A Cyclic Ruthenium Benzylidene Initiator Platform Enhances Reactivity for Ring-Expansion Metathesis Polymerization

Reactivity for Ring-Expansion Metathesis Polymerization

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A Cyclic Ruthenium Benzylidene Initiator Platform Enhances

Teng-Wei Wang, Pin-Ruei Huang, Jayme L. Chow, Werner Kaminsky, and Matthew R. Golder*

Cite This: J. Am. Chem. Soc. 2021, 143, 7314 −7319

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sı Supporting Information

ABSTRACT: Ring-expansion metathesis polymerization (REMP) has shown potential as an eﬃcient strategy to access cyclic

macromolecules. Current approaches that utilize cyclic oleﬁn feedstocks suﬀer from poor functional group tolerance, low initiator

stability, and slow reaction kinetics. Improvements to current initiators will address these issues in order to develop more versatile

and user-friendly technologies. Herein, we report a reinvigorated tethered ruthenium−benzylidene initiator, CB6, that utilizes design

features from ubiquitous Grubbs-type initiators that are regularly applied in linear polymerizations. We report the controlled

synthesis of functionalized cyclic poly(norbornene)s and demonstrate that judicious ligand modiﬁcations not only greatly improve

kinetics but also lead to enhanced initiator stability. Overall, CB6 is an adaptable platform for the study and application of cyclic

macromolecules via REMP.

yclic polymers are promising building blocks for novel

organic materials; their “endless” hoop-like architec-

tures display drastically diﬀerent behaviors in solution and the

−5

solid-state compared to their linear counterparts.

Smaller

hydrodynamic radii and lack of chain ends contribute to

unique thermal

spanning biomaterials, energy,

provide the impetus to develop improved methodologies.

and physical properties; applications

10 11,12 13,14

and sustainability

5,16

Acyclic ring closures

or ring-expansion polymeriza-

(REP) account for most synthetic examples.

7−19

tions

With advanced applications in mind, the latter is arguably more

convenient as the cyclic initiator facilitates both monomer

insertion and templated ring closure. Though there are myriad

REP reaction classes, ring-expansion metathesis polymer-

ization (REMP) remains one of the most versatile ways to

Figure 1. (a) Structural features of Ru ROMP and (b) REMP

initiators alongside (c) improved cyclic benzylidene initiator, CB6.

access cyclic macromolecules. For acyclic polymers, advances

in W, Mo, and Ru ring-opening metathesis polymerization

2−25

(

ROMP) initiator design

have allowed for widespread

control, limited stability, and slow polymerizations. Herein, we

detail the ﬁrst design of a more stable cyclic benzylidene

initiator, CB6 (Figure 1c), and highlight its faster polymer-

izations and superior molar mass control compared to the

state-of-the-art.

Synthesis of envisaged cyclic initiator CB6 (Scheme 1)

featuring both a diaryl-NHC and a tethered benzylidene

commenced with alkylation of 4-hydroxybenzaldehyde and 4-

bromo-3,5-dimethylphenol with dibromohexane over two steps

adoption of this valuable methodology. W and Mo initiators

are quite active and desirable for certain applications, but often

initiators are derived from more air and functional group

6,27

28,29

tolerant

plexes, such as Grubbs second (G2)

generation scaﬀolds (Figure 1a). A sterically demanding NHC

Ru N-heterocyclic carbene

(NHC) com-

0−32

and third (G3)

4,35

imparts initiator stability,

while a bulky benzylidene

ensures facile ligand dissociation and thus faster initiation rates.

36−41

For cyclic polymers, W

and Mo metathesis initiators

(

2). Wittig oleﬁnation, followed by PdBrettPhosG3-medi-

can facilitate REMP of both alkene and alkyne monomers, but

examples are largely limited to hydrocarbon feedstocks.

Despite the overwhelming success of Ru ROMP initiators,

ated amination of aryl bromide 3 with 4 aﬀorded 5.

