols with Müllen's polyphenylene dendrons, we first prepared macromonomers containing small to large dendrons (Scheme 1). In general, these macromonomers were prepared by a convergent approach, specifically using a DielsAlder reaction between cyclopentadienone derivatives (1 and 2) and electron poor alkynes (3 and 4). The Diels-Alder reaction between building blocks 1 and 3, 1 and 4, and 2 and 4

yielded generation-zero (G0), first-generation (G1), and second-generation (G2) dendrons, respectively, which each contained an ester moiety (5) as a handle for further modification. Subsequently, reduction and bromination (6) followed by a simple substitution reaction with 7, yielded the final TD macromonomers (TD-G0, TD-G1, and TD-G2). Notably, macromonomers, along with all precursors, were purified by flash column chromatography and characterized by NMR spectroscopy and matrix-assisted laser desorption/ionization mass spectrometry (MALDI) to ensure the production of defect-free dendrons. ROMP of TD-G0 and TD-G1 (Scheme 2) was conducted using the fast-initiating Grubbs 3rd-generation catalyst at room temperature. We observed controlled polymerizations in which we could target specific DPs by varying the monomer to initiator ratio. The resulting PTD-G0 and PTD-G1 polymers were prepared with a wide range of DPws (90 – 564, determined by dividing the weight average molecular weight, Mw, by the monomer molecular weight, Mmon) and with narrow dispersity (1.02 – 1.15). Having successfully polymerized TDG0/G1, we sought to polymerize the much bulkier TD-G2. However, initial attempts using Grubbs 3rd-generation catalyst at room temperature only gave low molecular weight polymer, presumably due to low reactivity and stability of the propagating species,29,33 limiting efficient propagation of this



challenging macromonomer. Fortunately, by switching to Grubbs 2nd-generation catalyst and increasing the temperature to 50 ºC, we overcame these issues and successfully obtained high-M PTD-G2 with DPws in the range of 91 – 465 and with acceptable dispersity (1.18 – 1.29). In addition, for comparison, we prepared a linear polymer series (PTD-EH) having relatively small 2-ethyl-1-hexyl side chains, according to literature protocols.34 These four series of polymers (summarized in Table 1) represented the core polymers for investigating mechanochemical degradation kinetics. In order to test the generality of any observed kinetic trends, we also synthesized three PTDs, again by following reported protocols,30,34 bearing an octadecyl side chain (PTD-OD), a third-generation ester dendron (PTD-eG3), or a fourthgeneration ester dendron (PTD-eG4). Ultrasonication and Kinetic Analysis. To begin the study of denpol degradation, dilute solutions of each polymer were subjected to ultrasonication in tetrahydrofuran (THF), and aliquots were removed at various time points over the course of the experiment for analysis by size exclusion chromatography (SEC) with multi-angle light scattering (MALS). Kinetic analyses were conducted using the method initially described by Florea,35 which has been used to determine degradation rate constants for linear and star polymers.8d,36 In short, first order rate constants (kRI) were obtained by monitoring the refractive index (RI) signal at a single retention time (Pmax) corresponding to the peak maximum of the parent polymer (pre-sonication) over the course of the sonication experiment (see Supporting Information for more details). With narrow ÐMs and adequate SEC column separation, this method is more effective at distinguishing the degradation of the parent polymer from daughter fragments than molecular weight-based kinetics analyses.8d To confirm that the daughter fragments were not significantly contributing to the RI signal at Pmax, nonlinear regression analysis was used to resolve each SEC trace. Using the resolved peaks, we calculated rate constants for scission of the parent polymer based upon the resolved Pmax RI intensity (kres, see Table 1) and total peak area (karea) (see Supporting Information Table S1). Good agreement between kRI, kres, and karea values support that the concentration of parent polymer determined by a single retention time is a good representation of the total parent concentration. In all cases, except for PTDG2202, PTD-G2341, and PTD-G2465 (the subscripts indicate the DPw), the kres value fell within three standard deviation of the average kRI value (determined from three independent sonication experiments), indicating minimal influence of the daughter fragments overall. For the three polymers in which that wasn't the case, we calculated the average and standard deviation using the resolved SEC traces and have

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Table 1. Polymer Series Molecular Weight Characterization and Mechanochemical Degradation Rate Constants. a

