Phosphazene-Catalyzed Regioselective Ring-Opening

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Phosphazene-Catalyzed Regioselective Ring-Opening Polymerization of rac-1-Methyl Trimethylene Carbonate: Colder and Less is Better Geng Hua,† Johan Franzén,‡ and Karin Odelius*,† †

Department of Fibre and Polymer Technology and ‡Department of Organic Chemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden

Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on March 27, 2019 at 04:06:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The regioselective organocatalytic ring-opening polymerization (ROP) of a 6-membered cyclic carbonate, rac1-methyl trimethylene carbonate, was studied using phosphazene base (t-BuP2) as the principle catalyst. The influence on the reaction kinetics caused by the reaction temperature (−74−60 °C), catalyst loading (0.5−2.5%), and reaction solvent (toluene and tetrahydrofuran) was systematically tuned and followed by 1H NMR. All studied reactions reached close to or above 90% monomer conversion in 3 h, and all exhibited typical equilibrium polymerization behavior that is inherent to 6-membered cyclic carbonates. Good control over the molecular weight and distribution of the polycarbonate product was obtained in most studied conditions, with Mn ranging from ∼4k to ∼20k and Đ < 1.2. The regioregularity (Xreg) of the resulting polycarbonate was thoroughly studied using various NMR techniques, with the highest Xreg obtained being 0.90. The major influence from the reaction conditions on both the ROP kinetics and Xreg are as follows: higher reaction temperature resulted in a decrease of both; higher catalyst loading resulted in a faster ROP reaction but a slight decrease in Xreg; and toluene being a better solvent resulted in both faster reaction and higher Xreg. Throughout this study, we have demonstrated the possibility to synthesize regioregular aliphatic polycarbonate using an organic base as the ROP catalyst, contrary to the existing studies on similar systems where only metal−base catalysts were in focus and our systems showed similar high Xreg of the product.

■

INTRODUCTION Aliphatic polycarbonates with well-defined structures have found widespread applications in the biomedical field, with examples including degradable elastomers, hydrogels, and fiber meshes.1−3 The regioregularity and stereoregularity of a polymer chain induces its crystallization behavior and crystallinity, which can greatly influence the melting temperature, mechanical properties, and degradation profile of a polymer.4−7 Thus, as a presynthetic consideration for degradable aliphatic polycarbonates, optimizing the regularity of the chains is of great interest. One of the most common way to synthesize aliphatic polycarbonates is the ring-opening polymerization (ROP) of the corresponding cyclic carbonates.8−11 For asymetrically monosubstituted 5-membered (5CC) and 6-membered cyclic carbonates (6CC), the carbonate moieties can be seen as two structurally diverse ester groups with a shared carbonyl: one ester group connects to carbon with the substituent (hindered) and the other ester group connected the methylene carbon (unhindered). During the enchainment of the cyclic carbonates in ROP, the acyloxygen cleavage site can be either the hindered or the unhindered ester side. This generates three possible types of microstructures of the repeating units: head-to-head (HH), head-to-tail (HT), and tail-to-tail (TT). When the HT © XXXX American Chemical Society

microstructure is prevalent, the polycarbonate can be regarded as a regioregular polymer. Organometallic catalysts have been developed and used to control the regioregularity of polycarbonates during polymerization. This includes a large variety of chromium-, zinc-, aluminum-, and cobalt-based salen-type catalysts for the ringopening copolymerization of carbon dioxide with propylene oxide and cyclohexane, respectively, with moderate to high degree of regularity (Xreg) of up to 0.99.12−15 On the contrary, the direct ROP route of cyclic carbonates is much less studied. Some recent examples include the use of heteroleptic magnesium and a zinc silylamido complex to polymerize rac1-methyltrimethylene carbonate (rac-1-MeTMC), resulting in a degree of regularity (Xreg) between 0.78 and 0.98;16 the use of zinc β-diiminate complex to polymerize rac-1-MeTMC as well as α- and β-methyltetramethylene carbonates with varying Xreg from 0 to higher than 0.98.17,18 In the same studies, the use of several nonmetal organic base catalysts such as 4-N,Ndimethylaminopyridine, 1.5.7-triazabicyclo-[4.4.0]dec-5-ene (TBD), and 2-tert-butylimino-2-diethylamino-1,3-dimethylperReceived: December 5, 2018 Revised: March 12, 2019

A

DOI: 10.1021/acs.macromol.8b02591 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Summary of Polymerization Entries entry

solvent

Ta (°C)

ratiob [I]/[cat.]/[M]0

timec (min)

conv.d (%)

Mnd (NMR)

Mne (theory)

Mnf (SEC)

Đf

1 2 3 4g 5 6 7 8 9 10 11 12 13h 14h 15i 16i

THF THF THF THF THF THF THF THF PhMe PhMe PhMe PhMe PhMe PhMe PhMe PhMe

−74 −40 −40 −40 −40 −40 RT 60 −40 −40 RT RT −40 RT −40 RT

1:1:40 1:1:40 1:1:200 1:1:200 1:5:200 5:1:200 1:1:40 1:1:40 1:1:40 1:1:200 1:1:40 1:1:200 1:1:200 1:1:200 1:1:200 1:1:200

60 30 180 180 180 180 30 60 1 10 30 30 180 180 180 180

94 95 88 89 94 93 86 72 95 94 91 91 39 81 5.7 3.7

4000 3800 10 600 10 500 12 800 3400 3400 2800 3800 15 100 3400 19 100 5400 12 200 ndj nd

4100 4100 13 700 13 900 14 700 4000 3700 3200 4100 14 600 4000 14 200 6100 12 600 nd nd

3600 3600 4900 4800 5600 3400 2600 2300 3100 6000 3300 5100 1900 3000 nd nd

1.19 1.18 1.17 1.13 1.12 1.18 1.28 1.50 1.21 1.13 1.26 1.14 1.12 1.18 nd nd

A fluctuation of ±2 °C existed for reactions conducted at −74 and −40 °C, room temperature (RT) was around 20 °C. bDesigned loadings, actual measured value was 1.1:1:40 for all the 1:1:40 entries and 1.5:1:200 for the 1:1:200 entries, 1.5:1:200 for entry 5 and 5.5:1:200 for entry 6. c Recorded as the time closest to or having reached equilibrium. dCalculated through raw sample NMR of the quenched aliquots, the results were rounded up or down to the nearest multiple of 10. eCalculated through the actual measured amounts of the reactants multiplied by the monomer conversion obtained through 1H NMR, the results were rounded up or down to the nearest multiple of hundred. fObtained from size exclusion chromatography (SEC) using chloroform as the eluent and the results were plotted against polystyrene standards with narrow distribution. gThe initial monomer concentration [M]0 was 2 M for this entry. hCatalyzed by TBD. iCatalyzed by DBU. jNot determined. a

