Highly Efficient ROP Polymerization of ε-Caprolactone Catalyzed by

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Highly Efficient ROP Polymerization of ε‑Caprolactone Catalyzed by Nanoporous Alumina Membranes. How the Confinement Affects the Progress and Product of ROP Reaction Magdalena Tarnacka,*,†,‡ Andrzej Dzienia,‡,§ Paulina Maksym,†,‡ Agnieszka Talik,†,‡ Andrzej Zięba,∥ Rafał Bielas,⊥ Kamil Kaminski,*,†,‡ and Marian Paluch†,‡ †

Institute of Physics, University of Silesia, 75 Pulku Piechoty 1, 41-500 Chorzow, Poland Silesian Center of Education and Interdisciplinary Research, University of Silesia, 75 Pulku Piechoty 1A, 41-500 Chorzow, Poland § Institute of Chemistry, University of Silesia, Szkolna 9, 40-007 Katowice, Poland ∥ Department of Organic Chemistry, School of Pharmacy with the Division of Laboratory Medicine in Sosnowiec, Medical University of Silesia in Katowice, Jagiellonska 4, 41-200 Sosnowiec, Poland ⊥ Department of Physical Chemistry and Technology of Polymers, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland ‡

ABSTRACT: Efficient ROP polymerization of ε-caprolactone, resulting in products characterized by high molecular weight of low dispersities, remains a challenging task and is currently an important matter of ongoing research, mostly due to the accompanying side reactions, i.e., hydrolysis and transesterification. Herein, we studied in detail the impact of 2D hard confinement on the progress and product of ring-opening polymerization (ROP) of ε-caprolactone with and without water acting as initiator in comparison to the macroscale conditions, where various forms (powder, sheet) of alumina were used. It turned out that applied aluminum oxide nanotemplates act as both catalyst and initiator (INICAT) of ROP nanopolymerization and seem to operate accordingly to the pseudoliving coordination− insertion mechanism, resulting in nanowires of PCL characterized by Mn up to 53.5 kg/mol. Additionally, due to the applied confinement, the side reactions were successfully suppressed, resulting in macromolecules of moderate molecular weight distribution (Đ = 1.27−1.41) and unimodal GPC peaks. It can be related to the extremely short nanopolymerization time (around 40 min), which successfully counteracts side reactions to occur. Moreover, our data clearly indicated that nanoporous membranes favor the growth of polymer chains of similar length, resulting in low dispersity of produced macromolecules. We believe that presented data significantly broaden the current state of knowledge and allow for a better understanding of the processes taking place under confinement, which seem to be crucial in any further nanotechnology development.

I. INTRODUCTION Because of its unique properties (i.e., degradability, miscibility with other polymers, and biocompatibility), poly(ε-caprolactone) (PCL) is one of the most important industrial polyesters that finds versatility of applications in medicine, pharmacy, electronics, and packaging.1−5 Increasing demand for biodegradable polymers of tailored-made properties requires synthesis of materials with well-defined structures and specific properties accordingly to the purpose, which remains a challenge (especially the synthesis of PCL of high molecular weights, Mn) and is currently an important matter of ongoing research. PCL might be produced by the two ways: (i) polycondensation of 6-hydroxyhexanoic acid and (ii) ring-opening polymerization (ROP) of ε-caprolactone (CL). The latter method is preferred because it leads to macromolecules of higher Mn and lower dispersity, Đ. Thus, the majority of work devoted to the synthesis of PCL is focused on the development of the new ROP catalysts, i.e., organometallic compounds, nucleophilic organocatalysts, or enzymes.6−8 However, in view of potential biomedical application of produced PCL’s, the drawbacks in the © XXXX American Chemical Society

application of those complex catalytic systems are their high toxicity and difficulties in product purification. Hence, great effort is put to develop an environmentally friendly (green) synthesis of biodegradable materials. An innovative and very promising catalytic alternative that satisfies the requirements of the green fabrication of tailored-made polymers seems to be performing the polymerization reaction in the reduced space of nanoreactors. The application of two-dimensional (2D) spatially restricted spaces provides the new possibility of obtaining polymers of unique morphologies and defined properties on the nanometric length scale9−15 with potential applications in the field of nanotechnology. As reported, the reduced space of nanoreactors usually shortens the reaction time and might lead to macromolecules of higher molecular weight and/or lower dispersity,10,11,16−19 although the opposite effect leading to wider molecular weight distribution is also reported.11 Received: February 23, 2018 Revised: May 10, 2018

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DOI: 10.1021/acs.macromol.8b00409 Macromolecules XXXX, XXX, XXX−XXX

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product of ROP reaction. Note that up to now major papers devoted to nanopolymerization are focused on both radical and step growth reaction,9−19 and there are only a few papers studying ROP polymerization. In this context, one can recall papers published two decades ago, where mesoporous zeolites materials were successfully used as a surface catalyst in ROP of lactones (δ-valerolactone and ε-caprolactone). It was shown that macromolecules of narrow molecular weight distribution were produced via monomer activated mechanism due to a combination of Lewis acidic aluminum and Bronsted acidic silanol functionalities on the interior wall of applied templates.23,24 In our paper, we explored further and more deeply the impact of 2D confinement conditions on the progress and product of ROP reactions, which seems to be crucial especially in the context of any industrial application in the field of biomedical nanotechnology.