Received: April 3, 2021

Published: May 7, 2021

the Ru REMP initiators developed by Fu

rstner and

4,45

Grubbs

G2 and G3 (Figure 1b, UC5 and SC5).

have never been optimized to the same extent as

44,45

Using inspiration

from G2 and G3 design principles, there is an opportunity to

develop a tunable cyclic Ru-NHC platform for REMP that

12,46−50

improves upon the current

poor molecular weight

2021 American Chemical Society

J. Am. Chem. Soc. 2021, 143, 7314−7319

314

Journal of the American Chemical Society

Communication

19.4 ppm and phosphine ³¹P resonance at δ = 28.7 ppm

conﬁrmed the presence of a lone organometallic species. For

both G2 and CB6, the benzylidene resonances (C2−H)

Scheme 1. Concise Synthesis of REMP Initiator CB6

appear as singlets, suggesting that these moieties are nearly

orthogonal to the PCy ligand and thus have a negligible J

P−H

value. Cyclic alkylidene complexes SC5 and UC5, on the other

hand, adopt geometries that orient the alkylidene hydrogens

away from orthogonality ( J

≈ 5−10 Hz). Additional

P−H

information was gleaned through solid-state X-ray analysis of

CB6 crystals (Scheme 1). Several stark diﬀerences exist

(

Figure S2 ) between dihedral angles in CB6 compared to

those of G2 and representative cyclic alkylidene complexes

SC5 and UC5. For example, the dihedral angle (φ) outlined by

Cl2−Ru−C2−C3 places the phenyl ring of G2 oﬀ center from

the chlorides by ∼12°, while the alkyl chains emanating from

the Ru center in SC5 and UC5 are oﬀset by ∼20° (Table S6).

Remarkably, the benzylidene ring in CB6 (Scheme 1) is almost

coplanar with the chloride ligands (φ = 0.08°). Further

evidence for the structural rigidity of CB6 is revealed through

density functional theory (DFT) calculations conducted at the

B3LYP/6-31G(d)//LANL2DZ level of theory. CB6 has 15

kcal/mol of strain energy (Figure S4) as assessed by

homodesmotic reactions compared to the more ﬂexible alkyl-

linked UC5 complex with just 6 kcal/mol of strain energy

(

Figure S5). We anticipated that these composite Ru-NHC

X-ray structure ellipsoids shown at 50% probability.

initiator structural data would manifest themselves in faster

reactions and improved molar mass control within REMP.

In proof of concept experiments, we investigated the activity

of initiator (I) CB6 in REMP using two substituted

norbornene-imide (Nb) monomers (M): AcNb and BnNb

(Table 1). Despite ROMP polymers derived from Nb ﬁnding

Formation of the requisite diaryl imidazolium salt 6 could then

be accomplished in one ﬂask (HCl then triethyl orthoformate).

Deprotonation of 6 with sodium hydroxide in chloroform led

to a protected NHC-adduct that formed the desired initiator

CB6 upon ligand exchange and cyclization with Grubbs ﬁrst

generation catalyst (G1). CB6 could be readily puriﬁed by

washing with Et O/pentane (Figure S1).

broad utility in drug delivery, imaging, self-assembly, and

photonic applications, ₄these monomer structures have been

8,49,57−59

historically challenging

for REMP. Only Ru initiators

The structure of CB6 was ﬁrst evaluated using solution-state

NMR spectroscopy; a single benzylidene H resonance at δ =

(i.e., UC5) have been utilized, and in these speciﬁc cases,

uncontrolled growth of high-molecular weight macrocycles was

Table 1. REMP of AcNb and BnNb

[I] /[M]

M_n_,_t_h_e_o_r(kDa)

M_n(kDa)

M_w(kDa)

AcNb

BnNb

UC5

CB6

UC5

CB6

UC5

CB6

UC5

CB6

1:50

12.5

49.9

62.3

79.8

137

25.3

50.6

127

421

43.6

1220

183

193

245

393

954

133

1380

148

170

200

572

65.0

1760

258

274

348

539

1240

182

1740

213

251

1.36

1.49

1.44

1.41

1.42

1.37

1.30

1.37

1.26

1.44

1.48

1.49

1:200

1:250

1:320

1:550

1:100

1:200

1:500

1:800

202

298

Polymerization reactions were stirred at 55 °C in DCE for 12 h under nitrogen. Detemined by GPC-MALS-IV (CHCl , 35 °C).

315

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Communication

Figure 2. Accepted REMP mechanism depicting initiation (i),

propagation (ii), and uncontrolled ring closure (iii).

observed. Polymerizations were initiated (Figure 2i) with CB6

by addition to monomers in DCE. Reactions were stirred

under nitrogen at 55 °C; the elevated temperature, previously

30,33

shown to facilitate chain-transfer

in poly(norbornene)s,

was employed to promote backbiting for ring formation

9,58

(Figure 2iii).

Ideally CB6 is regenerated upon release of

cyclic polymer (m = 0), but in actuality the ring-closure step is

not well controlled and a variety of sizes are produced.

