1 Cationic Copolymerization of o‑Phthalaldehyde and Functional2 Aliphatic Aldehydes3 Anthony Engler, Oluwadamilola Phillips, Ryan C. Miller, Cassidy Tobin, and Paul A. Kohl*

4 School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

5 *S Supporting Information

6 ABSTRACT: There has been a resurgence of research related to7 poly(phthalaldehyde) (PPHA) in the past several years because of the8 demonstration of long room-temperature lifetime and its low ceiling9 temperature facilitating the ability to rapidly depolymerize the polymer10 at ambient conditions. This rapid depolymerization upon triggering of11 PPHA mixtures makes them well suited for fabricating transient devices12 or stimuli-responsive materials. The copolymerization of phthalaldehyde13 (PHA) with other aldehydes provides a route to incorporate chemical or14 physical functionality into PHA-based polymers while still maintaining the favorable degradation properties. Although the15 anionic copolymerization of PHA with benzaldehydes has been described, only a limited number of examples have been16 reported on the cationic copolymerization of PHA with other aldehydes. In this study, the synthesis and reactivity behavior of17 PHA copolymers is reported. Aliphatic aldehydes were chosen for their prevalence and favorable degradation properties.18 Dichloromethane solvent and the boron trifluoride diethyl etherate Lewis acid catalyst showed the best monomer conversions19 and highest polymer molecular weights. It was found that the aliphatic aldehyde reactivity for copolymerization increased with20 the electron-withdrawing nature of the aldehyde, which correlates with the aldehyde hydration equilibrium constant. The21 molecular weight and copolymer yield decrease with an increase in the aliphatic aldehyde feed concentration. Results indicate22 that the polymerization conditions used in this study (ca. −78 °C) are above the ceiling temperature of the aliphatic aldehyde23 comonomers, but this does not prevent copolymerization of the comonomer with PHA. Postpolymerization modifications were24 performed to introduce functional groups into the PHA-based copolymer that are incompatible with the polymerization25 chemistry. PPHA copolymers were cross-linked using a radiation-induced thiol−ene click chemistry to show that the26 mechanical properties can be improved even though the copolymer has a lower molecular weight.

27 ■ INTRODUCTION

28 Degradable polymers are of interest because they can be used29 in transient-device applications,1,2 stimuli-responsive materi-30 als,3,4 advanced lithography,5−7 and closed loop polymer31 recycling.8 Low-ceiling-temperature polymers can be above TC

32 as long as the active ends have been removed, via end-capping33 or polymer cyclization, because depolymerization occurs most34 readily from these active sites.9 A single chemical event capable35 of breaking a bond in the polymer backbone can initiate the36 polymer unzipping reaction with a small activation energy37 because the monomer form is the thermodynamically favored38 state at temperatures above TC. Polyacetals prepared by the39 addition polymerization of aldehydes can have TC values from40 −60 to 50 °C.10 The low TC is due to the relatively small41 enthalpy change for polymerization of carbon−oxygen double42 bonds compared to polymerization of carbon−carbon double43 bonds.9 Thus, a low polymerization temperature is required to44 overcome the entropy loss in the system. Dainton and Ivin45 derived eq 1 to describe TC in terms of polymerization46 thermodynamics and initial monomer concentration, [M]0.

11

= ΔΔ ° + [ ]

TH

S R ln MC047 (1)

48In eq 1, ΔH and ΔS° are the enthalpy and standard entropy49of polymerization, respectively, and R is the ideal gas constant.50Throughout this work the term ceiling temperature is used in51reference to the specific conditions in which the polymer-52ization is conducted, that is, the reported monomer53concentration and solvent medium at atmospheric pressure.54Recent polyaldehyde publications have focused on poly-55(phthalaldehyde) (PPHA) and poly(glyoxylates).12−14 PPHA56and its derivatives and copolymers were originally investigated57as dry-develop photoresist films for lithography.15−17 More58recently, PPHA-based materials have been used as stimuli-59responsive materials for a variety of applications because the60materials rapidly depolymerize back to monomers at ambient61conditions.18−26 PPHA has been used as a structural material62for applications where device recovery is not desired and the63material needs to disappear into the surroundings.27 There is64interest in phthalaldehyde (PHA)-based copolymers with65improved transient and mechanical properties compared to66PPHA. For example, the incorporation of monomers with67higher vapor pressure than PHA (e.g., aliphatic aldehydes) into

Received: April 12, 2019Revised: May 15, 2019

Article

pubs.acs.org/Macromolecules

© XXXX American Chemical Society A DOI: 10.1021/acs.macromol.9b00740Macromolecules XXXX, XXX, XXX−XXX

* Unknown * | ACSJCA | JCA11.1.4300/W Library-x64 | research.3f (R4.1.i3 HF01:4938 | 2.1) 2018/08/24 11:08:00 | PROD-WS-118 | rq_794084 | 5/25/2019 09:34:05 | 10 | JCA-DEFAULT

