Synthesis and Polymerization of Norbornenyl-Terminated Multiblock

Article pubs.acs.org/Macromolecules

Synthesis and Polymerization of Norbornenyl-Terminated Multiblock Poly(cyclohexene carbonate)s: A Consecutive Ring-Opening Polymerization Route to Multisegmented Graft Polycarbonates Jeung Gon Kim and Geoffrey W. Coates* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States S Supporting Information *

ABSTRACT: We report a method for the synthesis of multisegmented polycarbonate graft copolymers. Using a β-diiminate zinc catalyst with a norbornene carboxylate initiator, we achieved living block copolymerizations of functionalized cyclohexene oxides and CO2, yielding norbornenyl-terminated macromonomers with variable block sequences. Subsequent ring-opening metathesis polymerization of the norbornenyl-terminated macromonomers produced segmented graft copolymers. This method provides a facile route to core−shell structures with readily controllable molecular parameters.

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INTRODUCTION The compact and confined structures of graft copolymers lead to unique properties with potential applications in nanoscience, drug delivery systems, and photonics.1 Graft copolymers can be prepared by three strategies:1c,d,2 (1) graf ting onto: the attachment of side chains onto a linear backbone; (2) graf ting f rom: the growth of side chains from a linear backbone; and (3) graf ting through: the polymerization of macromonomers. The “grafting through” method allows full graft saturation and high structural uniformity. Controlled/living polymerization methods can be used for both macromonomer syntheses and “grafting through” processes, regulating the structure and length of side chains and backbone. Examples of “grafting through” approaches3 using controlled radical,4 anionic,5 metalcatalyzed α-olefin,6 and metathesis7 polymerizations of macromonomers have been previously reported. Recently, ring-opening metathesis polymerization (ROMP) of norbornenyl-terminated macromonomers by Ru alkylidene catalysts with pyridine ligands has allowed controlled brush copolymer synthesis (Figure 1).8 Prior to the development of these metathesis catalysts 1 and 2, only Bowden and co-workers achieved high DP and low Mw/Mn “grafting through” by ROMP of α-norbornenyl polylactic macromonomer using a Grubbs first-generation catalyst.9 Catalysts 1 and 2 exhibit fast initiation, high reactivity, and good functional group tolerance (high DP and low Mw/Mn).10 Grubbs and co-workers explored © 2012 American Chemical Society

Figure 1. Synthesis of graft copolymers via ring-opening metathesis polymerization of norbornene-terminated polymers.

“grafting through” with various norbornenyl-terminated macromonomers. Subsequent ROMP by 1 produced functional graft copolymers with high DP and narrow Mw/Mn.11 The resulting molecular brushes display unique properties11d,e in selfassembly behavior8b,11a and drug delivery systems.11b,c Fontaine et al. also utilized click chemistry and sequential ROMP by 2 to synthesize hydrophilic graft copolymers from exo-oxanorbornenyl-terminated poly(ethylene oxide).12 Wooley and coworkers prepared macromonomers using reversible addition− fragmentation chain transfer (RAFT) polymerization, beginning from a norbornenyl-functionalized chain transfer reReceived: June 4, 2012 Revised: August 13, 2012 Published: September 20, 2012 7878

dx.doi.org/10.1021/ma301137q | Macromolecules 2012, 45, 7878−7883

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Scheme 1. Synthesis of Norbornenyl-Terminated Multiblock Poly(cyclohexene carbonate)s

Table 1. Polymerization Data: Synthesis of Norbornenyl-Terminated Multiblock Poly(cyclohexene carbonate)sa entry

macromonomer

Mn(theo)b (kg/mol)

Mn(MALLS)c (kg/mol)

Mn(RI)d (kg/mol)

Mw/Mnd

yielde (%)

