Air- and Moisture-Stable Indium Salan Catalysts for Living Multiblock

Aug 25, 2017 - We introduce an air- and moisture-stable hydroxy-bridged indium salan complex as a highly active and controlled catalyst for the ring-o...
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Air- and Moisture-Stable Indium Salan Catalysts for Living Multiblock PLA Formation in Air Tannaz Ebrahimi,†,‡ Dinesh C. Aluthge,† Brian O. Patrick,† Savvas G. Hatzikiriakos,‡ and Parisa Mehrkhodavandi*,† †

Department of Chemistry, University of British Columbia, Vancouver, BC Canada, V6T1Z1 Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC Canada, V6T1Z3

‡

S Supporting Information *

ABSTRACT: We introduce an air- and moisture-stable hydroxy-bridged indium salan complex as a highly active and controlled catalyst for the ringopening polymerization of cyclic esters in air. The reversible activation of this complex with linear and branched alcohols leads to immortal polymerization, allowing the controlled formation of block copolymers in air. It is the only reported example of a living catalyst that remains controlled after multiple exposures to ambient air at high temperatures. Although the prevalent catalyst for ring-opening polymerization, tin octanoate, is robust, it does not promote controlled polymerization. Our indium catalyst is exceptional in being both robust and controlled.

KEYWORDS: lactide, PHB, air stable, indium, PLA, block copolymer

B

achieved after 24−48 h) and suffer from extensive transesterification reactions, which limits control over polymer macrostructure.9,10 Indium catalysts bearing pyridine bisphenol ligands polymerize 100 equiv of lactide in air at 80 °C over the course of 24 h with 86% conversion.11 Air-stable catalysts with Al,12 Cu,13 Ti,14 Y,15 and Mg−Na/Li9 have been reported; however, in these studies, polymerizations were carried out under inert conditions. Magnesium,16 and Ti-based17 catalysts for the polymerization of caprolactone in air have been reported. To our knowledge, there are no examples of metalbased catalysts for the highly controlled polymerization and block copolymerization of lactide in air. Herein we report the first air- and moisture-stable indium18 catalyst supported for the ring-opening copolymerization of lactide in air. This complex catalyzes the rapid ring opening of lactide, in solution or in the melt, to form linear and star-shaped high-molecular-weight PLA triblock copolymers and PLA− PHB block copolymers with an unprecedented combination of high activity and control over molecular weight and dispersity. We show that this reactivity is a direct result of the tetradentate aminophenolate supporting ligand on indium; previously reported indium complexes for living and immortal polymerization in our group supported by tridentate aminophenolate (A)19 or bisimino-bisphenolate (salen) (B)20 ligands do not always show similar reactivity (Figure 1). Since our first

iocompatible polyesters such as poly(lactic acid) (PLA) and other poly(hydroxyalkanoates) are increasingly important components of biomedical and drug delivery systems.1 These important polymers can be synthesized through well-explored organocatalytic,2 as well as main group and transition-metal-based, ring-opening polymerization.3 However, despite these concentrated efforts in catalyst development, tin(II) 2-ethylhexanoate (tin octanoate, Sn(Oct)2) remains the most common metal-based catalyst used for ring-opening polymerization (ROP) in industrial applications and pharmaceutical studies.2,3e,f,h−j,m,4 Tin octanoate allows the ROP of lactide (LA) in the melt without the use of inert atmosphere or ultrapure monomers. However, the facility of reactivity comes at a price: Sn(Oct)2 lacks control over polymer molecular weight and dispersity, which prohibits formation of complex polymer morphologies such as multiblock copolymers. Nor is it active for some lactones such as βhydroxybutyrate (BBL).3q,5 Transesterification limits the range of polymers readily prepared with Sn(Oct)2 for pharmaceutical or other industrial applications. Despite the recent focus in the literature on developing air/ water stable catalysts for a range of catalytic applications,6 fewer efforts have been made to develop air- and moisture-resistant metal-based catalysts for ROP of cyclic esters that can be used under industrially relevant conditions.7 Zinc complexes bearing guanidine-pyridine ligands,8 and magnesium−sodium/lithium heterobimetallic complexes9 are active for the ring-opening polymerization (ROP) of lactide under these conditions; however, the polymerizations are sluggish (high conversion is © XXXX American Chemical Society

Received: June 14, 2017 Revised: August 17, 2017

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structures of 1 and 2, derived by single-crystal X-ray diffraction, are similar to dinuclear complexes A and hydroxyl-bridged analogue [(NNHO)InCl]2(μ-Cl)(μ−OH),25 respectively, and show distorted octahedral indium centers asymmetrically bridged by chloride and ethoxy (or hydroxy) ligands (Figure 2, Tables S1,S2). The PGSE-derived diffusion coefficients of