99.8%), sodium hydride (NaH, 60% dispersion in mineral oil), calcium hydride (CaH2, 99.9%), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, 98%), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, ≥99%), 1tert-butyl-2,2,4,4,4-pentakis-(dimethylamino)-2λ 5 ,4λ 5 -catenadi(phosphazene), (t-BuP2, ∼2.0 M in THF), benzoic acid (PhCOOH, ≥99.5%), and trichloroacetyl isocyanate (≥97.0%) were all purchased from Sigma-Aldrich and used as received. Chloroform-d (CDCl3, 99.5%) was purchased from Cambridge Isotope Laboratories. Syntheses of rac-1-Methyl Trimethylene Carbonate (rac-1MeTMC). rac-1-MeTMC was synthesized according to a ring-closing depolymerization procedure.26 The obtained product was passed through an Isolera 4 (Biotage, Sweden) for flash purification (1:3, v/v, hexane/ethyl acetate), redistilled over CaH2, and followed by three freeze−pump−thaw cycles using a Schlenk apparatus right before transferring the monomer into a glovebox (Mbraun MB 150-GI, Germany). Detailed 1H and 13C NMR of the monomer is given in the Supporting Information (Figures S1 and S2). General Polymerization Kinetics Procedure. All measurements and weightings were carried out inside a glovebox. For a typical setup (entry 2, Table 1), approximately 464 mg (4 mmol) of rac-1MeTMC was measured directly into a 1 mL disposable syringe and put into a self-sealing bag. A 1.1 mL (1.1 mmol) stock solution of BnOH (1 M) in anhydrous THF was transferred into a 20 mL headspace vial equipped with a magnetic stirrer. Then, 50 μL (1 mmol) of t-BuP2 (∼2 M THF solution) was injected into the vial, followed by the addition of 2.5 mL (to adjust the monomer concentration to 1 M) anhydrous THF. The vial was sealed and taken out of the glovebox together with the monomer-containing syringe. The vial was placed in a cooling bath held at −40 °C for 10 min before the monomers were quickly injected under vigorous stirring. Aliquots of the reaction mixture were taken out after the desired time points through chilled disposable syringes and quickly quenched in excess amount of PhCOOH (THF solution). The quenched mixtures were air-dried inside a fume hood and stored at 4 °C for further analysis. Nuclear Magnetic Resonance (NMR) Spectroscopy. Both 1H NMR (400.13 MHz) and 13C NMR (100.61 MHz) spectra of the crude samples were acquired from a Bruker Avance DPX-400 NMR instrument. The samples (∼15 mg) were dissolved in ∼1 mL of CDCl3 with 0.1% TMS (v/v %, δ 0 ppm) as the internal standard. The monomer conversions in the kinetics plots were calculated as the

hydro-1,3,2-diazaphosphorine, obtaining poly(rac-1-MeTMC) under similar conditions is described with Xreg around 0.39.17 As with many other cyclic monomers, the ROP behavior of 6CC is greatly influenced by the reaction conditions such as temperature, solvent, monomer concentration, and catalyst type.11,19−23 Temperature is an especially influential factor due to comparably low ceiling temperature of the 6CC.24 A common scenario is that a comparably higher 6CC monomer conversion is obtained at a lower temperature compared to a higher temperature,21 and previous studies have shown a decreased regioselectivity during polymerization with increased reaction temperature.18 Combining these two thoughts, we hypothesized that tuning the reaction temperature and thus changing the thermodynamic factors could be an effective tool toward a high regioregular polycarbonate using organic catalysts. To carry out the ROP of 6CC at low temperature, a highly effective catalyst is required. The commercially available phosphazene base (t-BuP2) was chosen as the model catalyst, with recent studies showing high activity to polymerize the low ceiling temperature γ-butyrolactone at −40 °C.25 Hence, the aim of this proof-of-concept study is to tune the regioselective ROP of an asymetrically substituted 6CC using rac-1-MeTMC as the model monomer and phosphazene base (t-BuP2) as the catalyst. The influence on the regioregularity of the formed poly(rac-1-MeTMC) caused by the reaction temperature, solvent, monomer concentration, catalyst loading, and reaction time will be thoroughly evaluated through detailed ROP kinetics. The Xreg of the final product and the chain-end of the poly(rac-1-MeTMC) will be examined and discussed in detail through a combination of various NMR techniques.

■

EXPERIMENTAL SECTION

Materials. Tetrahydrofuran (THF, anhydrous, ≥99.9%), toluene (PhMe, anhydrous, 99.8%), (±)-1,3-butanediol (99.5%), diethyl carbonate (anhydrous, ≥99%), benzyl alcohol (BnOH, anhydrous, B


Article

Macromolecules

Figure 1. (a) Temperature dependence of the ROP of rac-1-MeTMC using THF as a solvent and t-BuP2 as a catalyst, [I]/[cat.]/[M]0 = 1:1:40 with [M]0 = 1 M. (b) Influence on ROP of rac-1-MeTMC by initial monomer, catalyst, and initiator loadings. (c) Quenched raw sample showing the converted and unconverted monomer peaks in 1H NMR with a monomer conversion of 44%.

resulting polycarbonate product was examined through 13C NMR and the reaction conditions were subsequently optimized to yield a ROP system that comprised very fast polymerization rate and high regioregularity. The ROP reaction and the structure of the studied catalysts are summarized in Scheme 1.

ratios between the integrals of polycarbonate (δ 4.18−4.28 ppm) and (polycarbonate + rac-1-MeTMC (δ 4.36−4.50 ppm)), see Figure 1c for detailed spectra and calculation. The apparent rate constant of polymerization (kpapp) was calculated using the following equation kpapp = ln[([M]0 − [M]eq)/([M] − [M]eq)]/t, where [M]0 was the initial monomer concentration, [M]eq was the equilibrium monomer concentration, and [M] was the monomer concentration at time t. Distortionless enhancement by polarization transfer (13C DEPT), heteronuclear single quantum coherence (1H−13C HSQC), and heteronuclear multiple bond correlation (1H−13C HMBC) were used together to determine the chain-end of the polycarbonate products. Size Exclusion Chromatography (SEC). A Verotech PL-GPC 50 Plus system equipped with two PLgel 5 μm MIXED-D (300 × 7.5 mm2) columns from Varian and a PL-RI Detector was used to analyze the samples. Chloroform was used as an eluent (1 mL/min, 30 °C). The sample solutions were injected through a PL-AS RT Autosampler. Polystyrene standards (160−371 000 g/mol) with narrow molecular weight distributions were used for the calibration curve. Toluene was used as the internal standard for flow rate fluctuation corrections. All data were processed through Cirrus GPC Software.