1g 2g 3f 4g 5g 6g 7g 8g 9g 10g 11g 12g

NB-p(CHC)30 NB-p(CHC)40 NB-p(Hex-CHC)20 NB-p(DEG-CHC)20 NB-[p(CHC)15-b-p(F-CHC)15] NB-[p(F-CHC)15-b-p(CHC)15] NB-[p(CHC)15-b-p(Hex-CHC)15] NB-[p(Hex-CHC)15-b-p(CHC)15] NB-[p(CHC)10-b-p(DEG-CHC)10-b-p(CHC)10] NB-[p(CHC)10-b-p(F-CHC)10-b-p(Hex-CHC)10] NB-[p(Hex-CHC)10-b-p(CHC)10-b-p(F-CHC)10] NB-[p(Hex-CHC)10-b-p(DEG-CHC)10-b-p(F-CHC)10]

4.35 5.78 5.21 5.57 8.02 8.02 6.07 6.07 5.68 7.93 7.93 9.26

5.34 7.46 5.10 5.57 11.00 9.85 7.08 7.20 7.79 9.08 9.12 9.19

5.72 7.24 6.07 5.97 7.96 6.95 7.61 7.82 6.48 7.96 7.80 8.56

1.08 1.11 1.10 1.19 1.08 1.10 1.11 1.09 1.13 1.08 1.10 1.12

46 63 82 97 93 99 77 84 82 82 69 90

a Polymerization conditions: toluene, 50 °C, 100 psi CO2, [rac-3]initial = 50 mM. All polymerizations proceeded to >99% conversion by 1H NMR spectroscopy. bBased on 100% conversion. cMeasured in THF. dDetermined by GPC calibrated with polystyrene standards in chloroform at 40 °C. e Isolated yield. f[rac-3]initial = 67 mM. g[rac-3]initial = 33 mM.

agent.13 The ROMP of a triblock macromonomer of norbornene-terminated polystyrene-b-poly(methyl acrylate)-bpoly(tert-butyl acrylate) resulted in a triple-layered core−shell molecular brush with dynamic stimuli-responsive properties.14 Cheng and co-workers recently utilized ROMP of functional macromonomers to the syntheses of Janus nanomaterials15a and acid-triggered drug release system.15b Norbornenyl-terminated macromonomers for well-defined molecular brushes can be synthesized by controlled/living radical13,14,16 and anionic7c−k,z polymerizations or postpolymerization7a,b,l−o,u,8b,11a−d,g,12 modifications. Controlled/living radical and anionic polymerizations with chain initiation or termination by norbornenyl-functionalized reagents allow the formation of norbornene-terminated macromonomers with narrow Mw/Mn in minimal steps. However, in radical polymerizations, the norbornenyl functionality could polymerize and produce undesired branched polymers.13a,16 In the case of anionic polymerizations, the highly reactive nature of propagating anions limits the number of compatible functional groups. Thus, postpolymerization modification strategies have mainly been used despite requiring additional steps. The successful “grafting through” of block macromonomers with variable sequences would allow the systematic study of core−shell brush copolymers with layer variations.7h−k,17 However, the lack of a mild and efficient multiblock macromonomer synthesis has led to the use of other grafting techniques, mostly “grafting from”, despite lower grafting density and control.18 Previously, we employed (BDI)ZnOAc19 to copolymerize cyclohexene oxide derivatives with CO2 to produce multiblock poly(cyclohexene carbonate)s [p(CHC)s] with narrow molecular weight distributions.20 The sequential addition of functionalized cyclohexene oxides allows the

synthesis of up to hexablock copolymers of controlled block length and varied block sequence containing multiple functionalities. We hypothesized that the use of a norbornene carboxylate initiator could produce well-defined α-norbornenyl block polycarbonates for a “grafting through” brush polymer synthesis (Scheme 1). With control of macromonomer block sequences, this method would yield core−shell molecular brushes with a control over layer compositions. Herein, we describe the facile synthesis of α-norbornenyl block poly(cyclohexene carbonate)s [NB-p(CHC)s] and their “grafting through” by ROMP. β-Diiminate (BDI) zinc-catalyzed block copolymerization of functionalized epoxides and CO2 with a norbornenyl initiator and subsequent ROMP present good control for the synthesis of molecular brushes with variable layer compositions.