68 a copolymer could improve the overall rate of monomer69 evaporation. Postpolymerization reactions, such as cross-70 linking, may improve the toughness of the transient polymer.71 Cationic polymerization of cyclic PPHA is preferred over72 anionic polymerization because of (i) the ease of synthesis, (ii)73 the formation of higher-molecular-weight polymers (i.e.,74 improved mechanical properties), (iii) improved thermal75 stability above TC, and (iv) elimination of end-capping76 reaction during synthesis.28−31 Anionically polymerized77 aliphatic aldehydes are also highly isotactic and precipitate78 from the reaction solution.10,32 Cationic polymerizations of79 aldehydes typically suffer from higher dispersities and lower80 control over molecular weight than anionic polymerizations,81 which rely on initiator stoichiometry. The anionic copoly-82 merization of PHA with benzaldehydes was discussed by Kaitz83 and Moore,18 who also explored the cationic copolymerization84 of PHA and ethylglyoxylate.19 Schwartz et al. studied PHA-85 butanal copolymers, including their degradation properties.33

86 The aim of this study is to extensively examine the synthesis87 and characteristics of the cationic copolymerization of PHA88 with a variety of aliphatic aldehydes. Strategic choice of the89 copolymerized aldehyde permits the functionalization of PHA-90 based copolymers as a means of introducing cross-linkable91 moieties and other functional groups into the polymer, which92 are incompatible with the cationic polymerization chemistry.

93 ■ EXPERIMENTAL SECTION94 A detailed account of the materials, instrumentation, and synthetic95 methods can be found in the Supporting Information. Aldehyde96 monomers readily form diol products on contact with water, so97 proper drying, purification, and storage are necessary for reproducible98 copolymer syntheses. o-PHA (purity > 99.7%) was purchased from99 TCI and used as received and stored in a nitrogen-rich glovebox.100 Aliphatic aldehyde monomers were purified by distillation over101 desiccant to remove acidic and water impurities. Propanal (PA) was102 distilled under inert, atmospheric pressure over calcium hydride.103 Larger aliphatic aldehydes were distilled at reduced pressure over104 anhydrous calcium sulfate. All monomer containers were filled with105 argon gas, sealed, and stored in a nitrogen-rich glovebox.106 Polymerizations were prepared in a glovebox using glassware that107 had been cleaned with dichloromethane (DCM) and dried in an oven108 prior to use. To a 100 mL round-bottom flask was added a desired109 amount of PHA. Anhydrous DCM was added to bring the total110 monomer concentration to the desired level, typically 0.75 M. Next,111 the comonomer was added to the solution and the flask sealed. This112 order of addition helps prevent vaporization of volatile comonomers113 like PA. A diluted catalyst solution was prepared in a separate vial with114 the Lewis acid and anhydrous DCM. A volume less than 0.5 mL of115 this solution was added to the reaction flask via a syringe. Reactions116 took place at −78 °C for a desired length of time. Pyridine, 67 mol117 excess to Lewis acid, was injected to quench the polymerization. The118 reaction was allowed to mix with pyridine for 30−90 min before being119 diluted and precipitated dropwise into vigorously stirred methanol.120 The precipitation bath was stirred for >1.5 h before filtering and121 allowing the white solid polymer to air-dry overnight. Polymer122 conversions were calculated based on the gravimetric yield of isolated123 material and the composition of the copolymer, as measured by124 nuclear magnetic resonance (NMR).125 NMR spectra were collected using CDCl3 as the solvent and the126 residual solvent peak (δ = 7.26 ppm for 1H and δ = 77.16 ppm for127

13C) as the reference for chemical shifts. Molecular weights were128 measured by gel permeation chromatography (GPC) using129 tetrahydrofuran (THF) eluent at 30 °C, and calibrated against a130 series of linear polystyrene standards. Dynamic thermal gravimetric131 analysis (TGA) was performed at a heating rate of 5 °C/min.132 Isothermal TGA runs were heated at a rate of 5 °C/min until 10 °C133 before the desired temperature, followed by a 1 °C/min ramp rate.

134Dynamic mechanical analysis (DMA) was performed, measuring the135film tension in the frequency scan mode, oscillating at 0.01% strain to136test the mechanical properties of cross-linked polymer films at 30 °C.137Polymer films for cross-linking were prepared by dissolving 250 mg138of copolymer, thiols, and a photoradical generator in THF and placed139on a rolling mixer until homogeneous. The formulation was cast onto140polytetrafluoroethylene-coated foil that was molded into a rectangular141shape: 32 × 12 × 0.5 mm. The films were exposed to an Oriel142Instruments flood exposure source with a 1000 W Hg(Xe) lamp143filtered to 248 nm radiation for a specified length of time. After cross-144linking, the films were allowed to slowly dry in a semi-rich THF145environment to help minimize bubble defects in the films. After DMA146analysis, swelling ratio experiments were performed by allowing the147films to sit in excess of THF. Solvent-swollen films were periodically148weighed until a constant mass was achieved, and then the swelling149ratio was taken as the final mass divided by the initial mass. In general,150high cross-link densities can prevent the polymer from swelling and151produce swelling ratio values near 1. Films with a low cross-link152density tend to easily swell or completely dissolve in the solvent.