Scheme 1. Summary of the Reaction

■

Polymerization Kinetics Studies. An initial assessment of the influence of reaction temperature on the ROP kinetics of rac-1-MeTMC catalyzed by t-BuP2 was carried out in THF. Temperatures spanning a wide range, i.e., −74, −40 °C, RT (∼22 °C), and 60 °C, were used, and for all temperatures, very high catalytic activities were seen from the monomer

RESULTS AND DISCUSSION The influences of reaction temperature, solvent, monomer concentration, catalyst type, catalyst loading, and reaction time on the ROP reaction of rac-1-MeTMC were followed through a series of comparative kinetics. The regioregularity of the C


Article

Macromolecules

Figure 2. Polymerization kinetics influenced by catalyst loadings and solvent. (a) Influence on ROP of rac-1-MeTMC from initial monomer, catalyst, and initiator loadings. The reactions were carried out at −40 °C and RT in toluene using t-BuP2 as a catalyst with [M]0 = 1 M. (b) Influence on ROP of rac-1-MeTMC from solvent and temperature. The reactions were carried out either in toluene or THF under −40 or 20 °C, [I]/[cat.]/[M]0 = 1:1:40 with [M]0 = 1 M.

the primary hydroxyl group was formed and acted as the initiating site upon mixing the phosphazene base with BnOH.28 Therefore, a modification of the initiator-tocatalyst-to-monomer ratio should influence the rate of polymerization due to the alteration in the number of initiating sites.29 To verify a mechanism with an H-bonding activation path and the influence of the number of initiating sites, kinetics of ROP with different monomer, initiator, and catalyst loadings were performed in THF at −40 °C (Figure 1b). When the catalyst-to-monomer ratio was fixed at 1:40, a higher loading of the initiator (cf. entries 2 and 5) increased the kpapp from 1.85 × 10−3 to 2.74 × 10−3 s−1. In addition, when the initiator-tomonomer ratio was fixed at 1:40, a higher catalyst loading (0.5 vs 2.5%, entries 6 and 2) increased the kpapp from 0.93 × 10−3 to 2.74 × 10−3 s−1. Finally, when the reactant loading ([I]/ [cat.]/[M]0) was increased from 1:1:40 to 1:1:200 (entries 3 and 1, Table 1), the corresponding kpapp was ∼6.85 × 10−4 s−1, which was over fourfold lower than 2.74 × 10−3 s−1 (1:1:40, entry 2, Table 1). Hence, an increase in catalyst and initiator loadings both positively influenced the ROP reaction rate. Increasing the amount of the catalyst will accelerate the reaction rate more than increasing the initiator loading, since the proposed active site is the alcohol chain-ends with phosphazene pairs.30,31 The choice of solvent has a large influence on the ROP kinetics due to factors such as polarity, solvation of the monomers, and formation of complexes with active sites.32−35 For each solvent−monomer combination, different polymerization rate and equilibrium monomer concentration are expected. Hence, toluene with a low dielectric constant, compared to THF, was used as a solvent to determine the influence on ROP kinetics for rac-1-MeTMC. Four different systems using toluene or THF at −40 °C or RT (entries 2, 7, 9, and 11) with a fixed 2.5% catalyst loading were compared. Extremely fast kinetics was observed in toluene at −40 °C, where the equilibrium was reached within 1 min and with a high monomer conversion of ∼95% (Figure 2b). Even when the reaction was carried out at RT, a ∼90% monomer conversion was observed within 10 min after monomer injection. In both cases, the ROP reaction proceeded faster than the corresponding reactions in THF and the equilibrium

conversions (%) against time (Figure 1). Within 1 h or less after monomer injection, the polymerization equilibria were reached for all reactions. By decreasing the temperature from 60 to −40 °C, not only did the polymerization reach a higher conversion but the reaction rate was also accelerated. This is evident by comparing the highest reaction temperature (60 °C, entry 8) to a lower reaction temperature (−40 °C, entry 2), where a ∼20% increase in equilibrium monomer conversion was seen at the lower temperature in a short time. This can be attributed to the low ceiling temperature for the polycarbonate systems, where a higher temperature negatively affects the overall monomer conversion when all other reaction parameters are retained. Slower reaction rates were also observed at 60 °C with an apparent rate constant of polymerization (kpapp) of 1.82 × 10−3 s−1 compared to 2.74 × 10−3 s−1 at −40 °C. So, a lower temperature is preferred for ROP of rac-1-MeTMC catalyzed by t-BuP2. When the reaction temperature was further decreased to −74 °C (entry 1, Table 1), the equilibrium monomer conversion was maintained at the same as that at −40 °C and a lower kpapp (1.09 × 10−3 s−1) was observed. It is clear that below a reaction temperature of −40 °C, the monomer conversion would no longer increase even when the temperature was further decreased (Figures 1a and S3), and a decrease in kpapp will appear most likely due to the partially insolubilized monomers at −74 °C. Considering both the monomer conversion and the rate of polymerization, −40 °C is considered as the optimal temperature for ROP of rac-1MeTMC in THF using t-BuP2 as the catalyst in this system. The temperature dependence of the [M]eq for the reactions with THF as solvent were also studied, and a detailed plot can be found in Figure S4. The Mn results obtained from NMR (Table 1) are close to the theoretical values. The Mns determined by SEC are only used for comparison between the synthesized polymers and often underestimated for aliphatic polyesters and polycarbonates18,27,28 because SEC measures a relative value from the hydrodynamic volume of the polymer sample against polystyrene standards. In this case, the values obtained from the RI detector are very different for polycarbonate and polystyrene. Benzyl alcohol (BnOH) was added as an initiator in all studied systems. As reported before, H-bonding activation of D


Article

Macromolecules

Figure 3. (a) Evaluation of catalytic activities of TBD and DBU for the ROP of rac-1-MeTMC. The reactions were conducted at 20 °C, [I]/[cat.]/ [M]0 = 1:1:200 with [M]0 = 1 M in toluene. (b) The reactions were conducted at −40 °C, [I]/[cat.]/[M]0 = 1:1:40 with [M]0 = 1 M in toluene.