Evaluation of a variety of [CB6/M] ratios revealed an increase

in absolute weight-average molecular weight (M ) with

increased monomer loading (Table 1) as would be expected

for a controlled polymerization. Backbiting kinetics and chain-

transfer equilibria likely lead to the observed nonideal, albeit

controlled, relationships between M and [M] (Figures S10

and S11). Importantly, neither monomer Lewis basicity nor

steric encumbrance aﬀected the activity of CB6 signiﬁcantly.

To compare against the current state-of-the-art, analogous

REMP reactions initiated with UC5 were performed. Although

saturated variants (i.e., SC5) are more active than unsaturated

ones, the former is a capricious complex; in our hands, we were

unable to isolate suﬃcient quantities of pure SC5. Hence, we

opted to compare against the more widely adop-

Figure 3. (a) Log(molar mass) versus retention time plot and (b)

Mark−Houwink−Sakurada plot obtained for REMP (purple) and

ROMP (green) of AcNb. GPC-MALS-IV data were collected in

CHCl at 35 °C.

2,18,42−44,52,54,55

ted

unsaturated UC5. Previous accounts of

Ru-REMP with Nb monomers led to high molecular weight

(Figure 3a and Figure S12). Evaluation of Mark−Houwink−

Sakurada (MHS) plots for the same groups of polymers reveals

that η_c_y_c_l_i_c/η_l_i_n_e_a_r≈ 0.93 for both AcNb and BnNb polymers

(Figure 3b and Figure S13); as expected, the cyclic polymers

have a lower intrinsic viscosity than the corresponding linear

polymers (η_c_y_c_l_i_c< η_l_i_n_e_a_r). Additionally, both pairs of polymers

have nearly identical MHS a values (0.754−0.783 for

poly(AcNb) and 0.742−0.735 for poly(BnNb)), which

conﬁrms similar backbone-independent conformations. Pre-

vious works report η_c_y_c_l_i_c/η_l_i_n_e_a_rratios between 0.33 and

7,59,61

products;

consistent results were obtained in a direct

comparison of polymers produced from CB6 with those

produced from UC5 (Table 1). Polymerizations initiated by

CB6 are more controlled and aﬀord molar masses closer to

theoretical values, suggesting improved initiator eﬃciency.

These diﬀerences may be due in part to stark diﬀerences in

kinetics (vide infra) and clearly demonstrate a signiﬁcant

advantage of CB6 over UC5 as a REMP initiator.

To evaluate the cyclic polymer architecture accessed from

CB6, polymers derived from G2 and CB6 with the same

,37−41,62−69

0.92;

it is unclear if these discrepancies are due

number-average molecular weight (M ) were analyzed by

to sample purity, backbone-related phenomena, or M_n

dependence on viscosity.

GPC-MALS-IV. A comparison between linear and cyclic

polymers with identical molar masses reveals the macrocycle’s

smaller hydrodynamic radius (i.e., longer retention time) and

Polymerization kinetics (Figure 2i,ii) of AcNb using either

UC5, CB6, or G2 were studied in situ using H NMR

−3,60

lower intrinsic viscosity.

versus retention time for polymers initiated with G2 and CB6

M = 183 kDa for AcNb polymers and M = 170 kDa for

Indeed, plots of molar mass

spectroscopy (DCE-d , 55 °C). Reactions initiated with UC5

showed sluggish rates as expected based on previous work by

44,45

(

Grubbs

(Figure 4a). Excitingly, reactions initiated with

BnNb polymers) reveal that the cyclic structures have larger

molar masses than linear ones at any given retention time

CB6 showed a drastic increase in rate compared to that of

UC5. In under 5 min, full monomer conversion was observed

316

Journal of the American Chemical Society

J. Am. Chem. Soc. 2021, 143, 7314−7319

https://pubs.acs.org/doi/10.1021/jacs.1c03491.

Communication

robustness of CB6, REMP on the benchtop under air ([I/

AcNb] = 1:200) aﬀorded a cyclic poly(norbornene) as

determined by GPC-MALS-IV (Figure S14), albeit with

considerably higher M than expected, presumably due to

expedited initiator oxidation. Interestingly, the measured M_n

(

388 kDa) was still closer to the theoretical M than the M for

n n

the identical reaction conducted under an inert atmosphere

with UC5 (Table 1).