153■ RESULTS AND DISCUSSION154Polymerization Catalyst and Solvent. A number of155catalysts for the polymerization of PHA and aliphatic156aldehydes homopolymers have been reported.29,34 Lewis acid157catalysts were found to yield polyaldehydes with long room-158temperature shelf-life. It is thought that the macrocyclic159polymer conformations of PPHA could be maintained with the160addition of comonomers, such as with ethyl glyoxylate.19 PA161was chosen as the model comonomer for its structural162simplicity, ease of purification, and favorable solubility.163Copolymerization synthesis is carried out at low temperature164to help push the equilibrium in favor of polymerization165 s1(Scheme 1). The solvents were selected based on monomer

166and polymer solubility at the reaction temperature, ca. −78 °C,167and their freezing point. The solvent must also dissolve the168trimer form of the aliphatic aldehyde comonomer even though169the trimer itself does not homopolymerize.35 Precipitation of170the comonomer trimer would decrease the concentration of171free monomer from the reaction solution.172Copolymerization reactions were run at −78 °C to screen173 t1possible polymerization solvents, as shown in Table 1. The174monomer concentration was 0.75 M, the monomer-to-catalyst175molar ratio was 500:1, and the PHA-to-PA monomer feed176mole ratio was 1.5:1. The starting solution was a vibrant,177yellow color, which quickly converted to colorless at the178reaction temperature, showing the conversion of the yellow179PHA monomer to polymer. Some solutions became very180viscous upon reaction, making magnetic stirring difficult. The

Scheme 1. (a) General Copolymerization of PHA withAliphatic Aldehyde and (b) Trimerization of AliphaticAldehydes

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181 polymerization reaction was quenched with excess pyridine182 after 1 h followed by product precipitation into methanol to183 purify the polymer from the unreacted monomer, polymer-184 ization catalyst, and quenching reagent. No attempt was made185 to endcap the polymer chains. The Lewis acid catalysts and186 solvents are shown in Table 1 along with product yield,187 molecular weight, and percent incorporation of PHA and PA188 into the polymer.189 Boron trifluoride diethyl etherate (BF3OEt2) gave the190 highest yield of polymerizations among the Lewis acids tested.191 This catalyst has been reported as highly active to aldehyde192 polymerizations.28,35 Copolymer yields from chloroform and193 toluene were significantly lower than those from DCM. Other194 boron trihalides and triethyl aluminum catalysts did not195 catalyze the copolymerization reaction. Titanium(IV) halides196 were not fully soluble, and did not yield copolymers. The197 gallium trichloride catalyst gave a modest polymer yield in each198 solvent: DCM, chloroform, and toluene. It is important to note199 that polymerizations in toluene always resulted in the polymer200 precipitating from the reaction mixture during polymerization201 even to the extent of solidifying the entire polymerization202 medium. Chloroform showed similar but less-consistent results203 in terms of polymer solubility and solidification compared to204 the other solvents. The effectiveness of toluene as the205 polymerization solvent was demonstrated by performing a206 polymerization reaction at half the monomer concentration207 (0.38 M) that yielded a polymer product precipitating from the208 reaction mixture after only 25 min. Further dilution of the209 reactant concentration was detrimental because it would210 decrease the ceiling temperature of the monomer mixture, as211 shown by eq 1. Evidence from past reports suggests that the212 Lewis acids may require a co-catalyst to initiate the213 polymerization, which could come from acid impurities,214 adventitious water, or aldehyde hydrates.32 The BF3OEt2/215 DCM system was selected as the catalyst/solvent system in the216 remaining studies here because it produced copolymers with217 the highest yield, molecular weight, and monomer conversion.218 Copolymerization of the o-PHA and PA Model219 System. A series of PHA-PA copolymers were synthesized220 with monomer composition feeds ranging from 0 to 60 mol %221 to investigate PA reactivity. Feed loadings of ≥70 mol % PA222 resulted in little or no copolymer yield. Each reaction was223 performed in triplicate and the results are provided in the224 Supporting Information. The composition of the copolymers225 was measured by comparing integrations of the backbone

f1 226 protons in the 1H NMR spectra. Figure 1a shows the overlaid227 spectra of p(PHA-PA) copolymers between chemical shifts of228 δ = 4.7−7.2 ppm. Each curve represents a copolymerization229 with a higher PA mole percent in the monomer feed. The230 resulting copolymers had 3−23 mol % PA incorporation as231 seen by the growth of PA (peak B) with respect to the PHA

232(peak A). A deterioration of the well-defined PHA peaks can233also be seen as the PA incorporation increased. This234deterioration originates from the loss of the cis/trans235configuration of the PHA acetal protons (A″/A′, respec-236tively).29 The copolymerization appears to promote the cis237configuration for the PHA monomer, because the acetal peak238of the copolymer product shifts in favor of the cis configuration239as shown by the decrease in the trans peaks. The loss in well-240defined PHA tacticity is also observed in the acetal carbon241peaks in the 13C NMR spectrum, which is overlaid with the242PPHA homopolymer (blue) in Figure 1b.243The bimodal nature of the 1H NMR PA acetal peaks may244also be due to a cis/trans interaction with neighboring

Table 1. Synthetic Results for Copolymerizations of o-PHA with PA Using Various Lewis Acid Catalysts and Solventsa

Lewis acid solvent p(PHA-PA) yieldb (%) molecular weight (kDa); Đc PHA conversiond (%) PA conversiond (%)

BF3OEt2 DCM/CHCl3/Tol 77/54/57 60/43/26; 1.77/2.23/3.18 90/69/72 20/4/8BCl3 DCM/CHCl3/Tol 0/0/0BBr3 DCM/CHCl3/Tol 0/0/0TiF4

e DCM/CHCl3 0/0TiCl4

e DCM <1GaCl3 DCM/CHCl3/Tol 55/48/60 16/13/10; 2.31/2.17/1.93 69/59/74 10/12/13AlEt3 DCM/CHCl3 0/0

aTol = toluene. bGravimetric yield. cMeasured by GPC dCalculated from yield and composition by NMR. eNot fully soluble.