Figure 4. (a) Possible ring-opening sites of rac-1-MeTMC and the corresponding microstructures of the resulting polymers. (b) 13C NMR carbonyl region of the poly(rac-1-MeTMC) with varied Xreg, top: entry 8, 20 h, Xreg = 0; middle: entry 12, 1 h, Xreg = 0.62; bottom, entry 10, 5 min, Xreg = 0.90.

optimal reaction condition for t-BuP2 catalyzed ROP of rac-1MeTMC. Phosphazene bases along with TBD and DBU have become some of the most preferred base catalysts that offer comparably low catalyst loadings, high catalytic activity, and decent to good control for ROP of cyclic esters.36−38 These three catalysts have varied basicity with DBU (MeCNpKBH+ 24.3), TBD (MeCNpKBH+ 26), and t-BuP2 (MeCNpKBH+ 33.5). The basicity, the structure of the catalyst, and the activation species are factors that influence the polymerization rate.39 The ROP kinetics of rac-1-MeTMC catalyzed by TBD, DBU, and t-BuP2 were compared at −40 °C and RT under optimal conditions (1 M toluene, 0.5% catalyst loading, entries 13−16). The rate of polymerization decreased as the base became weaker, i.e., tBuP2-catalyzed reactions proceeded faster than TBD-catalyzed reactions and much faster than DBU-catalyzed reactions (Figure 3). The conversions of the monomer at the 3 h point followed the same trend (t-BuP2 > TBD ≫ DBU), both at −40 °C and RT The difference between the TBD- and tBuP2-catalyzed ROP is that, unlike the inverse correlation between temperature and reaction rate for t-BuP2, an increase in temperature for the TBD-catalyzed reaction increased the polymerization rate and the monomer conversion. We believe that in this case, the temperature influences the reactivity and solubility of the TBD catalyst. This is why, when the temperature was increased from −40 °C to RT, the reaction proceeded faster. We hypothesize that this inverse correlation for t-BuP2 is inherent to the fact that ROP of cyclic carbonates

monomer conversion was less affected by temperature in toluene (∼4% difference) than in THF (∼10% difference). To determine whether the fast ROP reaction remains when the catalyst/initiator loading was decreased, a catalyst loading of 0.5% at −40 °C and RT (entries 10 and 12, respectively) was conducted and compared (Figure 2a) to the counterparts with 2.5% catalyst loading (entries 9 and 11). As, expected, the fast reaction rate remained at a 0.5% catalyst loading and the monomer conversion reached ∼94% within 10 min (entry 10). In addition, the equilibrium monomer concentration agreed perfectly for each catalyst loading at the same temperature. The reaction conducted in toluene at RT with the lower catalyst loading (entry 12, Table 1) proceeded more slowly but equilibrium was reached within 1 h with a ∼91% monomer conversion. The difference in the reaction rate in toluene and THF has previously been found for ROP of ε-caprolactone catalyzed by t-BuP2 and was described to be caused by the slightly basic nature of the cyclic ether moiety in THF and the competing interference by coordination of the solvent to the activation site.30 From the above experiments, it is clear that toluene is a better solvent than THF if the goal is to achieve a fast polymerization rate at a low temperature and a low catalyst loading. Based on these kinetics and the Mn results concluded in Table 1 where a broadened dispersity of the polymer products was seen with elevated reaction temperature and increased amount of catalysts, the reaction conditions of −40 °C, with 0.5% catalyst loading in toluene was chosen as the E


Article

Macromolecules

Figure 5. (a) Calculated Xreg plotted against time when taking the reactant loadings into account. Reactions were carried out in 1 M toluene at −40 °C. Red dots from left to right: entry 10 (5 min, 1 h, 3 h); black square from left to right: entry 9 (1 min, 1 h, 3 h). (b) Calculated Xreg plotted against reactant loadings when taking the solvent into account. Reactions were carried out in 1 M solution at −40 °C. Red dots from left to right: entry 9 (1 min) and entry 10 (5 min); black square from left to right: entry 2 (30 min) and entry 3 (3 h). (c) Calculated Xreg plotted against reaction temperature. When taking the solvent into account, reactions were carried out in 1 M solution [I]/[cat.]/[M]0 = 1:1:40. Red dots from left to right: entry 9 (1 min) and entry 11 (30 min); black square from left to right: entry 1 (1 h), entry 2 (30 min), entry 7 (30 min), and entry 8 (1 and 20 h).

the 13C NMR. The carbonyl peak with the strongest intensity in the middle represented the regioregular HT enchainment (δ 153.5 ppm) of the monomer, and the other two peaks with lower intensities (δ ± 0.45 ppm) represented the regioirregular HH and TT enchainment of the monomers.44,45 A comparison of the 13C NMR carbonyl region between the different carbonate entries at equilibrium can be found in Figure 5b. The calculated Xreg for the three example samples were, from top to bottom, 0, 0.62, and 0.90, respectively. The complete regiorandom polymer (Xreg = 0, entry 8, Table 1, 20 h) and the highly regioregular polymer (Xreg = 0.90, entry 10, Table 1, 5 min) provide direct evidence that the regioregularity can be easily optimized when using t-BuP2 as the ROP catalyst. The influences of reaction time, catalyst loading (Figure 5a), solvent, (Figure 5b), and reaction temperature (Figure 5c) on the control of the polycarbonate microstructures were summarized and compared. In general, prolonged reaction time results in higher regiorandomness and lower catalyst loading, and lower temperature promotes higher regioregularity. It is crucial that equilibrium is reached when calculating Xreg, since at an early stage of the polymerization, Xreg can be very high; yet, the molecular weight of the polymer will be far off from the designated values. So to make fair comparisons, all Xreg calculations were made with samples that are either at the closest point to or during the equilibrium stage, as noted in Table 1. The influence on regioregularity of prolonged reaction times was examined on the ROP at −40 °C, 2.5% catalyst loading, and toluene solvent (entry 9, Table 1) with the fastest kinetics. The Xreg decreased from 0.85 (1 min) to 0.73 (1 h) and further to 0.61 (3 h) (Figure 5a). This is caused by extensive transesterification reactions occurring at high monomer conversion. At a lower catalyst loading (entry 10, 0.5% cat. loading), this increase in irregularity was also observed but was less intense, with a decrease of Xreg from 0.90 (5 min) to 0.85 (1 h) then to 0.78 (3 h). When the solvent was changed from toluene to THF (Figure 5b), the corresponding Xreg was 0.74 (2.5% cat. loading, entry 2) and 0.83 (0.5% cat. loading, entry 3), both of which were less regioregular than their respective counterparts in toluene. When the temperature was increased from −74 to −40 °C, a decrease of Xreg from 0.82 (entry 1, −74 °C) to 0.74 (entry 2, −40 °C) was