Generally discussed is that under 2D confinement (within nanopores), propagating radicals are stabilized and termination reactions are suppressed. As a consequence, polymerization performed under confinement reveals “pseudo-living” nature, resulting in a better control over the reaction.10,20,21 Moreover, under this condition, the increased control over stereoregularity and tacticity of produced polymers can be gained. The best illustration of this phenomenon are the systematic studies on poly(vinyl acetate), polystyrene, and poly(methyl methacrylate), where the content of isotactic units increases with decreasing pore size and variation in host−guest interactions.10 As presented, a counterbalance between both finite size effect and intramolecular interaction between host (nanotemplate) and guest (monomer) molecules seems to have a significant (often catalytic) effect on the polymerization kinetics and properties of the reaction products, including a possible lowering of the initiator decomposition temperature.22 Moreover, one should remember that those conditions give us also a remarkably easy opportunity to produce unique materials of morphologies controlled on the nanometer scale. Therefore, it is possible to synthesize macromolecules with tailored-made properties in the form of nanowires or nanotubules, which may have a numerous possible applications in new medical and electronic nanotechnologies. Herein, we examined water-initiated ring-opening polymerization (ROP) of CL carried out under 2D geometrical constraint due to the application of nanoporous aluminum oxide (AAO) membranes of various pore diameter, d = 150, 100, and 35 nm (see Figure 1c). Nanomaterials under 2D

II. EXPERIMENTAL SECTION Materials. ε-Caprolactone (97%) was purchased from SigmaAldrich and purified by distillation from CaH2 under high vacuum. Ultrapure water was purchased from Sigma-Aldrich and used as received. Aluminum oxide sheet (0.64 mm thickness, purity 96%) was purchased from Goodfellow. Aluminum oxide powder (Particle size 100−250 mesh, Camag Brockmann I netural) was purchased from Fisher Chemicals. The nanoporous aluminum oxide (AAO) membranes used in this study were supplied from Synkera Co. Details concerning porosity, pore distribution, etc., can be found on the Web page of the producer.25 The chemical structures of the investigated monomer and applied catalysts are presented in Figure 1. Methods. ROP Polymerization of CL at Macroscale. Added to pure ε-caprolactone or to a mixture of ε-caprolactone with water at molar concentrations 15/1 was the aluminum oxide sheet or powder in 1% or 5% weight ratio, calculated with respect to the amount of monomer. The reaction mixture placed in a round-bottom glass flask was degassed by inert gas flow (argon) and capped with a septum. Note that prior to the reaction, both alumina catalyst were kept in an oven at T = 200 °C overnight. After polymerization, the examined nanosystems were dissolved in chloroform, and then the polymer was isolated by precipitation into cold methanol or a methanol/water mixture (50/50 v/v), filtered, and then dried under vacuum to a constant mass. ROP Polymerization of CL under 2D Conditions. In the case of water-initiated reactions, we prepared a mixture of monomer to water in molar concentrations of 15/1. The mixture was transferred into the flask together with the AAO membrane. Note that prior to filling, AAO membranes were dried in an oven at T = 150 °C under vacuum to remove any volatile impurities from the nanochannels. Then, the whole system was maintained at T = 15 °C in a vacuum (10−2 bar) for 24 h to let the mixture flow into the nanocavities. After completing the infiltration process, the surface of AAO membrane was dried and the excess sample on the surface removed by use of a paper towel. In the experiment, we used membranes of different pore diameter: 150, 100, and 35 nm. The total amount of reaction mixture incorporated into the AAO membrane was found to be ∼3 mg. After polymerization, the examined nanosystems were diluted in chloroform; then the polymer was isolated by precipitation into a cold methanol or methanol/water mixture (50/50 v/v), filtered, and dried under vacuum to a constant mass. The extraction is a nondestructive method allowing the recovery of polymers, which is not covalently bonded to alumina template. Note that the part of the product was initiated by −OH groups attached to the alumina membrane walls and thus cannot be extracted. Nevertheless, the rest of polycaprolactone can be easily separated from the template. This repeated extractions procedure allowed us to extract about ≈90% of reaction product. It should be highlighted that alternatively we tried to remove alumina template using different recipes described in the literature (we used different kinds of acid and base); however, each time it was found that recovered polymers

Figure 1. Chemical structure of ε-caprolactone (a); structures of alumina sheet (b) and alumina templates (c). Data for the applied 2D catalyst were taken from the producer Web page.25

geometrical constraint are often characterized by different properties than the one observed for the macroscale materials, which is believed to be due to a counterbalance among several factors, i.e., surface interactions, finite size, and free volume. In order to explore the impact of hard confinement on ROP reactions, we performed an additional series of experiments at macroscale, which indicates typical surface catalysis, where no confinement and no above-mentioned effects are observed. We believe that the comparison of those “typical” geometrical conditions used in the modern catalyst together with the results obtained for confined reaction allows us to indicate and define the unique impact of confined spaces on the progress and B