Polymer

Mw

PTD-G0

a

kRI

c

d

kres

ÐM

39

124

1.01

0.09 ± 0.01

0.07

71

227

1.01

0.66 ± 0.02

0.67

118

378

1.01

3.02 ± 0.03

3.01

151

482

1.07

5.82 ± 0.20

5.86

202

646

1.03

11.62 ± 0.41

11.85

101

116

1.03

0.21 ± 0.01

0.21

187

213

1.02

1.09 ± 0.12

1.09

316

361

1.04

4.54 ± 0.04

4.62

391

447

1.08

8.29 ± 0.23

8.65

494

564

1.10

14.53 ± 0.96

14.87

130

90

1.02

0.15 ± 0.01

0.15

274

189

1.05

1.45 ± 0.07

1.42

468

324

1.04

5.42 ± 0.21

5.72

643

445

1.15

12.53 ± 0.55

13.16

290

91

1.29

0.29 ± 0.03

0.31

645

202

1.21

2.11 ± 0.01 2.29 ± 0.02

1088 341

1.28

8.29 ± 0.07 9.20 ± 0.07

1484 465

1.18

16.91 ± 0.33 18.65 ± 0.55

(kDa)

PTD-EH

b

DPw

-2

-1

-2

-1

(×10 min ) (×10 min )

PTD-G1

PTD-G2

a) Determined by SEC with MALS. b) Determined by dividing the Mw by Mmon (313, 876, 1444, and 3191 Da for PTD-EH, PTDG0, PTD-G1, and PTD-G2, respectively). c) Rate constants calculated from linear regression of the Ln(RI signal intensity) at the Pmax retention time of the parent polymer versus sonication time. Values are an average of three runs ± one standard deviation. d) Rate constants calculated in the same manner as kRI except using chromatograms that were resolved using non-linear regression to remove overlap of the daughter fragments at Pmax. Values are for a single run unless the single run value fell outside of three standard deviations from kRI, in which case the average and standard deviation of three runs were calculated.

used those values for subsequent analysis (and hence the use of just k to denote the rate constant in discussions below). The rate constants for mechanochemical scission were plotted as a function of Mw and DPw. First, examination of the Mw plot (Figure 1A) shows a substantially different dependence on MW for each polymer type. As the side chain size increases, the degradation rate constant decreases. For


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instance PTD-EH646 and PTD-G291 have comparable Mws of 202 and 290 kDa, respectively; however, PTD-EH646 has a ca. 40 times larger degradation rate. This result is consistent with the rate constant trends observed by Moore and coworkers for polymers bearing small side chains,7 and can be explained by the contour length, rather than the M, being a more important kinetic parameter for polymer degradation.13 For polymers with consistent MW, a polymer with larger side chains will have a shorter contour length and thus slower degradation. These results are also consistent with the results of star polymer degradation,8d in which the molecular weight of polymers alone cannot describe the degradation rate. The distribution of the molecular weight, via the polymer architecture, is also important. Next, examination of the DPw plot (Figure 1B) shows that the data for each polymer type does not converge in the same manner that Moore and coworkers observed with polymers having smaller side chains.7 Instead we see that the rate constant increases with increasing side chain size for a given DPw, which is consistent with the brush polymer trend observed by Sheiko and coworkers.14 By comparing polymers with similar DPws from each series, we see that in comparison to PTD-EH482, PTDG0446, PTD-G1445, PTD-G2465 have ca. 1.4, 2.2, and 2.9 times larger rate constants, respectively. Overall, these results suggest that the side chains, not just the contour length, play an important role in influencing the degradation rate. Bulk degradation rate constants in general represent "a complex convolution of probabilities", which at a minimum requires the polymer to be trapped in an elongational flow field and to experience sufficient strain to be stretched and have bond scission occur.1a By increasing the size of the side chain (keeping the DP the same), we expected many aspects of the polymer to change in manner that could increase the likelihood of these events occurring. For example, the physical size of the polymer might be larger, which we believe might make the polymer more likely to be near a cavitation bubble and be trapped in a flow field. Side chains at the termini of the polymer should extend out, effectively increasing the DP of the polymer (for the polymers in this study, we expect this to have minimal contribution). Relaxation times should increase, enabling elongation at lower strain rates (the critical strain rate scales with approximately M-2).1e Side chain repulsion should lead to the polymer being in an extended or rigid conformation in its unperturbed state,29,30,37 expediting further elongation. The hydrodynamic drag force associated with the side chain should increase,38 facilitating chain scission due to greater mid-chain forces. Any combination of these factors may also lead to the polymer being elongated into different conformations (dumbbell, half-dumbbell, kinked, and folded conformations are possible1i), which may have an effect on degradation rates.39 It would seem probable that the rate constant enhancement would be due to a combination of multiple of these factors. Nevertheless, we set out to try and determine the exact origin of the rate enhancement. It is well established that one of the defining features of denpols is that they have elongated or rigid conformations due to their large side chains.29,30,37 We predicted that this would be one of the primary factors (from those discussed above) influencing the degradation rate. To confirm the