is an equilibrium reaction and therefore reversible. Contrary to the exothermal ROP reactions, ring-closing depolymerization reactions are often endothermic.40,41 Since the reactions take place simultaneously, a strong ROP catalyst is also a strong depolymerization catalyst. The high catalytic activity of t-BuP2 was seen already at temperature as low as −74 °C, and elevating the temperature, in this case, significantly increases the rate of depolymerization rather than polymerization.24,42,43 Another explanation to this behavior can be the higher stability of the propagating center under a lower temperature. Since the proposed mechanism for t-BuP2-catalyzed ROP is alcohol/ chain-end activation through H-bonding, the stability of the activation site is supposedly higher at lower temperature than at higher temperature. In either case, the overall apparent rate of polymerization decreased with increasing temperature. Control over Regioregularity. The effect of the solvent’s dielectric constant, reaction temperature, basicity, and the amount of catalyst and reactants have been elucidated: toluene as a reaction medium at −40 °C with 0.5% t-BuP2 loading was determined as an optimal system for polymerization rate. The ROP was shown to be controlled with linearity of the semilogarithmic plots of ln[([M]0 − [M]eq)/([M] − [M]eq)] against time (Figure S3). Due to the methyl group on the 1position of the monomer, rac-1-MeTMC has two possible locations for the acyl-oxygen cleavage: either through the hindered bond (position i, Figure 4a) or through the unhindered bond (position ii, Figure 5a). Depending on the ring-opening position of the connecting monomer, three different types of microstructures may exist (Figure 4a): head-to-head (HH), head-to-tail (HT), or tail-to-tail (TT). In an ideal case of regioregular ROP of rac-1-MeTMC, only the HT configuration should exist in the microstructure and no HH or TT sequences should occur. The degree of regioregularity (Xreg) is calculated as Xreg = HT − (HH + TT), which means that in a complete regioregular polymer, Xreg = 1 (when the resonance intensity of the HT is normalized to 1), and in a completely regiorandom polymer, Xreg = 0. The respective peaks in 13C NMR diad analysis of the carbonyl and methine regions of poly(rac-1-MeTMC) have previously been determined, and the Xreg of the polymer can easily be calculated from the integrals of the corresponding peaks in F


Article

Macromolecules

Figure 6. Structures of possible chain-ends and NMR analysis. (a) Poly(rac-1-MeTMC) with a primary alcohol as chain-end. (b) Poly(rac-1MeTMC) with a secondary alcohol as chain-end. (c) Crude sample 1H NMR compilation. Numbers of the peaks corresponded to the numbers in (a) and (b). (d) 1H−13C HSQC NMR and cross-peaks were labeled in (1H, 13C) manner and the numbers corresponded to the numbers in (a) and (b). (e) 13C DEPT NMR, CH showing as positive peaks and CH2 as negative peaks. (f) 1H−13C HMBC NMR. The long-range correlations were color coded in the same way as in (a) and (b), with cross-peaks labeled in (1H, 13C) manner.

catalyzed ROP. A low-molecular-weight sample (DP ∼ 20, Xreg = 0.86) was prepared using BnOH as the (co)initiator and subsequently analyzed through a combination of 1H, 13C DEPT, 1H−13C HSQC, and 1H−13C HMBC NMR to determine the chain-end of the polymer upon full monomer conversion (Figure 6). The crude product was modified with trichloroacetyl isocyanate to determine the chemical shifts of the possible hydroxyl groups (Figure 6a,b). As shown in the stacked 1H NMR, five protons are shifted (numbered 1−5), meaning that both primary and secondary hydroxyl groups coexisted in the crude polymer as the chain-end (Figure 6c). 1 H−13C HSQC was used to examine the unmodified crude samples and the connected carbons of the hydroxyl groups were subsequently analyzed. The peaks of interest were labeled in the (1H, 13C) manner (Figure 6d), with δ 5.07 ppm corresponding to CH2 (6) from the initiator end. For the chain-end with primary alcohol, δ 3.59 ppm corresponded to CH2 (2) and δ 4.90 ppm corresponded to CH (5). For the chain-end with secondary alcohol, δ 1.15 ppm corresponded to CH3 (1), δ 3.86 ppm to CH (3), and δ 4.37 ppm to CH2 (4). These assignments were confirmed by the 13C DEPT NMR (Figure 6e), where CH shows as positive peaks and CH2 as negative peaks. Finally, the enchainment of alcohol ends was confirmed by 1H−13C HMBC. The correlations between the proton and the carbon were color coded and labeled in the (1H, 13C) manner shown in Figure 6a,b,f. For the chain-end with primary alcohol: long-range correlations between the CH proton of 5 (δ 4.90 ppm) to carbons 1′ (δ 19.3 ppm), 7 (δ 38.2 ppm), 2 (δ 57.6 ppm), and 8 (δ 153.5 ppm). For the chain-end with secondary alcohol: long-range correlations between the CH2 proton of 4 (δ 4.37 ppm) to carbons 7 (δ