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Figure 2. 1H NMR spectra of CL and PCL produced due to water-initiated ROP carried out at macroscale. and polydispersities (Mw/Mn) were obtained with a triple detection calibration with a narrow polystyrene standard. Note that in the case of ROP polymerization carried out without water the nascent polymer is chemically attached to the surface of applied alumina. However, due to the occurrence of transesterification reactions occurring within the system, we were able to dilute the reaction products from the reacting mixture. Note that ROP of lactones can be accompanied by both intermolecular and intramolecular transesterification reactions. The first one is a unimolecular backbiting reaction, releasing a fragment of a macromolecule in the cyclic form and leaving a shorter but still active macromolecule, whereas the second one takes place between two growing macromolecules without losing the activity of the chain end groups. The GPC measurements of polymers obtained at macroscale were performed with varied and known concentration in the range of 5−12 mg/mL, depending on the sample; for such experiments the refractive index increment (dn/dc) was calculated from data collected from three detectors, and its value (0.070−0.072) was similar to literature data for polycaprolactone (0.072). In the case of polymers produced under confinement, the concentrations of the samples were unknown. However, the concentration of the sample was measured with an RI detector by using a known value of the dn/dc parameter (0.072), and the data from three detectors provide information on molecular weight and its distribution. It should be also pointed out that the dn/dc refers to the rate of change of the refractive index with the concentration of a solution for a sample at given temperature, wavelength, and solvent, and above a certain molar mass (conventionally above Mn = 20.0 kg/ mol) the influence of end groups can be neglected. Therefore, the dn/ dc value of homogeneous polymers is constant and can be considered as a contrast factor. Nuclear Magnetic Resonance Spectroscopy (NMR). Proton nuclear magnetic resonance (1H NMR) spectra were recorded using a Bruker Ascend 600 spectrometer operating at 600 MHz in CDCl3 as a solvent. Standard experimental conditions and standard Bruker program were used. Typical 1H NMR spectra of PCL and its precursor CL are shown in Figure 2. 1H NMR of PCL: δ = 4.03 (t, 2H, CH2O), 3.63 (t, 2H, CH2−OH), 2.37 (t, 2H, CH2CO2H), 2.32 (t, 2H, CH2CO2), 1.65 (m, 4H, (CH2)2), 1.36 (q, 2H, CH2). Monomer conversions were

underwent rapid hydrolysis. In this context, one can recall that, as reported in the literature,26 PCL decomposes rapidly when pH ∼ 13 independently of Mn of the polymer. Therefore, this method was simply ruled out to obtain polymers for further NMR and GPC characterization. Differential Scanning Calorimetry (DSC). Calorimetric measurements of the isothermal reaction were carried out by a Mettler-Toledo DSC apparatus equipped with a liquid nitrogen cooling accessory and an HSS8 ceramic sensor (heat flux sensor with 120 thermocouples). Temperature and enthalpy calibrations were performed by using indium and zinc standards. The sample was prepared in an open aluminum crucible (40 μL) outside the DSC apparatus. Samples were heated to T = 80 °C at a heating rate of 10 °C/min and kept at this temperature for approximately 60 min. Each measurement at a given temperature was repeated three times. For each experiment a new sample was prepared. The kinetic plot of examined nanopolymerization was based on conversion estimated from DSC data according to the equation t dH

αDSC

∫t dt dt ΔH(t ) = = t0 ΔHreaction ∫t ∞ ddHt dt 0

(1)

where αDSC is the DSC conversion, ΔH(t) is the enthalpy variation as a function of the time spent at a given temperature condition, ΔHreaction is the total enthalpy of the reaction at the end of the process, dH/dt is the rate of heat evolution, and t0 and t∞ are the times when crystallization starts and ends.27−30 Note that the obtained αDSC was rescaled to real conversion, α, estimated from NMR studies. Gel Permeation Chromatography (GPC). Molecular weights and dispersities were determined by gel permeation chromatography (GPC) with a Viscotek GPC Max VE 2001 and a Viscotek TDA 305 triple detector (refractometer, viscosimeter, and low angle laser light scattering) used for data collecting and an OmniSec 5.12 for processing. Two T6000M general mixed columns were used for separation. The measurements were carried out in THF as the solvent at T = 35 °C with a flow rate of 1 mL/min. The apparatus was used in a triple detection mode, and absolute molecular weights (Mn and Mw) C

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Figure 3. Pseudo-first-order kinetic plot versus time for ROP of CL under different conditions. calculated using the integrations from the methylene protons of polymer (4.03 ppm [−CH2O−]) and monomer (4.24 ppm [−CH2O−]). Note that the applied alumina might act as an initiator of the reaction due to the presence of some −OH groups on its surface. In this context, we are not able to estimate the proper values of both the degree of polymerization (DP) and the theoretical molecular weight (Mn,th) of polymers due to the lack of alumina hydroxyl groups concentration required for those calculations. Additionally, in the case of the confined-synthesized polymers, we obtained comparable NMR spectra with the same main signals. Nevertheless, due to the small amount of the sample and impurities present within the mixture after the extraction, the quantity of their NMR spectra was lower than those measured for a system produced at the macroscale. Scanning Electron Microscopy (SEM). SEM was performed with a Phenom ProX microscope. In order to observe polymer morphology, the aluminum template was removed with procedure proposed by Salsamendi et al.31 A template with polymer was treated with mixture of HCl, CuCl2, and H2O (5 mL, 36%/50 mg/5 mL, respectively) for 20 min and next dissolved with 10 wt % NaOH. Insoluble polymer was then dried and coated with 5 nm gold nanoparticles prior to the analysis by a Q150R rotary-pumped sputter coater from Quorum Technologies. The template removal had been used just once to make a SEM measurements, where it was performed directly before the experiment and gave us very short period of time to act. Note that these measurements were carried out only to observe the nanowire morphology of produced polymers, and the possible hydrolysis was not a problem in this case.

pseudo-first-order kinetic plots of ln([M]t/[M]0) versus time indicate some discrepancy in the linear dependence due to a positive curvature (acceleration) of the kinetic plot, which suggests slow initiation (see Figure 3a). Interestingly, the surface-to-volume ratio of the applied alumina catalyst has no significant impact on the kinetics of the reaction, and a similar effect is observed for all studied systems. Note that a comparable scenario (slow initiation) was observed also in the case of water-initiated high pressure ROP of CL.32 Calculated values of CL conversion are summarized in Table 1. As it can be seen, water-initiated ROP polymerizations of CL in the presence of alumina powder or sheets proceeded from 25% up to completed monomer conversion and yielded PCL with low molecular weights (Mn = 820−3200 g mol−1) and narrow dispersities (Đ = 1.07−1.3). One can mention that in the presence of 1 wt % aluminum oxide sheet (no. 2) and Table 1. ROP Polymerization of CL at Macroscale