Figure 1. Representative NMR spectra of p(PHA-PA) in CDCl3: (a)1H NMR spectra of copolymer series with increasing PA content 3%(red), 6% (yellow), 9% (green), 12% (cyan), 19% (purple), 23%(blue), peaks A′ and A″ correspond to the trans and cis configurationsof PHA acetal protons; (b) 13C NMR spectra of p(PHA-PA) (red)and p(PHA) homopolymer (blue).

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245 monomers within the copolymer. Alternatively, it could be the246 result of varying monomer sequences within the polymer247 chains. A slight chemical shift would occur for PA acetal peaks248 flanked by PHA monomers (-PHA-PA-PHA- sequence)249 compared to two sequential PA monomers (-PHA-PA-PA-250 PHA- sequence). These copolymers likely have a random251 monomer sequence distribution because there is little evidence252 to support the existence of consecutive PA monomer253 sequences. If the copolymer was blocky in nature with -PA-254 PA- or -PHA-PHA- sequences, it is expected to better maintain255 the cis/trans configuration ratio. NMR results reported by256 Weideman et al. show that homopolymerization of aliphatic257 aldehydes (butanal in their case) produces broad acetal peaks258 centered around δ ≈ 4.8 ppm,36 which is not observed in these259 copolymers. This peak shift also corresponds to that of the260 aliphatic trimer, an impurity of which is the likely cause of the261 appearance of the triplet peak in Figure 1a at δ = 4.80 ppm (J =262 5.3 Hz) and the sharp peak in Figure 1b at δ = 102.6 ppm,37

263 based on the sharpness of the peak and the lack of trend264 compared to peak B.265 There are no apparent signs of endcaps existing within the266 copolymers synthesized here, including attempts to endcap267 with acetic anhydride during the pyridine quench, supporting268 the conclusion that the polymers are cyclic in nature. Lack of269 alkene signals in the NMR spectra suggests that α-elimination270 is not a favored pathway. 2D-NMR experiments did not show a271 strong correlation to other potential endcaps (see the272 Supporting Information), but this is expected with high-273 molecular-weight polymers because the concentration of274 endcaps is very low, if there were ends. Unfortunately,275 repeated matrix-assisted laser desorption ionization mass276 spectrometry analyses did not show well-resolved peaks to277 determine the mass of the copolymer chains.31 It is likely that278 the copolymers synthesized here are predominantly cyclic,279 formed by a transacetalization reaction from the polymer280 chain. This result would be similar to PPHA and p(PHA-281 ethylglyoxylate) polymerizations using BF3OEt2 in DCM.19,31

282 However, water or aldehyde hydrate impurities mights2 283 introduce a susceptible hemiacetal end group, Scheme 2.

284 Reactivity of Aliphatic Aldehydes toward Copoly-285 merization. PHA (monomer A) was copolymerized with a286 number of aliphatic aldehydes (monomer B) to investigate the287 relative reactivity of different monomers. Composition profiles288 for the copolymerization of PHA with PA, 2,2-dimethylpropa-289 nal (DMP), heptanal (HA), and phenylacetaldehyde (PAA)

f2 290 are shown in Figure 2a with data tables provided in the291 Supporting Information. In Figure 2a, the mol % of the

292aliphatic monomer in the feed ( f B) and resulting mol %293incorporation of aliphatic monomer in the copolymer (FB) are294shown. The copolymer uptake into the PHA-based copolymer295is less than the stoichiometric amount in the feed for each296comonomer. Kaitz and Moore introduced the incorporation297ratio, a simple, empirically derived parameter obtained from298the slope of the best-fit line of FB versus f B. It can be used to299compare the relative reactivity of different monomers with300PHA.18 Other common approaches to reactivity analysis are301not useful here because the low TC of the copolymer facilitates302the ability for monomers to depropagate during polymer-303ization. The Mayo−Lewis approach fails because their304assumption that the addition of a new monomer to a growing305polymer chain is irreversible does not hold.9 In the present306case, PHA units are likely free to reversibly shuttle in and out307of the growing polymer chain as evidenced by the polymer308scrambling studies on PHA derivatives.38 The extended309Kelen−Tudos model, which assumes no depropagation of310the growing chain, produces negative reactivity ratios for311aliphatic aldehydes and unusually large values for PHA (see the312Supporting Information).39 When the polymer is formed at313temperatures near TC, the depropagation of monomers is not314negligible and one must consider the reverse rate constants.315Although such models exist, they can be very nonlinear,316difficult to accurately calculate, and can utilize parameters that317would be unknown in this system.40−43