observed. The regioregularity continued to decrease when the reaction temperature was elevated from RT (entry 7, Xreg = 0.54) to 60 °C (entry 8, Xreg = 0.30) (Figure 5c). When the reaction was left at 60 °C for 20 h (entry 8), the Xreg decreased to 0, meaning the poly(rac-1-MeTMC) chains became complete regiorandom. Similar temperature effect was seen when the ROP was carried out in toluene (cf. entries 9 and 11), where a decrease in Xreg from 0.85 (−40 °C) to 0.64 (20 °C) was shown. It is thereby clear that a lower reaction temperature can promote regioregular ROP when conducted either in THF or toluene. Increasing the temperature greatly intensifies the regiorandomness both during ROP and at equilibrium. The change in Xreg during the early stages of ROP was also monitored; however, the corresponding carbonyl region portrayed numerous peaks, making precise interpretation difficult. The intensity of these peaks was greatly weakened as the polymerization proceeded and eventually became unobservable. We hypothesize that these peaks were from the initiating species, such as BnOH with one or a few ring-opened rac-1-MeTMC (Figure S5). In summary, the reactions conducted in toluene outperformed the ones in THF in terms of Xreg, which means that a fast polymerization rate at a low temperature with a lower catalyst loading is essential to reach high regioregularity. Chain-End Analysis and Robustness Toward Impurities. The acyl-oxygen cleavage can occur either at the i position, resulting in a secondary alcohol, or the ii position, resulting in a primary alcohol (Figure 5a), and the preferred opening site being the hinder bond or the unhindered bond been seen and reported previously.17,18,46 Chain-end analysis can provide insight into which cleavage is preferred for t-BuP2G


Article

Macromolecules

considered as the optimal polymerization condition with a high reaction rate. Good control over Mn and Đ was examined by 1 H NMR and SEC, with most of the entries having Đ less than 1.2, showing a “controlled polymerization” feature. The regioselective ring-opening character was examined by 13C NMR, with Xreg as high as 0.9, which is unprecedented for cyclic carbonates using organocatalysts. The chain-end analysis was carried out using a combination of carbamation, 1H, 13C DEPT, 1H−13C HSQC, and 1H−13C HMBC NMR, where a slight preference of secondary alcohol over primary alcohol as the product’s chain-ends was shown. The robustness of the reaction was confirmed, where a regioregular product was synthesized that showed a very slight influence of the existence of oligomeric impurities.

36.8 ppm), 3 (δ 63.4 ppm), and 8 (δ 153.5 ppm). Based on the chain-end analysis on the crude polycarbonates, only a slight preference over the cleavage site was seen, with ∼63% opened from the hindered bond and ∼37% opened from the unhindered bond (Figure S6). Since the regioregularity for the analyzed sample is moderately high (Xreg = 0.86), we hypothesize that the regioselectivity for the t-BuP2-catalyzed ROP of rac-1-MeTMC behaves in a similar manner as the stereoselectivity for the t-BuP2-catalyzed ROP of rac-lactide:47 the first monomer being opened has no preference over the acyl-oxygen cleavage site and can be from either the hindered bond or the unhindered bond. The opening site for the next monomer is then dominated by the previous one and is opened from the same acyl-oxygen bond as the last enchainment. Another possibility could be that this randomness occurred only at the chain-end. Since the ROP of cyclic carbonate is an equilibrium reaction, constant ring-opening/ ring-closing of the chain-end monomer with no preference over the acyl-oxygen cleavage site can appear when the reaction reaches a high conversion. Since this occurs only at each chain-end, the regioregularity of the main chain will not be interrupted, which will result in a moderately high regioregularity but both types of chain-ends in the product. Upon standing on the shelf in a sealed container over a long period (6 months in this study), some auto-oligomerization of the carbonate was noticed. This could be caused by an adventurous moisture-mediated decarboxylation pathway.48 The oligomers are generally considered as an impurity in ROP, having impact over the reactivity of some catalysts and causing poor control over polymerization. Organic base catalysts, however, generally withstands impurities better than metal complexes. Very high catalytic activity toward technical grade cyclic carbonates has been shown before using a variety of organic base catalyst with good control over the molecular weight and distributions of the polycarbonate products.49 We therefore would examine if the existence of oligomeric impurities has any influence on the regioselectivity of the polymerization. A reaction with the same parameters as entry 9 was carried out using the rac-1-MeTMC with 3% oligomeric impurities as monomers. The resulting raw product was first analyzed using matrix-assisted laser desorption ionization timeof-flight massspectrometry (MALDI-ToF MS), and the results showed that the impurities formed were diol or diol-capped oligomers without detectable cyclic impurities. These diol and diol-capped oligomers would act as additional initiators and decrease the molecular weight of the final product (Figure S7). The carbonyl region of the crude product was then subsequently analyzed through 13C NMR and only a slight decrease in the regioregularity from 0.85 to 0.82 was noticed This is yet an unexpected but outstanding feature of the studied t-BuP2 system, where the regioregularity of the polycarbonate product will not be greatly influenced even when around 3% of oligomeric impurities existed in the monomers.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02591. NMR of the purified monomer, semi-logarithmic kinetic plots, relationship between equilibrium monomer concentration and temperature, compiled 13C NMR of carbonyl region, chain-end analysis, and MALDI-ToF of raw product (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +46-8-790 80 76. ORCID

Geng Hua: 0000-0001-7304-6737 Karin Odelius: 0000-0002-5850-8873 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work is financially supported by the Swedish Research Council, VR (grant ID: 621201356 25). REFERENCES

(1) Park, S. H.; Choi, B. G.; Joo, M. K.; Han, D. K.; Sohn, Y. S.; Jeong, B. Temperature-Sensitive Poly(Caprolactone-Co-Trimethylene Carbonate)-Poly (Ethylene Glycol)-Poly(Caprolactone-Co-Trimethylene Carbonate) as in Situ Gel-Forming Biomaterial. Macromolecules 2008, 41, 6486−6492. (2) Pêgo, A. P.; Van Luyn, M. J. A.; Brouwer, L. A.; van Wachem, P. B.; Poot, A.; Grijpma, D. W.; Feijen, J. In Vivo Behavior of Poly(1,3Trimethylene Carbonate) and Copolymers of 1,3-Trimethylene Carbonate with D,L-Lactide or Epsilon-Caprolactone: Degradation and Tissue Response. J. Biomed. Mater. Res. 2003, 67A, 1044−1054. (3) Xu, J.; Feng, E.; Song, J. Renaissance of Aliphatic Polycarbonates: New Techniques and Biomedical Applications. J. Appl. Polym. Sci. 2014, 131, No. 39822. (4) Liu, B.; Zhao, X.; Wang, X.; Wang, F. Copolymerization of Carbon Dioxide and Propylene Oxide with Neodymium Trichloroacetate-Based Coordination Catalyst. Polymer 2003, 44, 1803−1808. (5) Arias, V.; Hö glund, A.; Odelius, K.; Albertsson, A. C. Polylactides with “Green” Plasticizers: Influence of Isomer Composition. J. Appl. Polym. Sci. 2013, 130, 2962−2970.