III. RESULTS AND DISCUSSION ROP Polymerization of CL at Macroscale. As first, we characterized the effect of aluminum oxide as the catalyst of the examined ROP reaction at macroscale, where no confinement effect is expected. A series of polymerizations were performed with constant monomer-to-initiator molar ratio ([CL]0/[H2O]0 = 15/1), but with different catalyst concentrations (1 and 5 wt %). Additionally, we applied alumina sheet and powder, characterized by different surface-to-volume ratio. For details, see the Experimental Section. The 1H NMR analysis confirmed the chemical structures of the produced PCL and enabled us to determine the monomer conversion, α, of each reacting systems. Typical 1H NMR spectra of PCL and monomer are shown in Figure 2. The kinetics of the water-initiated ROP polymerization of CL at T = 353 K is presented in Figure 3a. As illustrated, the

no.

catalyst concn [wt %]

1 2 3 4 5

1 1 5 1 5

6 7

1 5

8 9 10 11 12 13 14

5 5 5 5 5

temp [°C]

time [h]

conva [%]

Mnb [kg/mol]

[CL]0/[H2O]0 = 15/1 + alumina sheet 80 72 25 0.81 80 96 58 80 96 71 1.3 120 48 100 2.9 120 48 100 3.2 [CL]0/[H2O]0 = 15/1 + alumina powder 80 96 47 1.2 80 96 71 1.4 [CL]0/[H2O]0 = 15/1 (without alumina) 80 24 0 80 144 94 1.9 [CL] + alumina powder 80 24 1 80 96 4.5 80 144 15 2.0 120 24 10 120 144 92 5.5

Đb 1.059 1.028 1.308 1.286 1.045 1.074

1.049

1.7 1.624

a Determined by 1H NMR, CDCl3, 600 MHz. bDetermined by GPCLALLS, THF, 35 °C, calculated dn/dc = 0.075.

D

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Macromolecules powder (no. 6) at T = 80 °C, the former system is characterized by faster reaction rate and slightly higher CL conversion within 96 h (αsheet ∼ 58% vs αpowder ∼ 47%). However, for a 5 times higher amount of catalyst (5 wt %), no effect on the reaction rate and the properties of resulted polyesters can be seen after the same reaction time at T = 80 °C (see samples no. 3 and no. 7 in Table 1, where α ∼ 71%). Thus, it can be concluded that the variation in the form of alumina catalyst has a negligible effect on the ROP of CL. Because of the presence of nucleophilic reagent such as water in the examined systems, the ROP proceeds with the formation of neutral propagating chain ends, which react with lactone molecule yielding the formation of a polyester. In addition, the use of water molecules as initiating agent provides a simple way to obtain α-hydroxy-ω(carboxylic acid) PCL, which was confirmed by 1H NMR analysis for all studied systems containing aluminum oxide in both form powder and sheets. Note that similar telechelic hydroxyl and carboxylic/acidic terminated PCL were produced recently by our group in high pressure water-initiated ROP.32 Although in the case of high pressure studies, the resulting PCLs were characterized by moderate molecular weights (Mn = 1.6−19.0 kg mol−1) of low dispersities (Đ = 1.07−1.36). To better understand an impact of alumina on the examined ROP, we carried out one more reference reaction without water (CL + 5 wt % of alumina powder). Surprisingly, the reactions also took place in the absence of water, but CL conversions were small (no. 11). Note that it is 15 times lower than conversion of CL in the water-initiated system (no. 3) at T = 80 °C after 92 h (αCL ∼ 4.5% vs αCL+water ∼ 71%). The semilogarithmic kinetic plots of ln([M]t/[M]0) versus time for the ROP of CL without the presence of water reveal systematic deviation from the linear dependency, suggesting both slow initiation and the occurrence of termination reactions (see Figure 3b). However, an increase of polymerization temperature to 120 °C provides a much better process control, resulting in the reduction of the content of termination reaction and the increase of CL consumption (up to 92%). Polyesters produced at T = 120 °C have higher Mn (up to ∼5000 g mol−1) and dispersity comparable to those obtained at T = 80 °C (no. 14 in Table 1). As observed, the lack of initiating water molecules leads to a totally different type of behavior. Note that for threecomponent system (CL + H2O + alumina), we did not observe any termination effect, and the polymerization proceeded up to the total consumption of monomer (see Figure 3a). The reference reaction without alumina (only CL + H2O) was performed as well, where a similar occurrence of slow initiation and some termination process were observed (see Figure 3b). Interestingly for the reactions carried out without water, PCL of the highest Mn from all examined samples was produced. However, the polymers were characterized by broad molecular weight distributions (Đ = 1.62− 1.70), which is significantly larger when compared to waterinitiated systems (Đ = 1.05−1.30). Such increase of Đ might be due to the lack of control over polymerization and side reactions occurring within the reacting system. The absolute molecular weights and their distributions of polyesters were measured by means of GPC-LALLS. Representative GPC traces of PCL produced at macroscale at 80 °C and in the presence of aluminum oxide powder (with and without water) are presented in Figure 4. As shown, the GPC trace obtained for polymers produced in the presence of water is characterized by lower Mn (Mn = 1430 g/mol; no. 7:

Figure 4. GPC traces of the poly(ε-caprolactone) produced at macroscale at 80 °C in the presence of alumina powder with (gray solid line, no. 7) and without (black solid line, no. 12) water initiating agent.