To further conﬁrm that CB6 initiated REMP resulted in

cyclic polymers, we performed chain-transfer experiments

using 3-hexene as the chain-transfer agent (CTA). We

hypothesized that adding CTA would purposefully generate

linear impurities. The linear fragments should follow a MHS

relationship (η = KM ) as a function of CTA concentration;

deviation from this trend at [CTA] = 0 would only be

expected if there was a change in architecture. Accordingly,

ROMP of AcNb with G2 ([I/M] = 1:200) in the presence of

CTA ([CTA]/[Ru] = 0:1−10:1) a ﬀ orded linear polymers that

ﬁt the MHS relationship (Figure S17). On the other hand,

REMP of AcNb with CB6 and UC5 using identical CTA ratios

produced polymers that also ﬁt the MHS relationship, except

when CTA was omitted (Figures 4b and S18). Because cyclic

polymers have lower viscosities than their linear counterparts,

the observed deviations suggest a change in cyclic to acyclic

architecture upon addition of CTA. In other words, lower

viscosity (η_c_y_c_l_i_c/η_l_i_n_e_a_r= 0.77−0.85) polymers were only

generated in the absence of CTA; chain transfer results in

decreased molar masses and viscosities for linear structures and

topological changes when cyclic macromolecules are present.

In summary, we demonstrated the superior utility of a novel

cyclic Ru-NHC complex, CB6, to synthesize cyclic poly-

(

norbornene)s. Based on the drastic improvements in

molecular weight control, reaction kinetics, and initiator

stability, CB6 will serve as a building block for a platform of

REMP initiators. A thorough understanding of the intimate

relationship between initiator structure and subsequent

performance within the REMP mechanism (Figure 2) will

ultimately allow for broad control over cyclic macromolecule

design.

Figure 4. (a) AcNb monomer consumption ([AcNb] = 1 mM, [I/

AcNb] = 1:50) monitored by H NMR spectroscopy (55 °C in DCE-

d ) and (b) Mark−Houwink−Sakurada plot obtained during REMP

of AcNb with CB6 in the presence of 3-hexene.

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

using CB6, while UC5 eﬀected <10% monomer conversion.

The structural similarities between CB6 and G2 undoubtedly

led to the diﬀerence in kinetics. For polymerizations of AcNb

Washington, Seattle, Washington 98195, United States;

Figure S15 ) and BnNb (Figure S16 ) with CB6, a sharp

General experimental procedures and characterization,

NMR data, GPC traces, crystallography data for CCDC

2073591, and DFT experiments and coordinates (PDF)

increase in M occurred just after full monomer consumption,

followed by a quick decrease in M . The equilibrium-

controlled molar mass is nearly reached in 3 h, providing

expedited kinetics compared to that of UC5.

Accession Codes

CCDC 2073591 contains the supplementary crystallographic

data for this paper. These data can be obtained free of charge

via www.ccdc.cam.ac.uk/data_request/cif , or by emailing

data_request@ccdc.cam.ac.uk , or by contacting The Cam-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

In accordance with eﬀorts to make REMP a more accessible

technology, we sought to evaluate the stability of CB6.

Solution-state stability is also important because linear

impurities generated during REMP are often attributed to

initiator decomposition. We monitored the characteristic

benzylidene peak (or alkylidene peak in the case of UC5) of

CB6, UC5 , and G2 in degassed benzene-d as a function of

time (Figure S3 ) by H NMR spectroscopy. CB6 is less stable

AUTHOR INFORMATION

Corresponding Author

■

than benchmark G2 but persists longer in solution than UC5.

Perhaps more importantly, CB6 has a signiﬁcantly longer shelf

life (−20 °C under nitrogen) than UC5. CB6 remains

Matthew R. Golder − Department of Chemistry and

Molecular Engineering & Science Institute, University of

orcid.org/0000-0001-5848-7366; Email: goldermr@

uw.edu

unchanged by H NMR spectroscopy with similar activity for

at least 5 months, while UC5 decomposes over less than 3

months (Figures S6 and S7). As a ﬁnal testament to the

317

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Journal of the American Chemical Society

Authors

(11) Schappacher, M.; Deffieux, A. Synthesis of Macrocyclic

Communication

Copolymer Brushes and Their Self-Assembly into Supramolecular

Tubes. Science 2008, 319, 1512−1515.