318The copolymer compositions show near-linear profiles,319Figure 2a. The experimental incorporation ratios were320determined by applying a best-fit line to the data that extends321through the origin. A positive correlation was found with the322hydration equilibrium constants (KH) for the comonomer,323which is readily available for many aldehydes.44−46 KH is the324equilibrium constant for the addition reaction of water to an325aldehyde to form a gem-diol product, and a larger KH value326signifies that the aldehyde more readily undergoes an addition327reaction. The significance of this is apparent when considering328that aldehydes with higher KH values are more electron-329deficient, which is advantageous for the cationic aldehyde330polymerization mechanism, Scheme 1. The growing chain end331in a cationic aldehyde polymerization can be thought of as an332oxonium ion, where propagation occurs with nucleophilic333attack by an aldehyde monomer to the electrophile at the334polymer chain end. Creating a larger positive charge on the335aldehyde carbon through the use of nearby electron-with-336drawing groups improves its ability to act as an electrophile,337facilitates the transition from sp2 to sp3 configuration, and338ultimately shifts the equilibrium toward polymerization. These339results are consistent with the previous observation that340benzaldehyde reactivity to anionic copolymerization with PHA341improved with larger Hammett values.18

342The uptake of HA into the PHA-HA copolymer was linearly343related to the HA feed concentration for values ≤33 mol %.344Copolymerizations at higher HA feed sometimes resulted in a345heterogeneous solution with needle-like crystals that would346dissolve upon warming the solution to above −78 °C. The347crystals were separated from the cold reaction solution and348appeared to be almost entirely HA trimer, as determined by 1H349NMR analysis. The HA trimer NMR spectra was a sharp triplet350at δ = 4.80 ppm (J = 5.3 Hz) and no PHA protons. It is noted351that −78 °C is reportedly above the TC of the DMP352monomer,47 suggesting that there is some energetic benefit353to copolymerization.

Scheme 2. Possible Endcap Routes in PHA−AliphaticAldehyde Copolymers: (a) Formation of MacrocyclicTopologies through Transacetalization; (b) Hemiacetal EndGroups from Hydrate Impuritiesa

aCounter anions omitted.

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354 Copolymerization of a variety of aldehyde monomers with355 PHA was studied to probe the functional group tolerance of356 this polymerization chemistry. The copolymerization results

t2 357 are reported in Table 2, with the successful copolymers showns3 358 in Scheme 3. Electron-rich aldehydes are less reactive in PHA

359 copolymerization. It was found that electron-rich t-cinnamal-360 dehyde and methyl formate were not incorporated into the

361PHA copolymer. The presence of any of these monomers in362the reaction did not inhibit the homopolymerization of PPHA;363they simply did not participate. Copolymerization of PHA with364electron-deficient benzaldehydes also showed no incorporation365yielding only PPHA homopolymer. It is noted that 2,4-366dinitrobenzaldehyde was previously shown to have an367incorporation ratio of 0.71 under anionic copolymerization

Figure 2. Reactivity study of PHA-based copolymers with aliphatic aldehydes: (a) copolymer composition profiles, where the best fit line throughthe origin is taken as the experimental incorporation ratio; (b) correlation of aliphatic aldehydes’ incorporation ratios with their correspondinghydration equilibrium constant (KH).

44−46

Table 2. Synthetic Data for PHA Based Copolymers with Various Aldehydes

comonomer B f B (%) [M]/[I] time (min) Mn (kDa)a Đa FB

b (%) yield (%)c batch size (mmol)

2,3,5-trichlorobenzaldehyde 40 500 60 14 2.26 0 38 6.22,4-dinitrobenzaldehyde 40 100 60 124 1.69 0 41 15.03-bromo-5-nitrobenzaldehyde 40 500 60 39 1.57 0 30 18.7formyl ferrocene 50 100 60 0 15.0methyl formate 50 500 60 9.5 2.06 0 37 22.4butanal (BA) 50 750 60 33 2.10 10 53 22.42-ethylbutanal (EtBA) 40 380 60 64 1.93 9 42 24.62-ethylbutanal (EtBA) 40 380 60 44 2.15 9 40 24.62-ethylbutanal (EtBA) 40 750 60 51 1.97 9 38 24.6t-cinnamaldehyde 2050 500 60 5.3−17 1.62−2.28 0 18−28 22.44-pentenal (PE) 20 500 60 21 2.38 9 61 18.74-pentenal (PE) 40 500 60 13 1.75 14 48 6.24-pentenal (PE) 40 750 60 15 1.837 12 45 12.44-pentenal (PE) 40 500 60 11 1.92 13 51 18.74-pentynal (PY) 20 100 60 34 1.79 2 73 15.04-pentynal (PY) 40 100 60 15 1.54 4 56 15.04-pentynal (PY) 50 100 60 9 1.49 5 32 15.0norbornene-2-carboxaldehyde (NBA) 10 100 60 40 1.63 4 76 15.0norbornene-2-carboxaldehyde (NBA) 33 100 60 22 1.7 14 39 15.0norbornene-2-carboxaldehyde (NBA) 50 100 60 17 2.11 23 29 15.010-undecenal (UE) 25 767 1360 27 3.38 8 63 29.810-undecenal (UE) 32 500 8100 21 2.63 8 56 34910-undecenal (UE) 40 500 60 23 3.23 7 51 24.610-undecenal (UE) 40 500 1380 17 3.11 9 44 62.010-undecenal (UE) 40% 1000 2900 18 2.36 10 50 187.02-chlorobutanal (2CBA) 10−50 100 60 6−35 1.5−2.2 <1 30−80 15.02-chlorobutanal (2CBA) 10 100 1440 19 1.65 2 78 15.02-chlorobutanal (2CBA) 20 100 1440 5 2.05 8 67 15.02-chlorobutanal (2CBA) 33 100 1440 5 1.46 22 41 15.02-chlorobutanal (2CBA) 40 100 1440 2.4 1.65 17 30 15.02-chlorobutanal (2CBA) 50 100 1440 2.6 1.39 21 29 15.04-chlorobutanal (4CBA) 20 100 60 82 1.61 6 73 15.04-chlorobutanal (4CBA) 40 100 60 48 1.74 10 52 15.04-chlorobutanal (4CBA) 50 100 60 27 1.73 8 58 15.03-methylthiopropanal 20−60 500 60 0 22.44-tosyloxybutanal (TsBA) 20 100 60 75 1.66 4 64 15.0