Figure 1. Dinuclear indium complexes for ring-opening polymerization of lactide.

publication of complex A,19a many other indium catalysts for lactide polymerization have been reported.11,21 Of these, there is only one example that polymerizes low equivalents of lactide in air (discussed above).11 Asymmetrically bridged dinuclear salan indium alkoxy complexes (RR/RR)-[(ONHN H O)In] 2(μ-Cl)(μ-OEt) (1) were prepared from (RR)-N,N′-bis(3,5-ditertbutylsalicylidene)-1,2-cyclohexanediamine (RR-H2(ONHNHO) or salan)22 in two consecutive salt metathesis reactions (Scheme 1). Scheme 1. Synthesis of Dinuclear Indium Complexes

Figure 2. Molecular structures of (RR/RR)-1 (a, top) and (RR/RR)-2 (b, bottom) (depicted with thermal ellipsoids at 50% probability. H atoms as well as solvent molecules omitted for clarity).

complexes 1 (6.7(1) × 10−10 m2s−1) and 2 (6.5(6) × 10−10 m2s−1) are 28−30% lower than that of the proligand (9.4(3) × 10−10 m2s−1) and confirm dinuclear solution structures for both species (Figure S15, Table S6).26 Complex 2 is both air- and water-stable (see below) and shows unprecedented control over polymerization of unpurified rac-LA in air in the presence of chain transfer agents (Table 1). Reaction of 2 with lactide in toluene at 80 °C in the presence of up to 10 equiv of ethanol yields polymers with predictable molecular weights and controlled dispersities (Table 1, entries 1−3.) We can carry out the reaction in the melt when using a high boiling alcohol such as 1,3,5-tris(hydroxymethyl)benzene (THMB) as a CTA. Reaction of complex 2 with up to 10 000 equiv of LA in the presence of up to 100 equiv of THMB in the melt forms highly controlled star-PLAs (Table 1, entries 4−7). To our knowledge, complex 2 is the only system able to catalyze the formation of high molecular weight, monomodal star-PLAs from unpurified rac-LA in the melt, under air, and with high reactivity. The purity of the lactide was not a factor for reactivity: immortal polymerization reactions with THMB carried out with as-received commercial grade lactide and thrice-recrystallized lactide yielded identical results (Table 1, entries 8−9). Importantly, complex 2 catalyzes the formation of diblock and triblock star shaped PLA in the melt in air (Table 1, entries 10 and 11). As a general polymerization procedure, after weighing out the required amounts of (RR/RR)-2 and THMB in a Schlenk flask outside the dinitrogen box, L-LA was added and the reaction started by immersing the flask in oil bath at 130 °C. After full conversion, D-LA was added, and the

Although the reaction is similar to the synthesis of the salen analogue, (B), there are some significant differences. Deprotonation of H2(ONHNHO) with KOtBu forms the resulting K2(ONHNHO); however, the subsequent salt metathesis reaction requires greater than 2 equiv of InCl3 to form an indium-chloro intermediate which was not isolable. This species was reacted in situ with excess NaOEt to form complex 1 in 55% yield based on H2(ONHNHO). Although both racemic or enantiopure analogues can be synthesized (Figures S1−S5), we chose to proceed with the enantiopure analogue. Interestingly, the formation of an alkoxy-chloro bridged species was not possible with a related salan ligand bearing tertiary amine donors.21r This difference in reactivity may be due to the subtle but important role of tertiary vs secondary amines in aminophenolate systems.23 Significant reactivity differences have also been reported with “salalen”-supported complexes.24 Exposure of 1 to trace water in CH2Cl2 for 48 h forms asymmetrically bridged (RR/RR)-[(ONHNHO)In]2(μ-Cl)(μ− OH) (2) (Figures S6,S7) (95% yield). The solid-state 6414

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Table 1. ROP of Impure/Wet rac-LA, and Block Copolymerization of Industrially Relevant Recrystallized L-LA and D-LA Using Complex 2 in Aira 1 2 3 4b 5b 6b 7b 8 9 10b 11h 12h 13h 14 15

M1−M2−M3

CTA

[M]/[CTA]/[2]

solvent

temp (°C)

time (min)

Mn,theoc (g mol−1)

Mn,GPCd (g mol−1)

Đd

rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA LLA LLAf LLAf-DLAf LLAf-DLAf-LLAf LLAf-DLAf-LLAf LLA-DLA-LLAg BBLg BBL- rac-LAg

EtOH EtOH EtOH THMB THMB THMB THMB THMB THMB THMB THMB THMB THMB THMB THMB

1500/2/1 1500/5/1 1500/10/1 5000/10/1 5000/50/1 5000/100/1 10000/10/1 520/21/1 526/22/1 526 + 526/22/1 243 + 120 + 243/6/1 700 + 500 + 700/8/1 700 + 500 + 700/8/1 2500/10/1 2500 + 2500/10/1