■

CONCLUSIONS Through this study, the possibility of regioregular ROP of rac1-MeTMC using the phosphazene base t-BuP2 as a catalyst was demonstrated. The polymerizations were conducted under different temperatures varying from −74 to 60 °C using different solvents such as THF and toluene with different reactant loadings. The kinetic alterations were compared, and using toluene as the solvent at −40 °C with 0.5% catalyst was H


Article

Macromolecules

(26) Hua, G.; Olsén, P.; Franzén, J.; Odelius, K. Anionic Polycondensation and Equilibrium Driven Monomer Formation of Cyclic Aliphatic Carbonates. RSC Adv. 2018, 39022−39028. (27) Kricheldorf, H. R.; Eggerstedt, S. Macrocycles 2. Living Macrocyclic Polymerization Ofε-Caprolactone with 2,2-Dibutyl-2Stanna-1,3-Dioxepane as Initiator. Macromol. Chem. Phys. 1998, 199, 283−290. (28) Olsén, P.; Borke, T.; Odelius, K.; Albertsson, A.-C. εDecalactone: A Thermoresilient and Toughening Comonomer to Poly(l-Lactide). Biomacromolecules 2013, 14, 2883−2890. (29) Thomas, C.; Bibal, B. Hydrogen-Bonding Organocatalysts for Ring-Opening Polymerization. Green Chem. 2014, 16, 1687−1699. (30) Alamri, H.; Zhao, J.; Pahovnik, D.; Hadjichristidis, N. Phosphazene-Catalyzed Ring-Opening Polymerization of ε-Caprolactone: Influence of Solvents and Initiators. Polym. Chem. 2014, 5, 5471−5478. (31) Zhang, H.; Hu, S.; Zhao, J.; Zhang, G. Phosphazene-Catalyzed Alternating Copolymerization of Dihydrocoumarin and Ethylene Oxide: Weaker Is Better. Macromolecules 2017, 50, 4198−4205. (32) Dove, A. P.; Pratt, R. C.; Lohmeijer, B. G. G.; Waymouth, R. M.; Hedrick, J. L. Thiourea-Based Bifunctional Organocatalysis: Supramolecular Recognition for Living Polymerization. J. Am. Chem. Soc. 2005, 127, 13798−13799. (33) Kühling, S.; Keul, H.; Hocker, H. Copolymerization of 2, 2Dimethyltrimethylene Carbonate with Carbonate and with ECaprolactone Using Initiators on the Basis of Li, Al, Zn and Sn. Makromol. Chem. 1992, 1217, 1207−1217. (34) Keul, H.; Bächer, R.; Höcker, H. Anionic Ring-Opening Polymerization of 2,2-Dimethyltrimethylene Carbonate. Makromol. Chem. 1986, 187, 2579−2589. (35) Matyjaszewski, K.; Słomkowski, S.; Penczek, S. Kinetics and Mechanism of the Cationic Polymerization of Tetrahydrofuran in Solution. THF−CH2Cl2 and THF−CH2Cl2/CH3NO2 Systems. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2413−2422. (36) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M. Organocatalysis: Opportunities and Challenges for Polymer Synthesis. Macromolecules 2010, 43, 2093−2107. (37) Ottou, W. N.; Sardon, H.; Mecerreyes, D.; Vignolle, J.; Taton, D. Update and Challenges in Organo-Mediated Polymerization Reactions. Prog. Polym. Sci. 2016, 56, 64−115. (38) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Organocatalytic Ring-Opening Polymerization. Chem. Rev. 2007, 107, 5813−5840. (39) Pothupitiya, J. U.; Dharmaratne, N. U.; Jouaneh, T. M. M.; Fastnacht, K. V.; Coderre, D. N.; Kiesewetter, M. K. H-Bonding Organocatalysts for the Living, Solvent-Free Ring-Opening Polymerization of Lactones: Toward an All-Lactones, All-Conditions Approach. Macromolecules 2017, 50, 8948−8954. (40) Carothers, W. H.; Dorough, G. L.; Natta, F. J. van. Studies of Polymerization and Ring Formation. X. The Reversible Polymerization of Six-Membered Cyclic Esters. J. Am. Chem. Soc. 1932, 54, 761−772. (41) Hill, J. W.; Carothers, W. H. Studies of Polymerization and Ring Formation. XX. Many-Membered Cyclic Esters. J. Am. Chem. Soc. 1933, 55, 5031−5039. (42) Iannone, F.; Casiello, M.; Monopoli, A.; Cotugno, P.; Sportelli, M. C.; Picca, R. A.; Cioffi, N.; Dell’Anna, M. M.; Nacci, A. Ionic Liquids/ZnO Nanoparticles as Recyclable Catalyst for Polycarbonate Depolymerization. J. Mol. Catal. A: Chem. 2017, 426, 107−116. (43) Darensbourg, D. J.; Wei, S. H. Depolymerization of Polycarbonates Derived from Carbon Dioxide and Epoxides to Provide Cyclic Carbonates. A Kinetic Study. Macromolecules 2012, 45, 5916−5922. (44) Matsuo, J.; Sanda, F.; Endo, T. Substituent Effect on Cationic Ring-Opening Polymerization Behavior of Seven-Membered Cyclic Carbonates. Macromol. Chem. Phys. 2000, 201, 585−596. (45) Chisholm, M. H.; Navarro-Llobet, D.; Zhou, Z. Poly(Propylene Carbonate). 1. More about Poly(Propylene Carbonate) Formed from the Copolymerization of Propylene Oxide and Carbon Dioxide