aMeasured by GPC. bGravimetric yield. cMeasured by NMR.

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368 conditions.18 It is unclear whether steric hindrance, purity, or369 some other factor is responsible. The addition of BF3OEt2370 catalyst to the PHA copolymerization mixture containing 3-371 methylthiopropanal resulted in the immediate formation of a372 precipitate and no polymer formation. This is likely due to the373 Lewis acid−base reaction between the thioether and boron374 trifluoride. Aldehyde polymerizations have been cited as being375 intolerant of protic functional groups and impurities.35 The376 incompatibility of strong Lewis bases with the boron trifluoride377 Lewis acid catalyst in the cationic polymerization potentially378 prevents many heteroatom-based functional groups from being379 used and limits the choice of monomers in this system.380 Comonomer branching does not seem to significantly inhibit381 the polymerization for 2-ethylbutanal (EtBA) or norbornene-382 2-carboxaldehyde (NBA), but a quaternary carbon at the α-383 position becomes too electron-donating, as shown by the FB384 results for DMP. The monomer chain length affects the385 solubility of the comonomer trimer byproduct, which has been386 shown to limit incorporation of the aliphatic aldehyde, as387 observed with HA and 10-undecenal (UE) at high f B values.388 Aldehyde monomers with unsaturated functional groups can389 be polymerized as long as the unsaturation is not conjugated390 with the aldehyde group because they would become electron-391 donating. For example, 4-pentenal (PE), UE, NBA, and 4-392 pentynal (PY) were successfully copolymerized with PHA, as393 shown in Table 2. Alkyl halides are compatible with PHA394 cationic copolymerization, as shown by the copolymerization395 of 2- and 4-chlorobutanal with PHA (2CBA and 4CBA,396 respectively). The 2,2-dichlorobutanal impurity of the397 monochloro-aldehyde product also showed incorporation398 into the PHA copolymer, indicating that its steric hindrance399 is not an inhibiting factor. Sulfonate esters are compatible with400 the polymerization chemistry as shown by 4-tosyloxybutanal401 (TsBA), which can be useful for postpolymerization402 modifications.403 The copolymerization of 2CBA with PHA showed unusual404 polymerization kinetics. Whereas the other aliphatic aldehydes405 examples reacted to equilibrium within 1 h,37 the 2CBA-PHA406 reaction yielded a copolymer with less than 1 mol % 2CBA407 incorporation in 1 h. Extending the polymerization time to 24408 h significantly increased the 2CBA incorporation to 22 mol %

409at the same feed loading. Purifying the 2CBA monomer proved410to be difficult as the compound was prone to decompose411during distillation, which greatly affected the reactivity during412copolymerization. Although incorporation of this monomer is413favorable, it is limited by the tendency to decompose into414impurities that inhibit the overall polymerization kinetics.415Following the arguments from the discussion on PA416copolymers, none of the copolymer NMR results indicate417that the comonomer is incorporated as long consecutive units418in the polymer. Consecutive comonomer blocks within the419polymer chain would have a broad 1H NMR peak centered420around δ = 4.8 ppm, for the case of simple aliphatic421aldehydes.36 One explanation for the lack of consecutive422aliphatic monomer blocks is that they enable back-biting423within the polymer chain, which leads to trioxane formation, as424 s4shown in Scheme 4. This reaction may be a kinetic product of

425the polymerization reaction if the rate of the intramolecular426backbiting occurs faster than the propagation of a new427monomer to the chain end. Once formed, the trioxane428compound is inactive toward polymerization.429The copolymerization results for the aldehydes investigated430in this study share a common trend where both the molecular431weight and yield decrease with the amount of comonomer432 f3incorporation into the PHA copolymer, FB, as seen in Figure 3.433A similar trend of low-molecular-weight polymer accompanied434by low polymer yield was observed for PPHA homopolymer435synthesis.28,30 Given that the conversion of monomer to436polymer has been shown to track the polymerization437temperature,37 and the lower copolymerization yield occurred438at higher feed values, f B, it is possible that the formation of439copolymers here is thermodynamically limited. That is, the440results indicate that the aliphatic aldehydes are behaving as if441they are still above their TC, even though reports place the TC442value of PA in pentane at −48 °C.48 The suggestion that the443comonomer TC is lower than previously reported is supported444by the evidence that PA-based copolymerizations do not yield445the polymer when f B > 70%. The low TC value is also446supported by the lack of consecutive (i.e., blocky) PA447monomer units in the copolymer, as shown in the448spectroscopic analysis. The inability to polymerize aliphatic449aldehydes can be explained by the impact that the solvent has450on eq 1.49 The concentration of the aliphatic comonomer is451lowered because some of the material resides as a trimer which452is in equilibrium with the free monomer. The initial monomer453concentration term, [M]0, should be replaced with the activity454of the monomer, and the activity coefficients of the monomers455are affected by the polarity of the solvent. Changing the solvent456from pentane to DCM can lower the activity coefficients,457which together with other factors can decrease the PA activity458and thus make the denominator smaller (i.e., a larger459magnitude negative number). This results in lowering TC for460the comonomer, even below −78 °C, the reaction temperature

