Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
pubs.acs.org/Macromolecules
Cationic Copolymerization of o‑Phthalaldehyde and Functional Aliphatic Aldehydes Anthony Engler, Oluwadamilola Phillips, Ryan C. Miller, Cassidy Tobin, and Paul A. Kohl* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
Downloaded by UNIV PIERRE ET MARIE CURIE at 02:08:48:526 on May 30, 2019 from https://pubs.acs.org/doi/10.1021/acs.macromol.9b00740.
S Supporting Information *
ABSTRACT: There has been a resurgence of research related to poly(phthalaldehyde) (PPHA) in the past several years because of the demonstration of long room-temperature lifetime and its low ceiling temperature facilitating the ability to rapidly depolymerize the polymer at ambient conditions. This rapid depolymerization upon triggering of PPHA mixtures makes them well suited for fabricating transient devices or stimuli-responsive materials. The copolymerization of phthalaldehyde (PHA) with other aldehydes provides a route to incorporate chemical or physical functionality into PHA-based polymers while still maintaining the favorable degradation properties. Although the anionic copolymerization of PHA with benzaldehydes has been described, only a limited number of examples have been reported on the cationic copolymerization of PHA with other aldehydes. In this study, the synthesis and reactivity behavior of PHA copolymers is reported. Aliphatic aldehydes were chosen for their prevalence and favorable degradation properties. Dichloromethane solvent and the boron trifluoride diethyl etherate Lewis acid catalyst showed the best monomer conversions and highest polymer molecular weights. It was found that the aliphatic aldehyde reactivity for copolymerization increased with the electron-withdrawing nature of the aldehyde, which correlates with the aldehyde hydration equilibrium constant. The molecular weight and copolymer yield decrease with an increase in the aliphatic aldehyde feed concentration. Results indicate that the polymerization conditions used in this study (ca. −78 °C) are above the ceiling temperature of the aliphatic aldehyde comonomers, but this does not prevent copolymerization of the comonomer with PHA. Postpolymerization modifications were performed to introduce functional groups into the PHA-based copolymer that are incompatible with the polymerization chemistry. PPHA copolymers were cross-linked using a radiation-induced thiol−ene click chemistry to show that the mechanical properties can be improved even though the copolymer has a lower molecular weight.
■
In eq 1, ΔH and ΔS° are the enthalpy and standard entropy of polymerization, respectively, and R is the ideal gas constant. Throughout this work the term ceiling temperature is used in reference to the specific conditions in which the polymerization is conducted, that is, the reported monomer concentration and solvent medium at atmospheric pressure. Recent polyaldehyde publications have focused on poly(phthalaldehyde) (PPHA) and poly(glyoxylates).12−14 PPHA and its derivatives and copolymers were originally investigated as dry-develop photoresist films for lithography.15−17 More recently, PPHA-based materials have been used as stimuliresponsive materials for a variety of applications because the materials rapidly depolymerize back to monomers at ambient conditions.18−26 PPHA has been used as a structural material for applications where device recovery is not desired and the material needs to disappear into the surroundings.27 There is interest in phthalaldehyde (PHA)-based copolymers with improved transient and mechanical properties compared to PPHA. For example, the incorporation of monomers with higher vapor pressure than PHA (e.g., aliphatic aldehydes) into
INTRODUCTION
Degradable polymers are of interest because they can be used in transient-device applications,1,2 stimuli-responsive materials,3,4 advanced lithography,5−7 and closed loop polymer recycling.8 Low-ceiling-temperature polymers can be above TC as long as the active ends have been removed, via end-capping or polymer cyclization, because depolymerization occurs most readily from these active sites.9 A single chemical event capable of breaking a bond in the polymer backbone can initiate the polymer unzipping reaction with a small activation energy because the monomer form is the thermodynamically favored state at temperatures above TC. Polyacetals prepared by the addition polymerization of aldehydes can have TC values from −60 to 50 °C.10 The low TC is due to the relatively small enthalpy change for polymerization of carbon−oxygen double bonds compared to polymerization of carbon−carbon double bonds.9 Thus, a low polymerization temperature is required to overcome the entropy loss in the system. Dainton and Ivin derived eq 1 to describe TC in terms of polymerization thermodynamics and initial monomer concentration, [M]0.11 TC =
ΔH ΔS° + R ln[M]0
Received: April 12, 2019 Revised: May 15, 2019
(1) © XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.9b00740 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Dynamic mechanical analysis (DMA) was performed, measuring the film tension in the frequency scan mode, oscillating at 0.01% strain to test the mechanical properties of cross-linked polymer films at 30 °C. Polymer films for cross-linking were prepared by dissolving 250 mg of copolymer, thiols, and a photoradical generator in THF and placed on a rolling mixer until homogeneous. The formulation was cast onto polytetrafluoroethylene-coated foil that was molded into a rectangular shape: 32 × 12 × 0.5 mm. The films were exposed to an Oriel Instruments flood exposure source with a 1000 W Hg(Xe) lamp filtered to 248 nm radiation for a specified length of time. After crosslinking, the films were allowed to slowly dry in a semi-rich THF environment to help minimize bubble defects in the films. After DMA analysis, swelling ratio experiments were performed by allowing the films to sit in excess of THF. Solvent-swollen films were periodically weighed until a constant mass was achieved, and then the swelling ratio was taken as the final mass divided by the initial mass. In general, high cross-link densities can prevent the polymer from swelling and produce swelling ratio values near 1. Films with a low cross-link density tend to easily swell or completely dissolve in the solvent.