Figure 1. Rate constants (k) for the mechanochemical degradation of polymers as a function of Mw (A), and DPw (B). Dashed lines are for visual aid only. Each data point is the average rate constant determined from three independent experiments and error bars represent ± one standard deviation.

elongated nature of polyphenylene-based denpols, we estimated the Flory exponent (ν) from log-log plots of the zaverage radius of gyration (Rgz) versus Mw (see Supporting Information Figure S2), obtained from SEC-MALS data. The parameter ν is related to the conformation of the polymer, with a value of 0.6 indicating a random coil in good solvent,40 and 1.0 indicating a rigid rod.41 We obtained values of 0.47, 0.57, 0.79, and 0.81 for PTD-EH, PTD-G0, PTD-G1, and PTD-G2, respectively, indicating a significant increase in polymer rigidity with increasing side chain size. To further characterize the rigidity of each polymer type, we estimated the persistence length (lp) for each series using the Benoit-Doty law (see the Supporting Information for more details).42 We calculated lp values of 3.3 ± 0.2, 6.2 ± 0.1, 11.1 ± 0.3, and 38.5 ± 0.3 nm for PTD-EH, PTD-G0, PTD-G1, and PTD-G2, respectively, further indicating that the polymers with larger side chains were in more extended conformations. Surprisingly, we found that for each polymer, the natural log of the lp value was almost exactly equal to the degradation rate enhancement (how many times greater the rate constant trend was than PTD-EH, see Supporting Information Figure S3), such that if k was divided by Ln(lp), the rate constant trends for each polymer converged onto the PTD-EH trend line (Figure 2A). This result suggested that we could accurately determine the



ultrasonic degradation rate of any PTD polymer using only two factors, its contour length and rigidity (when all other factors are constant, such as ultrasonic power intensity, solvent, temperature, etc.). Despite how well this trend fits the data, we want to reiterate that the lp values are estimates, the error associated with the calculation is expected to be quite large (at least ±15%), and in general, there is some controversy associated with the measurement and interpretation of lp values.43 Thus, we expect that lp values calculated using other theories or methods may not have the same correlation with our rate enhancement. We therefore also sought to find another parameter that could be exactly measured and was proportional to the rate enhancement. We found that to be the square root of Mmon (Figure 2B), such that plotting k/Mmon0.5 versus DPw leads to convergence of our rate constant trend lines. While we do not have an explanation for the exponent in this relationship, Mmon itself is a useful estimation of side chain size (as it eliminates the need for the somewhat arbitrary assignment of where a side chain starts and where the backbone ends) and we therefore would expect the rate enhancement to increase with increasing Mmon (based on the discussion above). Testing the Generality of Our Results. As we were able to find relationships that led to convergence of our rate constant trends, we envisioned using these plots as "master curves" from which we would be able to predict the degradation rate constants of other polymers, such as PTD-OD, PTD-eG3, and PTD-eG4. We expected that these polymers might have different properties than the polyphenylene-based denpols, and would provide a good assessment of the generality of our master curves. Using the DPw and lp or Mmon as input variables, we predicted rate constants (kp1, based on the relationship with lp, and kp2, based on the relationship with Mmon) for each polymer (Table 2), and then sonicated each polymer and determined experimental rate constants for comparison. The predicted rate constants, in both cases, were in good agreement with the experimentally determined rate constants; and the experimentally determined k/Ln(lp) or k/Mmon0.5 values fell on the same trend as their respective master curves (see Figure 2). This indifference to the side chain structure is consistent with the theoretical prediction that, in a good solvent, the apparent lp is independent of the rigidity or degree of branching of a dendron (see Supporting Information for further discussion).44 Given the diversity of the side chains in this study (short and long alkyl chains, polyphenylene dendrons, and flexible ester-type dendrons), it appears that our master curve is quite general and would likely be able to predict the degradation rate of other PTD-based polymers over the same DP range. Furthermore, we believe these findings will be useful in the development of new theoretical models pertaining to the ultrasonic degradation of polymers. Degradation of Daughter Fragments. Up to this point, we have solely focused on the degradation of the