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RESULTS AND DISCUSSION Macromonomer Synthesis: Norbornenyl Block Poly(cyclohexene carbonate)s. A (BDI)Zn norbornene carboxylate was synthesized to generate polycarbonate macromonomers. The highly strained norbornenyl functionality exhibits high activity and living behavior in ROMP.10 exoNorbornenyl groups display higher reactivity than their endoisomers due to less steric hindrance around the reactive olefin group;21 thus, we pursued (BDI)Zn catalysts bearing an exonorbornenyl initiator. Following a procedure analogous to (BDI)ZnOAc,19c exo-5-norbornene-carboxylic acid was reacted with (BDI)ZnEt in toluene, producing the desired (BDI)Zn exo-norbornene-carboxylate (rac-3) (Scheme 1). A series of macromonomers were synthesized using catalyst rac-3 (Scheme 1 and Table 1). The norbornene carboxylate initiator displayed similar catalytic activity to its acetate 7879


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variant.19b As shown in Table 1, various polycarbonate blocks such as p(CHC), lipophilic p(Hex-CHC), hydrophilic p(DEGCHC), and fluorophilic p(F-CHC) (Figure 2) were assembled

Despite considerable efforts to study the syntheses and applications of core−shell brush copolymers,22 only few pursue isomeric core and shell compositions due to synthetic limitations.7h−k,17 Since our synthetic method produces not only block macromonomers but also their sequential isomers, their subsequent ROMP provides access to core−shell brushes with variable layer compositions. Thus, we investigated the “grafting through” of α-norbornenyl block polycarbonate macromonomers (Table 3). The ROMP of di- and triblock macromonomers of Table 1 resulted in highly controlled core− shell brushes (Table 3) with high DPs (up to 196) and narrow to moderate polydispersities (1.12−1.66). All polymerizations of macromonomers showed high conversions; however, Mw/ Mn broadened as [monomer]/[2] ratios increased. The layerisomeric core−shell molecular brushes were obtained using block isomers (entries 2/3, 4/7, 5/8, 6/9, 11/13, and 12/14). For example, triblock isomers of NB-[p(Hex-CHC)10-bp(CHC) 10 -b-p(F-CHC) 10 ] and NB-[p(CHC) 10 -b-p(FCHC) 10-b-p(Hex-CHC)10] reacted with catalyst 2 and produced core−shell brush copolymers, which are only different in layer orders. The kind and order of polycarbonate blocks showed a minimal effect on both α-norbornenyl block polycarbonate macromonomer synthesis and ROMP. Our strategy, consecutive living polymerizations by 2 and rac-3, produced highly controlled graft copolymers including layerisomeric core−shell brushes in two steps from monomers (Schemes 1 and 2). While other methods for layer alternations require significant changes in synthetic schemes or conditions, our strategy only needs adjustment in monomer addition sequences. Grubbs11a and Wooley13c synthesized heterografted diblock molecular brushes via ROMP, in which different polymeric side chains were grafted sequentially along a backbone. Using our system, we were able to build block architectures not only along the backbone but also along the side chains. Sequential ROMP of diblock NB-p(CHC)15-b-p(F-CHC)15 and triblock NBp(CHC)10-b-p(F-CHC)10-b-p(Hex-CHC)10 macromonomers yielded a heterografted diblock core−shell brush copolymer, PNB-g-[p(CHC)15-b-p(F-CHC)15]50-b-PNB-g-[p(CHC)10-bp(F-CHC)10-b-p(Hex-CHC)10]50 (Scheme 3). Both block formations showed >98% macromonomer consumption, increase in Mn, and slight broadening of Mw/Mn (Figure 4). To our knowledge, the resulting product represents the first example of block core−shell brushes using a “grafting through” method1 via sequential one-pot living block copolymerizations.