Tol Tol Tol

80 80 80 120 120 120 120 130 130 155 155 155 155 25 25

240 240 240 120 120 120 120 30 30 60 120 300 300 960 960

106 970 42 810 21 430 52 730 11 540 5570 100 970 3570 3460 6360 11 220 31 000 34 370 19 520 53 200

73 960 36 200 24 980 50 080 11 800 6900e 91 200 3510e 3800e 6100e 10 200 28 500 30 300 16 890 48 600

1.33 1.36 1.32 1.07 1.06 1.34

THF THF

1.05 1.10 1.05 1.02 1.01

a

All reactions performed in ambient atmospheric conditions without any protection of inert gases and wet/impure lactide was used for polymerization. Monomer conversion, determined by 1H NMR spectroscopy and unless indicated 90−99%. bReactions stopped after 120 min, 70− 80% conversion. cCalculated from [M]o/[initiator] × monomer conversion × MM + MCTA (MLA = 144.13 g/mol, MBBL = 86.09 g/mol, MEtOH = 46 g/mol, MTHMB = 168.19 g/mol). dDetermined by GPC-MALS, dn/dc = 0.044 and 0.068 mL/g, respectively, for PLA and PHB in THF. Chloroform was used as the GPC solvent for PLLA, PLLA−PDLA, and PLLA−PDLA-PLLA, dn/dc = 0.029 mL/g). eNMR/MALDI-TOF molecular weight. f Recrystallized lactide. gPurified monomers, reactions carried out under nitrogen using in situ formed THMB bridged complex. hRacemic complex 2 was used.

temperature was raised to 155 °C along with the addition of few drops of toluene to ease stirring. After full conversion of the monomer, another batch of L-LA was fed to the reaction to form triblock copolymer of PLLA−PDLA-PLLA in 2 h. Both 1 H NMR and MALDI-TOF results confirm the presence of −OH and −OTHMB chain ends, indicating that the polymerization proceeds via a coordination−insertion under these conditions (Figures S24−S27). The agreement between experimental and theoretical molecular weights of the resulting block copolymers, along with very narrow molecular weight distributions, and the fact that sequential monomer addition leads to complete conversion of the monomer and increase of the molecular weight (Table 1, entries 9 and 10) imply excellent control in these systems and the living characteristic of the polymerization. To the best of our knowledge this is the only reported example of a living catalyst that remains controlled after multiple exposures to ambient air at high temperatures (Figure 3). In air, complex 2 forms high molecular weight stereotriblock copolymers of PLA with narrow molecular weight distribution (Table 1 entry 12, Figure 3, Figures S35−S37) and Tm > 200 °C (Figures S41−S45) in one pot after 5 h, without significant racemization (Figures S29,S30). In contrast, similar highmolecular-weight PLLA−PDLA block copolymers can be synthesized with Sn(Oct)2 through two-step synthesis under inert atmosphere after 7 days.27 Importantly, the polymerization results for complex 2 are identical regardless of whether the polymerization was carried out under air or N2 atmosphere (Table 1, entry 13, Figures S28, 32−34, 38−40, and 44−45).28 Finally, complex 2 is also active for the immortal polymerization of β-butyrolactone (BBL) to form PHB, or with sequential addition of rac-LA, star shaped PHB-b-PLA (Table 1 entries 14,15).29 Reaction of 2500 equiv of BBL with in situ formed −OTHMB bridged complex in THF at room temperature yields symmetric 3-arm star PHB homopolymer with an Mn value of 17 kDa at room temperature (Figure S46a).

Figure 3. GPC overlaid chromatograms of 3-arm star PLLA obtained from the polymerization with [L-LA]//[THMB]/[2] ratios of 700/8/ 1 (Mn = 16500, Đ = 1.06), 3-arm star diblock copolymers of PLLA− PDLA obtained from the polymerization with [L-LA+D-LA]/ [THMB]/[1] ratios of 700 + 500/8/1(Mn = 24160, Đ = 1.07), and 3-arm star triblock copolymers of PLLA−PDLA-PLLA obtained from the polymerization with [L-LA+D-LA+L-LA]/[THMB]/[1] ratios of 700 + 500 + 700/8/1(Mn = 28500, Đ = 1.10) in neat at 155 °C (Table 1, entry 12). The slight broadening of the chromatogram is due to the lack of homogeneous stirring at higher molecular weights.