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Figure 2. Plots of k/Ln(lp) (degradation rate constant divided by the natural log of the persistence length) as a function of DPw (A), and k/Mmon0.5 (degradation rate constant divided by the square root of the monomer molecular weight) as a function of DPw (B), showing convergence of the rate constant trends for all denpols and linear polymers studied. Dashed lines are for visual aid only.

parent polymer. As long as the DP of daughter fragments is above the limiting DP (from Figure 1 we roughly estimate that value to be near 100 for PTDs), the daughter fragments will also undergo chain scission. To study the degradation of daughter fragments, we examined four parent polymers (PTDEH482, PTD-G0446, PTD-G1445, and PTD-G2465), with comparable DPws, in more detail. These parent polymers should degrade to give daughter fragments with half their original DPw (we refer to these fragments as 1/2 daughters from here on), and the 1/2 daughters should degrade to smaller daughter fragments, with one quarter the DPw of the parent polymer (referred to as 1/4 daughters from here on). We expected that the degradation rate of daughter fragments (once they have left the flow field around a collapsing bubble) would be equivalent to that of a parent polymer with equivalent DPw. Therefore, we also expected that since the ratio of the degradation rate constants for a polymer with a DPw of ca. 450 and a polymer with a DPw of ca. 250 is essentially the same for each polymer series (determined from the master curves), that the relative amount of 1/4 daughter produced would be consistent for each polymer type. However, upon examination of the weight fraction (wi) plots over the course of sonication (see Supporting Information Figure S4), obtained via MALS analysis (see Supporting Information for more details), we observed that the production of 1/4 daughters appeared to be


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enhanced as the side chain size was increased. For instance, when we compared the ratio of the 1/4 daughter wi versus the parent polymer wi, we saw that the 1/4 daughter wi increased at a faster rate for polymers with larger side chains (Figure 3). We believe these results are most consistent with the hypothesis that the parent polymer may undergo multiple scission events before it can escape the flow field around a collapsing cavitation bubble, effectively increasing the degradation rate of 1/2 daughters. Table 2. Predicted vs Experimental Mechanochemical Degradation Rate Constants. kp1c Mwa b a Polymer DPw ÐM (×10-2 (kDa) min-1)

kp2d

polymer undergoing scission to 1/2 daughters, then some portion of those fragments undergoing scission to 1/4 daughters, before escaping the flow field (i.e. during a single scission cycle). Using the multi-scission model, we were able to tune the amount of 1/4 daughters produced so that the predicted distribution for PTD-G2465 visually matched the experimental distribution (Figure 4e) and the predicted ratio of 1/4 daughters to parent polymer was nearly

ke

(×10-2 min-1)

(×10 min-1)

-2

PTD-OD

114

251

1.02

1.3

1.4

1.19 ± 0.05f

PTD-eG3

603

450

1.03

10.0

10.8

9.94 ± 0.09g

PTD-eG4 1124

462

1.17

15.2

15.5

15.96 ± 0.86f

a) Determined by SEC with MALS. b) Determined by dividing Mw by Mmon (453, 1342, and 2430 Da for PTD-OD, PTD-eG3, and PTD-eG4, respectively). c) Rate constants predicted from the Ln(lp) master curve. The DPw was used to determine the expected k/Ln(lp) value from the polynomial fit of the master curve data, which was then multiplied by Ln(lp) to obtain kp1 d) Rate constants predicted from the Mmon0.5 master curve. The DPw was used to determine the expected k/Mmon0.5 value from the polynomial fit of the master curve data, which was then multiplied by Mmon0.5 to obtain kp2. e) Values are an average of three runs ± one standard deviation. f) kRI value. g) kres value.