gray solid line) and much lower dispersity (manifested as narrower peak in GPC) when compared to the one recorded for polymer produced without water (Mn = 2000 g/mol; no. 12: black solid line). It must be stressed that all GPC traces display unimodal peaks without any tailing. As presented above, the applied alumina materials might also be in the same way treated as an initiator, which operates simultaneously as a catalyst (so-called INICATs) of the ROP of CL. Although it is not an effective system. In this context, one can recall that recently hydroxyalkylated strong organic bases were successfully applied as INICATs in polymerizations of Llactide, resulting in a living and controlled process.33 We suppose that INICAT properties of alumina powder/sheet might be related to the presence of some −OH groups on the surface, which in the absence of water interacts with CL initiating the reaction, while Al atoms might coordinate a hydroxyl group to the CL carbonyl carbon. Additionally, aluminum oxide accelerates polymerization and provides a more controlled process up to the end of reaction. ROP Polymerization of CL under 2D Confinement Conditions. As a next step, the examined ROP reaction was carried out within AAO templates of different pore diameter, d = 150, 100, and 35 nm. One can recall that the application of constrained geometries is at the moment one of the most investigating and interesting research idea, enabling us to perform controlled polymerization reaction at the nanoscale and obtain materials of unique morphologies. Among the variety of available and explored constrained geometries, examples include the meso- and microporous zeolites, inclusion complexes, liquid and organic crystals, and micelles.34 Herein, we applied the constrain medium made of aluminum oxide (Al2O3) composed of uniaxial channels (open from both sides) with well-defined pore size, d, as a constraint media (see Figure 1c). The examined matrix is extremely stable (independently of the applied reaction conditions) and might be reused after the proper purification as an easy and cheap catalytic system, which makes it beneficial from an economical point of view. Additionally, it should be highlighted that this type of membrane was successfully applied in the case of reversible E

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constraint and interaction within the system. It should be added that a comparable behavior was observed also in the case of ROP of CL + H2O carried out at macroscale (see Figure 3a). On the other hand, for reactions performed without water, we observed a totally different pattern of behavior. The examined dependence shows a deviation in the linear dependence due to a negative (slowing down) curvature of the kinetic plot, indicating some termination of the reaction and the end of the process. In this case, the biggest deviation can be seen for the smallest pore diameter, d = 35 nm, which might be due to the strongest host−guest interactions. Note that a similar occurrence of termination reaction was observed previously for CL + alumina powder reaction at macroscale, where the obtained product was characterized by high dispersity, Đ = 1.62−1.7 (see Table 1). As shown, the nanoreactions carried out in the presence and absence of water behaves differently. Moreover, there is some connection/similarities in the kinetics of ROP carried out at macroscale and in nanotemplates for both investigated systems. In this context, one can suppose that the reported difference might be caused by the AAO templates playing double roles as INICAT (initiator and catalyst), where the efficiency of the examined reaction might be lower when compared to the one with water acting as initiator. As a consequence, in the system catalyzed solely by alumina termination of the reaction occurs. After extraction of produced poly(ε-caprolactone) (PCL) from the AAO templates with the use of THF, we performed NMR and GPC analysis to determine α, Mn, and Đ (see Table 2). Note that the product of the reaction carried out under 2D

addition−fragmentation chain transfer (RAFT) nanopolymerization, where polymeric nanowires of tailored-made properties were obtained (Mn up to 266.0 kg mol−1 and Đ = 1.21−1.38).35 In this study, we examined ROP nanopolymerization of two systems: CL + H2O ([CL]0/[H2O]0 = 15/1) and CL without water as initiating agent at T = 80 °C. The progress of the examined nanoreactions was monitored by means of differential scanning calorimetry (DSC). Raw DSC data obtained for the both studied systems are presented in Figure 5, where the

Figure 5. Raw data collected for the nanopolymerization of CL mixed with (a) and without (b) water carried out at T = 80 °C. As an inset in panels (a) and (b), pseudo-first-order kinetic plot versus time for ROP of CL with and without water, respectively.

Table 2. ROP Polymerization of CL under 2D Conditions (Within AAO Templates)

exothermic peak of polymerization can be observed, reaching endset after approximately 40 min for all samples, independently of the examined system and pore diameter. Note that the nanopolymerization reaches the end point after approximately 100 times shorter time when compared to the macroscale conditions (see Table 1). Moreover, the enthalpy of the reaction, ΔHreaction, seems to be the highest for the nanopolymerization carried out within AAO templates of d = 35 nm (for details see the Experimental Section). However, it should be highlighted that ΔHreaction for polymerization within alumina membranes is comparable for all applied d for the CL + H2O system. Note that the obtained DSC data were shifted vertically for clearer presentation (see Figure 5a). On the other hand, some deviation can be seen for nanopolymerization of CL, where ΔHreaction seems to differ dependently on the pore diameter, which might be due to different system reactivity and conversion reached at nanochannels of varying d (see Figure 5b). The kinetics plots of the ROP nanopolymerization of CL at T = 353 K are presented as insets in Figures 5a and 5b, for water initiated and pure monomers, respectively. Note that the kinetic plot was based on conversion estimated from DSC data (see Experimental Section). As illustrated for CL + H2O, the pseudo-first-order kinetic plots of ln([M]t/[M]0) versus time exhibit some deviations from the linear dependence due to a positive curvature (acceleration) of the kinetic plot, which suggests slow initiation for all the applied pore diameters (see Figure 5a). However, the smallest acceleration (the most linear trend) can be seen for d = 150 nm, indicating the most controlled reaction. This discrepancy might be a result of the most suitable counterbalance between the applied geometrical

no.