Once the reaction reached >90% conversion, a further 2500 equiv of rac-LA were added to the reaction mixture to form 3arm star copolymer PHB-b-PLA with predictable molecular weight and distribution (Figure S46b). It should be noted that reactions with BBL must be carried out under an inert atmosphere with purified monomer. Complex 2 is an air-stable surrogate for 1 which is an excellent catalyst, with reactivity analogous to the extensively studied complex A in both living and immortal polymerization. Polymerization of up to 2000 equiv of rac-LA with (RR/RR)-1 6415

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ACS Catalysis shows a linear relationship between Mn and added monomer as well as low dispersity, which is indicative of a highly controlled living system (Figure S51a, Table S7). The MALDI−TOF spectra of PLA oligomers made with 1 show peaks corresponding to [H(C6H8O2)n(OEt)H]+ separated by m/z = 144, which indicates the absence of any transesterification reactions (Figure S47). The rate of polymerization for complex 1 is first order in LA concentration (kobs = 3.1(8)× 10−4 s−1) and is comparable to those for A (6.2(2) × 10−4 s−1)19b and B (4.6(9) × 10−4 s−1)20a (Figure S51b, Table S9). Polymerizations of rac-LA in the presence of up to 100 equiv of ethanol, catalyzed by (RR/RR)-1, generates monodispersed PLAs with excellent control of molecular weight (Table S8, Figures S48, 49). Chain-end analyses of these polymers by MALDI-TOF mass spectroscopy shows good agreement between theoretical and experimental molecular weights, indicative of the controlled and immortal nature of the catalytic process with the (RR/RR)-1 in the presence of ethanol (Figure S50). The stereoselectivity of (RR/RR)-1 for the polymerization of rac-LA differs from that of complex A.19b A comparison of the ROP rates for D- and L-LA with (RR/RR)-1 shows a kL/kD value of ∼3 which is lower than the value of ∼14 observed for (RR/RR)-A (Table S9).19b In addition, polymerization of racLA with (RR/RR)-1 yields heterotactically enriched PLA (Pr = 0.72) and syndiotactic PLA (Ps = 0.83) from the polymerization of rac- and meso-LA as determined by 1H{1H} NMR spectroscopy (Figure S52). Complex 2 does not convert to the hydroxy bridged dimer [(ONHNHO)In(μ−OH)]2 (3) in the presence of water (Scheme 1). A solid sample of complex 2 was exposed to air for over 30 days and was monitored using 1H NMR spectroscopy during and after this period. These spectra show that complex 2 remains unchanged (Figure S16). Under more forcing conditions, for example exposure to air for more than two months or addition of 100 equiv of water, the 1H NMR spectrum of the mixture shows only the presence of 2 and traces of free ligand (Figure S17). Complex 3 can be prepared independently from H2(ONHNHO) after consecutive reactions with KOtBu, excess InCl3, and excess NaOH and characterized in solution and in the solid state (Figuress S8, S13, S15, Tables S3, S5, S6). The lack of reactivity of complex 2 with water is unexpected and contrasts with previously reported systems (Scheme 2). Complex A reacts with water to form bis-hydroxylated complex [(NNHO)In)(μ−OH)]2 (C)25 analogous to 3, and when B is

dissolved in wet hexanes for 1 h, it undergoes significant decomposition to form one major metalated species and the salen proligand (Figures S18, S19). Single crystals of bishydroxy complex [(ONNO)In(μ−OH)]2 (4) can be isolated from a mixture of decomposition products (Figure S14). The formation of complexes C and 4 is irreversible. In contrast, formation of complex 2 is reversible with addition of alcohol. Reaction of complex 2 in neat ethanol followed by removal of the alcohol reforms 1 (Scheme 1, Figure S20). Salan-supported dinuclear indium complex 2 is an air and water stable, highly active, and highly controlled catalyst for the polymerization of lactide in air. We show that changing the ligand support for indium from a diaminophenolate (A) or salen (B) system favors the formation of a mixed hydroxyl/ chloro bridged indium complex (2) which can convert to an active alkoxy/chloro bridged complex (1) to allow reactivity. Complex 1 is synthesized in three steps from a simple salan ligand and can be converted to 2 quantitatively by exposure to moist air. Industrial production of PLA takes place in melt state. Most of the isoselective catalysts reported in the literature (including complexes A and B), lose their selectivity toward polymerization of rac-LA in melt state. Hence, in order to achieve highly crystalline PLA, development of catalytic systems which do not undergo transesterification/racemization in melt state is vital. We believe that complex 2 can be a real solution to the need for a stable catalyst, capable of high activity and strict macrostructure control, in research and industrial environments without inert atmosphere facilities. We are exploring the commercial possibilities of this, and other indium complexes in the family.30

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b01939. Experimental details, solution and solid-state characterization of compounds, polymerization kinetics, and polymer characterization (PDF) X-ray data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Parisa Mehrkhodavandi: 0000-0002-3879-5131

Scheme 2. Reactivity of Complexes A and B with Water

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS TE has been supported by a UBC 4YF scholarship. REFERENCES

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