This multi-scission behavior was previously hypothesized by Nguyen and coworkers to described the ultrasonic degradation of PS in the 106 Da molecular weight range.45 They found that they could model the degradation of PS with a M less than 100 kDa using only a single scission event per elongation cycle, but they needed to invoke multiple scission events to fit the distributions of larger polymers. While their work has been met with criticism due to their manipulation of SEC chromatograms,1a their overall results appeared to be in good agreement with ours. Therefore, we sought to model the molecular weight distributions (MWD) of denpols to further support the multi-scission hypothesis. We used a model developed by Glynn et al. (a different model than what was used by Nguyen and coworkers), which has been shown to accurately predict the MWD of various linear and star polymers.46 We began by modeling the DP distribution of PTD-EH482 (Figure 4a) with the unmodified single scission model (Figure 4b). The model gives an excellent match between the expected ratio of 1/4 daughters to parent polymer as the parent polymer degrades (Figure 4c). When we applied the same single scission model to PTD-G2465 (See Supporting Information Figure S5), we saw that visually the predicted distribution was not well matched to Figure 4d and that the predicted ratio of 1/4 daughters to parent polymer was underestimated. Therefore, we modified the model (see Supporting Information for more details) to include a tunable parameter to describe the probability of the parent polymer producing 1/4 daughters in addition to 1/2 daughters (we will refer to this as the multi-scission model here on). The addition of the 1/4 daughter fragmentation probability is not to be interpreted as the probability of a given polymer directly breaking into 1/4 fragments, but rather the probability of a

Figure 3. Ratio of the 1/4 daughter weight fraction (wi) to the parent polymer wi over the course of the parent polymer degradation. Dashed lines are for visual aid only. As the side chain size is increased, the rate at which 1/4 daughters are produced is also increased.

identical to the experimentally determined ratios (Figure 4f). When we applied the multi-scission model to PTD-G0 and PTD-G1, we could very accurately predict their 1/4 daughter to parent ratios as well (see Supporting Information Figure S6), and importantly, the modelled probability of 1/4 daughter production increased with increasing side chain size, as would be expected from the results in Figure 3. While our modeling provided additional support for the multi-scission hypothesis, the question remained why larger side chains would facilitate this behavior. Nguyen and coworkers observed multi-scission while exclusively studying linear polymers and found it to be a molecular weightdependent phenomena.45 This suggests that multi-scission was observed at lower DPs (for polymers with large side chains) simply due to the larger side chains increasing the total M. To confirm, we prepared a PTD-EH with a DPw of 2173 and we observed that, upon sonication, it produced 1/4 daughters at an accelerated rate compared to PTD-EH482 (see Supporting Information Figure S7). Thus, it appears that multi-scission behavior is not specific to large-side-chain architectures. While we don't have enough data to describe why multiscission occurs, we speculate that high-M polymers would undergo elongation and scission at lower strain rates, meaning scission can occur at earlier stages of bubble collapse, giving 1/2 daughters an opportunity to react. As mentioned previously, the parent polymer can be elongated into various conformations and if scission occurs prior to full elongation, then the daughter fragments will still require



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Figure 4. Experimental weight fraction (wi) plots for PTD-EH482 over the course of sonication (A), modeled wi plots for PTD-EH482 using the single scission model (B), plots of the experimental and modeled ratio of 1/4 daughter wi to the parent polymer wi over the course of PTD-EH482 parent polymer degradation (C), experimental wi plots for PTD-G2465 over the course of sonication (D), modeled wi plots for PTD-G2465 using the multi-scission model (E), and plots of the experimental and modeled (single and multi-scission) ratio of 1/4 daughter wi to the parent polymer wi over the course of PTD-G2465 parent polymer degradation (F). Overall, these plots show that PTD-EH482 degradation is well described by the single scission model whereas PTD-G2465 degradation is better fit with the multi-scission model.

some length of time to undergo sufficient elongation themselves. As the greatest strain rates are generated at only the latest stage of bubble collapse,47 if scission of the parent polymer occurs at the latest stage (which should be more likely for low-M polymers, as they should have higher critical strain rates), then the daughter fragments may not remain in

the flow field long enough for further degradation. Presumably, if the parent polymer underwent scission fast enough (i.e. very high-M polymers), then all of the 1/2 daughters might undergo scission in the same elongation event as the parent and 1/2 daughters might not even be observable. To further complicate matters, it may also be that only a subset


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of the 1/2 daughters are properly situated in the flow field for further degradation. For instance, the "leading" 1/2 daughters (those closer to the bubble wall) might be more prone to further degradation than the "trailing" daughters. Regardless, one of the main take-aways from these results is that multiscission appears to be a molecular weight-dependent phenomena, and that it can occur at lower DPs for polymers with large side chains.