pore diam [nm]

2.1 2.2

150 35

2.3 2.4

150 35

temp [°C]

time [min]

conva [%]

[CL]0/[H2O]0 = 15/1 80 40 96 80 40 92 CL (without water) 80 40 60 80 40 56

Mnb [kg/mol]

Đb

45.1 53.5

1.27 1.30

8.6 10.7

1.41 1.30

a

Determined by 1H NMR, CDCl3, 600 MHz. bDetermined by GPCLALLS, THF, 35 °C, calculated dn/dc = 0.075.

confinement might be obtained according to the following methods: (i) extraction and/or (ii) template removal. The first approach enables to characterize the properties of confinedsynthesized polymers and perform, i.e., reliable GPC and NMR measurements, while the second one allows to observe the free polymer morphology and is often used in i.e. SEM studies (see Experimental Section and Figure 7).31 In our studies, we applied both methods. 1 H NMR analysis confirmed the chemical structure of the PCL synthesized in nanochannels and provided a possibility to determine the monomer conversion, α. NMR spectra obtained for PCL obtained at macroscale are presented in Figure 2. Note that due to the low quantity of NMR spectra of confinedsynthesized samples (small amount of the sample and impurities present after the extraction), they are not shown in this paper. As illustrated, the examined nanosystems are characterized by high conversion, reaching α ∼ 100% and α ∼ 60% for CL + H2O and CL nanoreactions, respectively (see samples no. 2.1 and no. 2.3 in Table 2). Surprisingly, no F

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Macromolecules pronounced difference can be observed for the reaction carried out at membranes of varying pore diameter (see samples no. 2.1 and no. 2.2 in Table 2). One can add that comparable results were also reported in the case of RAFT and free radical polymerization (FRP) of imidazolium-based ionic monomers under 2D confinement, where conversion reached αRAFT ∼ 96% and αFRP ∼ 60%, respectively.22,36 Note that high α is often indicated as one of the nanopolymerization advantage. Moreover, the obtained values of α are comparable to those estimated for macroscale reaction at the end of the reaction (see Table 1). Nevertheless, it should be emphasized that the high conversion of ROP nanopolymerization (α ∼ 56−96%) is reached after 40 min (see Figure 5), while in the case of macroscale reaction a similar α was achieved after more than 90 h. As presented in Table 2, the examined ring-opening polymerization jnder 2D confinement resulted in PCL with Mn up to 53.5 kg/mol of a moderate dispersion (Đ = 1.27− 1.41) and high monomer conversion α = 56−96%. Interestingly, Mn of produced macromolecules reaches similar values independently of the pore size for both studied systems, i.e., Mn ∼ 8−10 kg/mol in the case of nanopolymerization of CL (see Table 2). It should be added that in the literature different effects have been reported for the pore size dependence of Mn; i.e., in the case of radical polymerization, an increasing confinement (decrease of pore size) was generally reported to result in polymers of highest Mn and lowest Đ. Note that the diffusion of nascent polymer chains is discussed to be limited by the applied confinement conditions, which have no impact on the diffusion of monomer, leading to macromolecules of high Mn and narrow distributions.37,38 On the other hand, for controlled reaction as RAFT under confinement, the highest Mn was reported for the greater pore diameter.22 In this context, one can assume a different mechanism ongoing changes in examined ROP reaction, resulting in similar Mn independent of confinement. One can add that ROP nanopolymerization of lactones carried out within an aluminosilicate mesoporous zeolite (AlMCM-41) resulted in polylactones of Mn up to 18 kg/mol of narrow distribution (Đ = 1.07−1.28).23,24 It should be highlighted that the confined-synthesized PCL produced by us are characterized by significantly higher Mn than those synthesized at macroscale. In the case of CL + H2O systems, we obtained polylactones of more than 10 times higher molecular weight (longer polymer chains), while for CL reaction, we received twice higher Mn with smaller Đ in reaction, which instead of 120 h (5 days!) takes only 40 min. The Mn of polymers obtained under confinement are also higher than those produced at elevated pressure conditions for 96 h (Mn = 1.6−19.0 kg/mol and Đ ≈ 1.07−1.50).32 Nevertheless, it should be pointed out that when we compared ROP polymerization of CL without water at macroscale and under confinement, the only difference between these both systems is the geometry of applied alumina (the same material!), leading to clearly different products (where higher Mn and lower Đ were obtained under confinement). We assume that the restricted spaces of applied alumina membranes forces shorter and active polymer chains to grow homogeneously along nanochannels, and thus we obtained macromolecules of narrow molecular weight distribution. The representative GPC traces of poly(ε-caprolactone) (PCL) produced under 2D confinement recovered from AAO templates by dilution in THF are presented in Figure 6. As illustrated, the GPC traces of

Figure 6. GPC traces of the poly(ε-caprolactone) produced under 2D confinement at 150 nm pores with (no. 2.1) and without (no. 2.3) water molecule as initiator.