(6) Quan, Z.; Min, J.; Zhou, Q.; Xie, D.; Liu, J.; Wang, X.; Zhao, X.; Wang, F. Synthesis and Properties of Carbon Dioxide - Epoxides Copolymers from Rare Earth Metal Catalyst. In Macromolecular Symposia; Wiley-Blackwell, 2003; Vol. 195, pp 281−286. (7) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.; Coates, G. W. Stereoselective Epoxide Polymerization and Copolymerization. Chem. Rev. 2014, 114, 8129−8152. (8) Parzuchowski, P. G.; Jaroch, M.; Tryznowski, M.; Rokicki, G. Synthesis of New Glycerol-Based Hyperbranched Polycarbonates. Macromolecules 2008, 41, 3859−3865. (9) Weilandt, K.; Keul, H.; Höcker, H. Polymerization of 2Acetoxymethyl-2-Alkyltrimethylene Carbonates and of 2-Methoxycarbonyl-2-Methyltrimethylene Carbonate; a Comparison with the Polymerization. Macromol. Chem. Phys 1996, 197, 3851−3868. (10) Rokicki, G. Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers. Prog. Polym. Sci. 2000, 25, 259−342. (11) Olsén, P.; Odelius, K.; Albertsson, A.-C. Ring-Closing Depolymerization: A Powerful Tool for Synthesizing the AllyloxyFunctionalized Six-Membered Aliphatic Carbonate Monomer 2Allyloxymethyl-2-Ethyltrimethylene Carbonate. Macromolecules 2014, 47, 6189−6195. (12) Cohen, C. T.; Chu, T.; Coates, G. W. Cobalt Catalysts for the Alternating Copolymerization of Propylene Oxide and Carbon Dioxide: Combining High Activity and Selectivity. J. Am. Chem. Soc. 2005, 127, 10869−10878. (13) Luinstra, G. A. Poly(Propylene Carbonate), Old Copolymers of Propylene Oxide and Carbon Dioxide with New Interests: Catalysis and Material Properties. Polym. Rev. 2008, 48, 192−219. (14) Zhang, X.; Fevre, M.; Jones, G. O.; Waymouth, R. M. Catalysis as an Enabling Science for Sustainable Polymers. Chem. Rev. 2018, 118, 839−885. (15) Kember, M. R.; Buchard, A.; Williams, C. K. Catalysts for CO2/Epoxide Copolymerization. Chem. Commun. 2011, 47, 141− 163. (16) Huang, M.; Pan, C.; Ma, H. Ring-Opening Polymerization of Rac-Lactide and α-Methyltrimethylene Carbonate Catalyzed by Magnesium and Zinc Complexes Derived from Binaphthyl-Based Iminophenolate Ligands. Dalton Trans. 2015, 44, 12420−12431. (17) Brignou, P.; Carpentier, J.-F.; Guillaume, S. M. Metal- and Organo-Catalyzed Ring-Opening Polymerization of α-Methyl-Trimethylene Carbonate: Insights into the Microstructure of the Polycarbonate. Macromolecules 2011, 44, 5127−5135. (18) Brignou, P.; Priebe Gil, M.; Casagrande, O.; Carpentier, J.-F.; Guillaume, S. M. Polycarbonates Derived from Green Acids: RingOpening Polymerization of Seven-Membered Cyclic Carbonates. Macromolecules 2010, 43, 8007−8017. (19) Duda, A.; Libiszowski, J.; Mosnáček, J.; Penczek, S. Copolymerization of Cyclic Esters at the Living Polymer-Monomer Equilibrium. Macromol. Symp. 2005, 226, 109−120. (20) Hong, M.; Chen, E. Y.-X. Completely Recyclable Biopolymers with Linear and Cyclic Topologies via Ring-Opening Polymerization of γ-Butyrolactone. Nat. Chem. 2016, 8, 42−49. (21) Endo, T.; Kakimoto, K.; Ochiai, B.; Nagai, D. Synthesis and Chemical Recycling of a Polycarbonate Obtained by Anionic RingOpening Polymerization of a Bifunctional Cyclic Carbonate. Macromolecules 2005, 38, 8177−8182. (22) Hodge, P. Entropically Driven Ring-Opening Polymerization of Strainless Organic Macrocycles. Chem. Rev. 2014, 114, 2278−2312. (23) Höcker, H.; Keul, H. Ring-Opening Polymerization and RingClosing Depolymerization. Adv. Mater. 1994, 6, 21−36. (24) Olsén, P.; Undin, J.; Odelius, K.; Keul, H.; Albertsson, A.-C. Switching from Controlled Ring-Opening Polymerization (CROP) to Controlled Ring-Closing Depolymerization (CRCDP) by Adjusting the Reaction Parameters That Determine the Ceiling Temperature. Biomacromolecules 2016, 17, 3995−4002. (25) Hong, M.; Chen, E. Y.-X. Towards Truly Sustainable Polymers: A Metal-Free Recyclable Polyester from Biorenewable Non-Strained γ-Butyrolactone. Angew. Chem., Int. Ed. 2016, 55, 4188−4193. I


Article

Macromolecules Employing a Zinc Glutarate Catalyst. Macromolecules 2002, 35, 6494− 6504. (46) Ling, J.; Dai, Y.; Zhu, Y.; Sun, W.; Shen, Z. Ring-Opening Polymerization of 1-Methyltrimethylene Carbonate by Rare Earth Initiators. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 3807−3815. (47) Zhang, L.; Nederberg, F.; Messman, J. M.; Pratt, R. C.; Hedrick, J. L.; Wade, C. G. Organocatalytic Stereoselective RingOpening Polymerization of Lactide with Dimeric Phosphazene Bases. J. Am. Chem. Soc. 2007, 129, 12610−12611. (48) Barker, I. A.; Ablett, M. P.; Gilbert, H. T. J.; Leigh, S. J.; Covington, J. A.; Hoyland, J. A.; Richardson, S. M.; Dove, A. P. A Microstereolithography Resin Based on Thiol-Ene Chemistry: Towards Biocompatible 3D Extracellular Constructs for Tissue Engineering. Biomater. Sci. 2014, 2, 472−475. (49) Helou, M.; Miserque, O.; Brusson, J.-M.; Carpentier, J.-F.; Guillaume, S. M. Organocatalysts for the Controlled “Immortal” RingOpening Polymerization of Six-Membered-Ring Cyclic Carbonates: A Metal-Free, Green Process. Chem. - Eur. J. 2010, 16, 13805−13813.

J


Phosphazene-Catalyzed Regioselective Ring-Opening

Recommend Documents