Scheme 3. Polyaldehyde Copolymers Synthesized in ThisStudya

aComonomer acronyms provided below the structure.

Scheme 4. Proposed Mechanism for the Formation ofTrioxane Derivatives by Back-Bitinga

aCounter anion omitted.

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461 used in this study. Evidence from Mohamed et al. supports this462 equilibrium temperature shift in response to solvent effects by463 reporting more exothermic and exoentropic thermodynamic464 values for the trimerization of butanal in pentane versus465 DCM.50 The same authors also confirm the inability to466 homopolymerize butanal in DCM with BF3OEt2 at −95 °C.467 The improved polymerizability of copolymers in nonpolar468 solvents is in agreement with results from the anionic469 polymerization of aliphatic aldehydes.34 On the basis of this470 observation, the toluene-based polymerizations are expected to471 give a higher incorporation for PA, but the solubility issues472 previously discussed may have diminished the beneficial effect.473 The possibility of comonomer incorporation being thermo-474 dynamically limited can be tested by increasing the monomer475 concentration during the reaction. According to eq 1, an476 increase in comonomer activity helps overcome the change in477 entropy for the reaction to increase the TC of the monomer

f4 478 mixture.49 Figure 4 shows the molecular weight and

479 composition of a series of p(PHA-PAA) copolymers where480 the only difference was the concentration of the reaction

481solution. As [M]0 is quadrupled from 0.75 to 3 M, the482composition of the resulting copolymer increases from 24 to48333 mol % PAA. The increase in polymer molecular weight is484statistically significant when compared to the values shown in485Figure 3. If the homopolymerization of the aliphatic aldehydes486is not appreciable in this system, then there must be an487energetic benefit to copolymerizing aliphatic aldehydes with488PHA, or the incorporation of aliphatic aldehydes would not489occur. This is supported by the results from Schwartz et al.490who calculated the enthalpy of copolymerizing butanal with491PHA to be slightly more exothermic than the trimer formation492in DCM.37

493Reactions with Polyaldehyde Copolymers. Postpoly-494merization reactions can be used to introduce functional495groups into the copolymer that are incompatible with the496polymerization chemistry itself. The choice of chemistry for497postpolymerization reactions is somewhat limited to mild498acid−base or temperature conditions so as not to initiate499 s5polymer depolymerization. Scheme 5 shows examples of500polymer modifications carried out in this study. Epoxide501functional groups were created by oxidation of p(PHA-UE)502copolymers with m-chloroperoxybenzoic acid in the presence503of NaHCO3 to quench the acidic byproducts. 1H NMR504showed complete conversion of the terminal alkenes into505epoxide groups, and an isolated polymer yield of 50%. The506epoxide ring could be ring-opened by subsequent reactions507with amines, alcohols, carboxylic acids, or anhydrides.508Azide functional groups were introduced into the copolymer509via nucleophilic substitution of p(PHA-4CBA) and p(PHA-510TsBA) using NaN3 in dimethylformamide. Substitution of the511terminal chloride was limited to 15% conversion at 25 °C, and51250% conversion at 40 °C after reaction for 24 h. The tosylate is513an excellent leaving group, and was completely converted to

Figure 3. Trends of polyaldehyde copolymers with incorporation of the aliphatic aldehyde (FB): (a) Mn from GPC; (b) copolymerizationgravimetric yield. The legend in the middle applies to both plots.

Figure 4. p(PHA-PAA) copolymer series at different initial monomerconcentrations. All copolymerizations were charged with f B = 50%.

Scheme 5. Reactions on p(PHA-UE), Top, and p(PHA-TsBA), Bottom, Performed in This Study