polymers obtained for d = 150 nm with (no. 2.1) and without water (no. 2.3) display unimodal and narrow peaks, despite the fact that, as we suspect, this polymer is the result of intra- and intermolecular transesterifications involving incorporation of the growing species on shorter active polymer chain in nanochannels. As discussed above, the 2D ROP nanopolymerization performed within alumina membrane resulted in (i) shorter reaction time, (ii) higher Mn (up to 53.5 kg/mol) of moderate distribution (Đ = 1.27−1.41), and (iii) higher monomer conversion when compared to the macroscale polymerization. Note that all reactions (nanoscale and macroscale) were performed for the same conditions, where [CL]0/[H2O]0 = 15/ 1 and alumina catalyst were used. It should be highlighted that ROP polymerizations are often accompanied by side reaction, i.e., hydrolysis and chain transfer by transesterification (also designated as backbiting reactions). Nevertheless, herein due to the applied confinement, the side reactions were successfully suppressed, resulting in macromolecules of moderate molecular weight distribution and unimodal GPC peaks (see Figure 6). We believe that such unexpected outcomes are strictly related to the application of hard confinement conditions and the pace of the reaction. One can recall that three effects play major role in the behavior of soft matter under 2D geometrical restriction: finite size, surface interaction, and free volume, where a specific counterbalance between them leads to unusual behavior/ properties. As mentioned above, the nanometer size limits the diffusion of active polymer chains, resulting in polymers of high Mn and narrow distributions.37,38 Additionally, the shape and size of the applied membranes indicate the unique possibility of producing polylactones of highly designed morphology according to our desired purpose. Herein, due to the uniaxial structure of applied AAO membranes (see Figure 1c), we were able to produce nanowires and/or nanotubes macromolecules of PCL, which might be helpful in any nanobiomedical application. Figure 7 shows SEM images recorded for nanoscale and macroscalesynthesized PCL. As illustrated, the recovered polymer retains the wire/tubule shape of applied nanochannels. However, it appears that the obtained nanorods underwent aggregation in aqueous media and formed larger fibers, and thus their external size appears to be in the range of 246−542 nm (see Figure 7a). However, at higher magnification, also some nonaggregated polymers can be seen (d = 150 nm, Figure 7b). On the other G

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Figure 7. SEM images of PCL nanowires (a, b) and those synthesized at the macroscale (c, d).

hydroxyl or alkoxyl group to the CL carbonyl carbon occurs. Propagation step involves ring-opening by an anionic propagation mechanism to produce dimers and so on, resulting in telechelic PCL with acidic and hydroxyl end groups. Note that this mechanism requires the formation of Al alkoxides (Al−O−R) in the initiation step of the process. However, due to the INICAT properties of applied alumina, we assume that the surface −OH groups coordinates Al atom enough to trigger the reaction accordingly to this mechanism. It should be mentioned that the application of INICAT results in novel kinetic and mechanistic features within the reactive system.33 In this way, we obtained nanowires of PCL characterized by Mn up to 53.5 kg/mol of moderate distribution (Đ = 1.27−1.41). However, this issue requires further comprehensive studies. Note that in the case of mesoporous zeolites materials applied as a surface catalyst in ROP of lactones (δ-valerolactone and εcaprolactone), the process was reported to undergo via monomer activated mechanism due to a combination of Lewis acidic aluminum and Bronsted acidic silanol functionalities on the interior wall of applied templates, resulting in macromolecules of Mn up to 18 kg/mol of narrow dispersion (Đ = 1.07−1.28).23,24 Moreover, one cannot forget that the soft matter under 2D geometrical constraint is characterized by higher effective free volume than macroscale material.47 The comprehensive studies carried out for various glass-forming liquids incorporated into both silica and alumina nanoporous indicate that the reduced

hand, macroscale-synthesized PCL exhibits morphology of the spherical aggregates (Figure 7c) or wrinkled flakes (Figure 7d). As discussed above, application of AAO templates generates much stronger interactions with the catalyst than in the case of macroscale reaction, where no confinement effects are expected. Note that CL incorporated within alumina nanoporous is confined along nanochannels of defined pore diameter (see Figure 1c). Thus, the catalytic system might be more effective, leading to higher Mn (see Table 2). However, apart from this high surface-to-volume ratio of membranes, we believe that this higher efficiency also might be caused by the presence of −OH groups attached to the walls of applied AAO templates. Note that alumina mesoporous are prepared due to the two-step anodization process, in acidic environment/ solution, resulting in the presence of Al−O−H structure within the applied nanocavities.39 In this context, one recalls the structure of metal alkoxides (M−O−R), which are one of the most frequently applied catalysts of ROP of cyclic esters40 and operate by a pseudoliving coordination−insertion mechanism.41−43 Among many metals, aluminum-based compounds are characterized by high selectivity and low toxicity, and thus they are the most used ones.44−46 By analogy, we assume that a similar mechanism might take place in the case of 2D ROP nanopolymerization, which can occur as follows. CL exocyclic oxygen coordinates to the Al, making the carbonyl carbon of lactone more susceptible to nucleophilic attack. After the formation of coordinating species, nucleophilic addition of H