Figure 2. Molecular structures of polycarbonate blocks.

to form α-norbornenyl mono- (entries 1−4), di- (entries 5−8), and triblock (entries 9−12) copolymers with narrow polydispersities (Mw/Mn = 1.08−1.13). Several pairs of block isomers were prepared for use in the synthesis of layered isomeric core−shell molecular brushes (entries 5−6, 7−8, and 10−11). “Grafting Through” Brush Copolymer Synthesis. We applied catalyst 2 to perform “grafting through” ROMP of αnorbornenyl poly(cyclohexene carbonate)s (Scheme 2, Tables 2 and 3). Initially, the polymerization of homomacromonomers was investigated with different catalyst loadings (Table 2, entries 1−3). High conversions and linear Mn growth were observed for [monomer]/[2] ratios from 26 to 51 and 100; however, polydispersities broadened slightly with increasing proportions of macromonomer (Figure 3). Relative Mn measurements from a RI detector (calibrated with polystyrene standards) showed much lower Mn than theoretical values because brush polymers have relatively small hydrodynamic volumes. In contrast, multiangle laser light scattering (MALLS) analysis provides absolute Mn measurements and gave Mn values comparable to the theoretical values. Functionalized homomacromonomers such as hydrophilic NB-p(DEG-CHC) and lipophilic NB-p(Hex-CHC) were also grafted (Table 2, entries 5 and 6). While the polymerization of lipophilic NBp(Hex-CHC)20 went to near complete conversion (96%) with narrow polydisperity (Mw/Mn = 1.11), hydrophilic NB-p(DEGCHC)20 gave moderate conversion (69%) and broad molecular weight distribution (Mw/Mn = 2.14).

Scheme 2. “Grafting Through” Ring-Opening Metathesis Polymerization of a Norbornene-Terminated Multiblock p(CHC) with Catalyst 2

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Table 2. Polymerization Data: Ring-Opening Metathesis Polymerization of Norbornene-Terminated Poly(cyclohexene carbonate)sa entry

macromonomer

[monomer]/[2]

Mn(theo)b (105 g/mol)

Mn(MALLS)c (105 g/mol)

Mn(RI)d (105 g/mol)

Mw/Mnd

conve (%)

1 2 3 4 5 6

NB-p(CHC)30 NB-p(CHC)30 NB-p(CHC)30 NB-p(CHC)40 NB-p(Hex-CHC)20 NB-p(DEG-CHC)20

26 51 100 25 100 110

1.36 2.64 5.18 1.83 4.90 4.23

1.54 3.01 5.43 2.45 4.21 16.7

0.681 0.983 1.47 0.841 1.35 2.01

1.09 1.16 1.36 1.22 1.11 2.14

98 97 97 98 96 69

a Polymerization conditions: THF, rt, 1 h. bMn(theo) = Mn(monomer, MALLS) × [monomer]/[2] × % conversion. cDetermined by MALLS in THF. dDetermined by GPC calibrated with polystyrene standards in chloroform at 40 °C. eDetermined by GPC (RI detector).

Table 3. Polymerizations of Multiblock Norbornene-Terminated Macromonomersa entry

macromonomer

1 2 3 4 5 6 7 8 9 10 11

NB-[p(CHC)15-b-p(F-CHC)15] NB-[p(CHC)15-b-p(F-CHC)15] NB-[p(F-CHC)15-b-p(CHC)15] NB-[p(CHC)15-b-p(Hex-CHC)15] NB-[p(CHC)15-b-p(Hex-CHC)15] NB-[p(CHC)15-b-p(Hex-CHC)15] NB-[p(Hex-CHC)15-b-p(CHC)15] NB-[p(Hex-CHC)15-b-p(CHC)15] NB-[p(Hex-CHC)15-b-p(CHC)15] NB-[p(CHC)10-b-p(DEG-CHC)10-b-p(CHC)10] NB-[p(CHC)10-b-p(F-CHC)10-b-p(HexCHC)10] NB-[p(CHC)10-b-p(F-CHC)10-b-p(HexCHC)10] NB-[p(Hex-CHC)10-b-p(CHC)10-b-p(FCHC)10] NB-[p(Hex-CHC)10-b-p(CHC)10-b-p(FCHC)10] NB-[p(Hex-CHC)10-b-p(DEG-CHC)10-b-p(FCHC)10]