Conclusions In this study, we have synthesized a series of well-defined denpols with polyphenylene dendrons, for the first time, and systematically studied their solution-based ultrasonic degradation kinetics to further elucidate the role of side chain size in polymer mechanochemistry. We found that as the size of the side chain increased, the degradation rate constant also increased, in agreement with the majority of studies thus far (the studies that did not see a difference in side chain size were likely looking at too small of differences between their comparisons). More importantly, we found that the rate enhancement was proportional to Ln(lp) and Mmon0.5, and that these relationships could be used to generate master curves that describe the reaction rate trends of polymers bearing alkyl chains and rigid or flexible dendrons of various generations. These results are consistent with polymers having elongated and rigid conformations degrading more rapidly. We expect that these results will be helpful in the development of new theoretical models that describe the ultrasonic degradation of polymers. Testing these trends with other polymers consisting of different backbones and various other side chains will be a crucial next-step toward determining the universality of these relationships. However, as there is nothing particularly exotic about the PTD backbone, we expect these trends will be widely applicable. We also explored the degradation of daughter fragments and used modeling to support the hypothesis that multiple scission events can occur during a single scission cycle. This appears to be a molecular weightdependent phenomena; thus larger side chains facilitate multiscission even at lower DPs. This work provides new insight into the role of side chain size in polymer mechanochemistry and further supports the notion that polymer chains should not be treated simply as generic handles for inducing mechanochemical reactions. For instance, here we have used the polymer chain structure to augment mechanochemical reaction rates. Expansion of this work to polymers containing mechanophores may provide a means to boost their performance in various applications, such as mechanocatalysis. Another important area of future study will be to determine if side chain size also plays a significant role in solid-state polymer mechanochemistry.

ASSOCIATED CONTENT Supporting Information. Supporting figures (Figures S1-8), synthetic protocols, descriptions of sonication procedures, persistence length calculations, descriptions of MWD modeling and modifications, and NMR spectra are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions †These authors contributed equally to this work.

Funding Sources We are thankful for financial support from the NRF, Korea through the following grants: Creative Research Initiative Grant, Creative Material Discovery Program, and Young Investigator Grant (2018R1C1B6003054).

ACKNOWLEDGMENT This paper is dedicated to Professor Kookheon Char at SNU on his 60th birthday and his lifelong achievement in research and education. We would also like to thank Dr. Andrey V. Dobrynin for helpful discussions.

ABBREVIATIONS Denpol, dendronized polymer; ÐM, molecular weight dispersity; DP, degree of polymerization; DPw, weight average degree of polymerization; eG3, ester-based third-generation dendron; eG4, ester-based fourth-generation dendron; EH, 2-ethylhexyl; G0, zero-generation dendron; G1, first-generation dendron; G2, second-generation dendron; k, degradation rate constant; karea, degradation rate constant determined from the area of the resolved RI parent peak; kres, degradation rate constant determined from resolved RI traces at a single retention time; kp, predicted rate constant; kRI, degradation rate constant calculated from RI trace at a single retention time; lp, persistence length; M, molecular weight; MALS, multi-angle light scattering; Mmon, monomer molecular weight; Mw, weight average molecular weight; MWD, molecular weight distribution; OD, octadecyl; PMMA, poly(methyl methacrylate); PDDMA, poly(dodecyl methacrylate); Pmax, retention time associated with the RI trace peak maximum for the parent polymer; PNB, polynorbornene; PS, polystyrene; PTD, poly(endo-tricycle[4.2.2.0]deca-3,9-diene); RI, refractive index; ROMP, ring opening metathesis polymerization; RT, room temperature; SEC, size exclusion chromatography; TD, endotricycle[4.2.2.0]deca-3,9-diene; THF, tetrahydrofuran; ν, Flory exponent; wi, weight fraction.

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Mechanochemical Degradation of Denpols: Synthesis and Ultrasound

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