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(7) Dubois, P., Coulembier, O., Raquez, J. M., Eds.; Handbook of Ring-Opening Polymerization;Wiley-VCH: Weinheim, 2009. (8) Labet, M.; Thielemans, W. Synthesis of Polycaprolactone: a Review. Chem. Soc. Rev. 2009, 38, 3484. (9) Martin, C. R. Membrane-based Synthesis of Nanomaterials. Chem. Mater. 1996, 8, 1739−1746. (10) Uemura, T.; Ono, Y.; Kitagawa, K.; Kitagawa, S. Radical Polymerization of Vinyl Monomers in Porous Coordination Polymers: Nanochannel Size Effects on Reactivity, Molecular Weight, and Stereostructure. Macromolecules 2008, 41, 87−94. (11) Giussi, J. M.; Blaszczyk-Lezak, I.; Cortizo, M. S.; Mijangos, C. In-situ Polymerization of Styrene in AAO Nanocavities. Polymer 2013, 54, 6886−6893. (12) Ng, S. M.; Ogino, S.; Aida, T.; et al. Free Radical Polymerization within Mesoporous Zeolite Channels. Macromol. Rapid Commun. 1997, 18, 991−996. (13) Li, Q.; Simon, S. L. Curing of Bisphenol M dicyanate Ester under Nanoscale Constraint. Macromolecules 2008, 41, 1310−1317. (14) Lopez, E.; Simon, S. L. Trimerization Reaction Kinetics and Tg Depression of Polycyanurate under Nanoconfinement. Macromolecules 2015, 48, 4692−4701. (15) Tarnacka, M.; Dulski, M.; Starzonek, S.; Adrjanowicz, K.; Mapesa, E. U.; Kaminski, K.; Paluch, M. Following Kinetics and Dynamics of DGEBA-aniline Polymerization in Nanoporous Native Alumina Oxide Membranes−FTIR and Dielectric Studies. Polymer 2015, 68, 253−261. (16) Uemura, T.; Kitagawa, K.; Horike, S.; Kawamura, T.; Kitagawa, S.; Mizuno, M.; Endo, K. Radical Polymerisation of Styrene in Porous Coordination Polymers. Chem. Commun. 2005, 5968−5970. (17) Li, X.; King, T. A.; Pallikari-Viras, F. Characteristics of Composites Based on PMMA Modified Gel Silica Glasses. J. NonCryst. Solids 1994, 170, 243−249. (18) Pallikari-Viras, F.; Li, X.; King, T. A. Thermal Analysis of PMMA/Gel Silica Glass Composites. J. Sol-Gel Sci. Technol. 1996, 7, 203−209. (19) Kalogeras, I. M.; Neagu, E. R. Interplay of Surface and Confinement Effects on the Molecular Relaxation Dynamics of Nanoconfined Poly(methyl methacrylate) Chains. Eur. Phys. J. E: Soft Matter Biol. Phys. 2004, 14, 193−204. (20) Uemura, T.; Nakanishi, R.; Mochizuki, S.; Murata, Y.; Kitagawa, S. Radical Polymerization of 2, 3-dimethyl-1, 3-butadiene in Coordination Nanochannels. Chem. Commun. 2015, 51, 9892−9895. (21) Kawata, T.; Uekusa, H.; Ohba, S.; Furukawa, T.; Tokii, T.; Muto, Y.; Kato, M. Magneto-structural Correlation in Dimeric Copper (II) Benzoates. Acta Crystallogr., Sect. B: Struct. Sci. 1992, 48, 253−261. (22) Maksym, P.; Tarnacka, M.; Dzienia, A.; Erfurt, K.; Chrobok, A.; Zięba, A.; Kaminski, K.; Paluch, M. A Facile Route to Well-defined Imidazolium-based Poly(ionic liquid)s of Enhanced Conductivity via RAFT. Polym. Chem. 2017, 8, 5433−5443. (23) Kageyama, K.; Ogino, S.; Aida, T.; Tatsumi, T. Mesoporous Zeolite as a New Class of Catalyst for Controlled Polymerization of Lactones. Macromolecules 1998, 31, 4069−4073. (24) Kageyama, K.; Tatsumi, T.; Aida, T. Mesoporous Zeolite as a New Class of Catalyst for Polymerization of Lactones. Polym. J. 1999, 31, 1005−1008. (25) http://www.synkerainc.com/products-services/unikeraceramic-membranes/unikera-standard. (26) Jung, J. H.; Ree, M.; Kim, H. Acid- and Base-Catalyzed Hydrolyses of Aliphatic Polycarbonates and Polyesters. Catal. Today 2006, 115, 283−287. (27) Lorenzo, A. T.; Arnal, M. L.; Albuerne, J.; Muller, A. J. DSC Isothermal Polymer Crystallization Kinetics Measurements and the Use of the Avrami Equation to Fit the Data: Guidelines to Avoid Common Problems. Polym. Test. 2007, 26, 222−231. (28) Parthun, M. G.; Johari, G. P. Dielectric Spectroscopy of a Polymerizing Liquid and the Evolution of Molecular Dynamics with Increase in the Number of Covalent Bonds. J. Chem. Phys. 1995, 103, 440−450.

packing density is responsible for the enhancement of their molecular dynamics.48−50 Thus, the observed significant acceleration of reaction time might be by analogy caused by variation in free volume generated by the applied 2D hard confinement. We believe that all of these factors (finite size, surface interaction, and free volume) have their own contributions to the observed behavior, although the particular contribution of each factor requires further studies.

IV. CONCLUSION In conclusion, we report the impact of alumina in the form of sheets and powder templates as well as nanotemplates for ROP polymerization of ε-caprolactone. As presented, the effectiveness of aluminum oxide was the highest when it was used as nanoreactors for the reaction. Our studies indicated that it may operate accordingly to the pseudoliving coordination−insertion mechanism, resulting in nanowires of PCL characterized by Mn up to 53.5 kg/mol with moderate distribution (Đ = 1.27−1.41). We believe that the reported data open an unique and alternative way of easy and cheap processing of ROP reaction with industrial potential, resulting in polylactones with tailoredmade properties and morphologies.

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AUTHOR INFORMATION

Corresponding Authors

*(M.T.) E-mail [email protected]. *(K.K.) E-mail [email protected]. ORCID

Magdalena Tarnacka: 0000-0002-9444-3114 Andrzej Dzienia: 0000-0002-9628-0224 Paulina Maksym: 0000-0002-8506-7102 Agnieszka Talik: 0000-0001-7940-6967 Rafał Bielas: 0000-0002-5129-063X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.K., A.D., A.T., and M.T. are thankful for the financial support from the Polish National Science Centre within SONATA BIS 5 project (Dec 2015/18/E/ST4/00320).

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REFERENCES

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