a copolymer could improve the overall rate of monomer evaporation. Postpolymerization reactions, such as crosslinking, may improve the toughness of the transient polymer. Cationic polymerization of cyclic PPHA is preferred over anionic polymerization because of (i) the ease of synthesis, (ii) the formation of higher-molecular-weight polymers (i.e., improved mechanical properties), (iii) improved thermal stability above TC, and (iv) elimination of end-capping reaction during synthesis.28−31 Anionically polymerized aliphatic aldehydes are also highly isotactic and precipitate from the reaction solution.10,32 Cationic polymerizations of aldehydes typically suffer from higher dispersities and lower control over molecular weight than anionic polymerizations, which rely on initiator stoichiometry. The anionic copolymerization of PHA with benzaldehydes was discussed by Kaitz and Moore,18 who also explored the cationic copolymerization of PHA and ethylglyoxylate.19 Schwartz et al. studied PHAbutanal copolymers, including their degradation properties.33 The aim of this study is to extensively examine the synthesis and characteristics of the cationic copolymerization of PHA with a variety of aliphatic aldehydes. Strategic choice of the copolymerized aldehyde permits the functionalization of PHAbased copolymers as a means of introducing cross-linkable moieties and other functional groups into the polymer, which are incompatible with the cationic polymerization chemistry.
■
■
RESULTS AND DISCUSSION Polymerization Catalyst and Solvent. A number of catalysts for the polymerization of PHA and aliphatic aldehydes homopolymers have been reported.29,34 Lewis acid catalysts were found to yield polyaldehydes with long roomtemperature shelf-life. It is thought that the macrocyclic polymer conformations of PPHA could be maintained with the addition of comonomers, such as with ethyl glyoxylate.19 PA was chosen as the model comonomer for its structural simplicity, ease of purification, and favorable solubility. Copolymerization synthesis is carried out at low temperature to help push the equilibrium in favor of polymerization (Scheme 1). The solvents were selected based on monomer
EXPERIMENTAL SECTION
A detailed account of the materials, instrumentation, and synthetic methods can be found in the Supporting Information. Aldehyde monomers readily form diol products on contact with water, so proper drying, purification, and storage are necessary for reproducible copolymer syntheses. o-PHA (purity > 99.7%) was purchased from TCI and used as received and stored in a nitrogen-rich glovebox. Aliphatic aldehyde monomers were purified by distillation over desiccant to remove acidic and water impurities. Propanal (PA) was distilled under inert, atmospheric pressure over calcium hydride. Larger aliphatic aldehydes were distilled at reduced pressure over anhydrous calcium sulfate. All monomer containers were filled with argon gas, sealed, and stored in a nitrogen-rich glovebox. Polymerizations were prepared in a glovebox using glassware that had been cleaned with dichloromethane (DCM) and dried in an oven prior to use. To a 100 mL round-bottom flask was added a desired amount of PHA. Anhydrous DCM was added to bring the total monomer concentration to the desired level, typically 0.75 M. Next, the comonomer was added to the solution and the flask sealed. This order of addition helps prevent vaporization of volatile comonomers like PA. A diluted catalyst solution was prepared in a separate vial with the Lewis acid and anhydrous DCM. A volume less than 0.5 mL of this solution was added to the reaction flask via a syringe. Reactions took place at −78 °C for a desired length of time. Pyridine, 67 mol excess to Lewis acid, was injected to quench the polymerization. The reaction was allowed to mix with pyridine for 30−90 min before being diluted and precipitated dropwise into vigorously stirred methanol. The precipitation bath was stirred for >1.5 h before filtering and allowing the white solid polymer to air-dry overnight. Polymer conversions were calculated based on the gravimetric yield of isolated material and the composition of the copolymer, as measured by nuclear magnetic resonance (NMR). NMR spectra were collected using CDCl3 as the solvent and the residual solvent peak (δ = 7.26 ppm for 1H and δ = 77.16 ppm for 13 C) as the reference for chemical shifts. Molecular weights were measured by gel permeation chromatography (GPC) using tetrahydrofuran (THF) eluent at 30 °C, and calibrated against a series of linear polystyrene standards. Dynamic thermal gravimetric analysis (TGA) was performed at a heating rate of 5 °C/min. Isothermal TGA runs were heated at a rate of 5 °C/min until 10 °C before the desired temperature, followed by a 1 °C/min ramp rate.
Scheme 1. (a) General Copolymerization of PHA with Aliphatic Aldehyde and (b) Trimerization of Aliphatic Aldehydes
and polymer solubility at the reaction temperature, ca. −78 °C, and their freezing point. The solvent must also dissolve the trimer form of the aliphatic aldehyde comonomer even though the trimer itself does not homopolymerize.35 Precipitation of the comonomer trimer would decrease the concentration of free monomer from the reaction solution. Copolymerization reactions were run at −78 °C to screen possible polymerization solvents, as shown in Table 1. The monomer concentration was 0.75 M, the monomer-to-catalyst molar ratio was 500:1, and the PHA-to-PA monomer feed mole ratio was 1.5:1. The starting solution was a vibrant, yellow color, which quickly converted to colorless at the reaction temperature, showing the conversion of the yellow PHA monomer to polymer. Some solutions became very viscous upon reaction, making magnetic stirring difficult. The B
DOI: 10.1021/acs.macromol.9b00740 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
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 (%)
BF3OEt2 BCl3 BBr3 TiF4e TiCl4e GaCl3 AlEt3
DCM/CHCl3/Tol DCM/CHCl3/Tol DCM/CHCl3/Tol DCM/CHCl3 DCM DCM/CHCl3/Tol DCM/CHCl3
77/54/57 0/0/0 0/0/0 0/0