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514 the azide overnight at room temperature with a polymer yield515 of 35%.516 Thiol−ene click reactions can be used for polyaldehyde517 functionalization. The radical-based reaction was used to cross-518 link the polyaldehyde films in an effort to improve the519 mechanical properties of the low-molecular-weight copolymer520 films. Cross-linking parameters were optimized for the521 concentration of polymer, thiol-to-alkene ratio, photoradical522 initiator content, and 248 nm exposure dose using a system of523 p(PHA-UE) with pentaerythritol tetrakis(3-mercaptopropio-524 nate) in THF and 2,2-dimethoxy-2-phenylacetphenone525 (DMPA) as the photoradical generator. The p(PHA-UE)526 copolymer used for the cross-linking study had a composition527 of 8 mol % UE and an Mn = 21 kDa. The cross-linking was528 evaluated by measuring the swelling ratio of the cross-linked529 films in THF. Films that would fully dissolve in the THF were530 not considered cross-linked.531 Dried copolymer films did not show evidence of cross-532 linking, given by total dissolution in THF. One hypothesis for533 this result is that the glassy nature of the polyaldehydes does534 not permit sufficient chain mobility for thiol cross-linking535 agents to find the multiple alkene sites necessary to reach a536 cross-link density, which results in insoluble films. Dissolving537 the components in THF to improve chain mobility led to538 cross-linked, insoluble films at concentrations of 30−50 wt %539 p(PHA-UE) in THF. The film quality deteriorated with540 formulations greater than 40 wt% polymer, resulting in bubble541 defects throughout the film. Increasing the free radical542 generator loading beyond 0.5 mol ratio of DMPA-to-thiol at543 a constant UV exposure dose led to a higher degree of polymer544 depolymerization as observed by the yellowing of the films and545 PHA monomer odor. It was found that omission of the free546 radical generator still resulted in polyaldehyde cross-linking,547 presumably through the formation of thiyl radicals by 248 nm548 radiation.51 Omitting the thiols and relying on free radical549 alkene reactions did not cross-link the polymers at ambient550 temperatures. The thiol-to-alkene ratio of 1 showed the highest551 cross-link density, as given by the lowest degree of swelling.

f5 552 Figure 5 shows the storage modulus for a series of films with

553 varying degrees of 248 nm UV exposure. The maximum554 modulus occurred at an exposure dose of 3 J/cm2. Below this555 dose there is very little cross-linking, and above this dose the556 polymer showed signs of degradation, especially the sample at557 10 J/cm2. This increase in mechanical strength is indicative of558 an increase in molecular weight by chemical cross-linking.559 The tosylate group on the p(PHA-TsBA) copolymer can act560 as a thermal trigger for the decomposition of the polymer at561 Td,onset = 95 °C, compared to the rest of the copolymers that

562begin to thermally degrade at 150 ± 20 °C (see the Supporting563Information for representative TGA ramps). It is hypothesized564that p-toluenesulfonic acid is formed after the thermally565induced dissociation and elimination of the tosylate group,566 f6which is then able to attack and degrade the polymer.52 Figure567 f66 shows the isothermal TGA runs for a 4 mol % TsBA

568containing a copolymer at 50−75 °C. Isotherms for p(PHA)569and p(PHA-BA) with 10 mol % BA at 80 °C are shown for570comparison to highlight the difference in thermal stability571when adding the tosylate group. The 20 wt % residue572remaining after long times in Figure 6 is likely caused by TsOH573side reactions with the degraded products. The 20 wt %574remaining is not solely due to the TsBA monomer because it575represents only about 5 wt % of the copolymer.

576■ CONCLUSIONS577Cationic copolymerizations between o-PHA and aliphatic578aldehydes showed that the Lewis acid catalyst and solvent579choice have strong effects on the copolymer composition,580conversion, and final molecular weight. Aliphatic aldehyde581reactivity for copolymerization with PHA increases with the582electron-deficiency of the aldehydes. It has also been shown583that the comonomer reactivity correlates with the hydration584equilibrium constant of the aldehyde monomer, which can585provide a method to screen future aldehyde monomer586candidates. Under the conditions in this study, it is likely587that the aliphatic aldehydes are operating above their588respective ceiling temperatures, but are still able to589copolymerize with PHA. A photo-induced thiol−ene cross-590linking study examined the ability to improve the mechanical591properties of low-molecular-weight copolymers. These functio-592nalizable, metastable copolymers exhibit remarkable transient593abilities under mild conditions and other properties that make594them highly desirable material systems for various engineering595applications in transient technologies and stimuli-responsive596devices.

597■ ASSOCIATED CONTENT598*S Supporting Information599The Supporting Information is available free of charge on the600ACS Publications website at DOI: 10.1021/acs.macro-601mol.9b00740.

602General experimental details, synthetic procedures,603copolymer data, regression statistics for incorporation604ratios, TGA data, and 1H and 13C NMR spectra (PDF)

Figure 5. Storage modulus from DMA film tension frequency sweepof cross-linked p(PHA-UE) films exposed to different doses (mJ/cm2) of light centered at 248 nm.

Figure 6. Isothermal TGA traces for p(PHA-TsBA), dotted lines, atseveral temperatures compared to p(PHA) and p(PHA-BA), solidlines, at 80 °C.

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605 ■ AUTHOR INFORMATION606 Corresponding Author607 *E-mail: kohl@gatech.edu.608 ORCID609 Paul A. Kohl: 0000-0001-6267-3647610 Funding611 Financial support was provided through DARPA, contract #612 HR0011-16-C-0047 and HR0011-16-C-0086.613 Notes614 The authors declare no competing financial interest.

615 ■ ACKNOWLEDGMENTS616 The financial support of the Defense Advanced Research617 Projects Agency Inbound, Controlled, Air-Releasable, Unre-618 coverable Systems (ICARUS) program is gratefully acknowl-619 edged. The authors also thank Dr. Jared Schwartz and Dr. Jisu620 Jiang for fruitful discussions.

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