12 13 14 15

[monomer]/ [2]

Mn(theo)b (105 g/mol)

Mn(MALLS)c (105 g/mol)

Mn(RI)d (105 g/mol)

Mw/Mnd

conve (%)

76 150 170 50 100 200 50 100 200 74 51

8.03 15.8 15.7 3.43 6.80 13.7 3.53 7.06 14.1 5.13 4.45

9.06 25.0 15.2 5.10 9.86 19.1 5.13 10.1 15.9 4.52 3.96

1.81 3.61 2.69 1.19 2.05 3.98 1.19 2.06 3.51 1.21 1.09

1.16 1.27 1.45 1.15 1.28 1.66 1.14 1.29 1.57 1.28 1.12

96 96 93 97 96 97 98 98 98 89 96

100

8.63

7.02

1.62

1.39

95

51

4.56

6.02

1.09

1.21

98

100

8.85

1.90

1.54

97

50

4.14

1.31

1.34

90

12.5 5.01

a Polymerization conditions: THF, rt, 1 h. bMn(theo) = Mn(monomer, MALLS) × [monomer]/[2] × % conversion. cDetermined by MALLS in THF. dDetermined by GPC calibrated with polystyrene standards in chloroform at 40 °C. eDetermined by GPC (RI detector).

Figure 3. (a) Gel-permeation chromatographs (RI). (b) Plot of PNB-g-[p(CHC)30]n Mn (RI) (●), Mn (MALLS) (▲), and Mw/Mn (◆) versus the [macromonomer]/[Ru] ratio (Table 2, entries 1−3).

In summary, we report a method for facile α-norbornenyl polycarbonate macromonomer synthesis and their subsequent “grafting through” by ROMP, producing well-defined molecular brushes. The one-pot living multiblock copolymerization of

functionalized cyclohexene oxides and CO2 using catalyst rac-3 yields a set of α-norbornenyl macromonomers with variable block sequences. Highly reactive metathesis catalyst 2 polymerized the resulting macromonomers with generally 7881


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synthesis of core−shell molecular brushes, which could facilitate studies on phase separation, self-assembly, and nanostructure formation.

Scheme 3. Hetero-Grafted Diblock Core−Shell Brush Synthesis of PNB-g-[p(CHC)15-b-p(F-CHC)15]50-b-PNB-g[p(CHC)10-b-p(F-CHC)10-b-p(Hex-CHC)10]50a

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, polymer synthesis, and characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. William R. Dichtel and Mr. David Bunck for help with MALLS measurements and Materia Inc. for providing catalyst 2. We gratefully acknowledge financial support from the NSF (CHE-0809778 and CHE-1112278) as well as the Energy Material Center at Cornell (EMC2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Award DE-SC0001086.

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a

Mn and Mw/Mn were determined by GPC-RI calibrated with polystyrene standards in chloroform at 40 °C.

REFERENCES

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Figure 4. Gel-permeation chromatographs (RI) of NB-p(CHC)15-bp(F-CHC)15 (blue), NB-p(CHC)10-b-p(F-CHC)10-b-p(Hex-CHC)10 (red), PNB-g-[p(CHC)15-b-p(F-CHC)15]50 (black), and PNB-g-[p(CHC)15-b-p(F-CHC)15]50-b-PNB-g-[p(CHC)10-b-p(F-CHC)10-b-p(Hex-CHC)10]50 (green).

high conversions and narrow molecular weight distributions. We demonstrated the various molecular brush syntheses of homo, core−shell, and block core−shell structures. This method of “grafting through” of block macromonomers with interchangeable block sequences provides a highly controlled 7882


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