Teng-Wei Wang − Department of Chemistry, University of

Washington, Seattle, Washington 98195, United States

Pin-Ruei Huang − Department of Chemistry, University of

Washington, Seattle, Washington 98195, United States

Jayme L. Chow − Department of Chemistry, University of

Washington, Seattle, Washington 98195, United States

Werner Kaminsky − Department of Chemistry, University of

Washington, Seattle, Washington 98195, United States;

orcid.org/0000-0002-9100-4909

(

12) Turner, M.; Lidster, B.; Hirata, S.; Matsuda, S.; Yamamoto, T.;

Komanduri, V.; Kumar, D. R.; Tezuka, Y.; Vacha, M. Macrocyclic

Poly(p-Phenylenevinylene)s by Ring Expansion Metathesis Polymer-

isation and Their Characterization by Single-Molecule Spectroscopy.

Chem. Sci. 2018, 9, 2934−2941.

13) Kaitz, J. A.; Diesendruck, C. E.; Moore, J. S. End Group

Characterization of Poly(Phthalaldehyde): Surprising Discovery of a

Reversible, Cationic Macrocyclization Mechanism. J. Am. Chem. Soc.

013, 135, 12755−12761.

14) Feinberg, E. C.; Hernandez, H. L.; Plantz, C. L.; Mejia, E. B.;

https://pubs.acs.org/10.1021/jacs.1c03491

Complete contact information is available at:

Sottos, N. R.; White, S. R.; Moore, J. S. Cyclic Poly(Phthalaldehyde):

Thermoforming a Bulk Transient Material. ACS Macro Lett. 2018, 7,

Notes

7−52.

The authors declare the following competing ﬁnancial

interest(s): T-W.W. and M.R.G. are inventors on Provisional

Patent Application 63/171,474.

(15) Laurent, B. A.; Grayson, S. M. An Efficient Route to Well-

Defined Macrocyclic Polymers via “Click ” Cyclization. J. Am. Chem.

Soc. 2006, 128, 4238−4239.

(16) Voter, A. F.; Tillman, E. S. An Easy and Efficient Route to

Macrocyclic Polymers via Intramolecular Radical-Radical Coupling of

ACKNOWLEDGMENTS

Chain Ends. Macromolecules 2010, 43, 10304−10310.

■

(17) Kammiyada, H.; Konishi, A.; Ouchi, M.; Sawamoto, M. Ring-

We thank Prof. Jeremiah Johnson for numerous invaluable

discussions, Prof. Yan Xia for helpful advice, and Prof. Gojko

Lalic, Prof. Alshakim Nelson, and Prof. Forrest Michael for

feedback during manuscript preparation. Dr. Adrienne

Roehrich and Dr. Martin Sadilek are acknowledged for

assistance with VT-NMR experiments and HRMS experi-

ments, respectively. We also thank Dr. Adam Brooks (Wyatt

Technology) for helpful discussions and Umicore for a

generous gift of Grubbs 1st generation catalyst. This research

was supported by generous startup funds from the University

of Washington.

Expansion Living Cationic Polymerization via Reversible Activation of

a Hemiacetal Ester Bond. ACS Macro Lett. 2013, 2, 531 −534.

(18) Ouchi, M.; Kammiyada, H.; Sawamoto, M. Ring-Expansion

Cationic Polymerization of Vinyl Ethers. Polym. Chem. 2017, 8,

4970−4977.

(19) Chang, Y. A.; Waymouth, R. M. Recent Progress on the

Synthesis of Cyclic Polymers via Ring-Expansion Strategies. J. Polym.

Sci., Part A: Polym. Chem. 2017, 55 , 2892 −2902.

(20) Wang, T.-W.; Golder, M. R. Advancing Macromolecular Hoop

Construction: Recent Developments in Synthetic Cyclic Polymer

Chemistry. Polym. Chem. 2021, 12, 958−969.

(21) Kubota, H.; Yoshida, S.; Ouchi, M. Ring-Expansion Cationic

Cyclopolymerization for the Construction of Cyclic Cyclopolymers.

REFERENCES

Polym. Chem. 2020, 11 , 3964 −3971.

■

(22) Schrock, R. R. Recent Advances in High Oxidation State Mo

1) Laurent, B. A.; Grayson, S. M. Synthetic Approaches for the

Preparation of Cyclic Polymers. Chem. Soc. Rev. 2009 , 38 , 2202.

2) Haque, F. M.; Grayson, S. M. The Synthesis, Properties and

Potential Applications of Cyclic Polymers. Nat. Chem. 2020, 12, 433−

44.

3) Edwards, J. P.; Wolf, W. J.; Grubbs, R. H. The Synthesis of

Cyclic Polymers by Olefin Metathesis: Achievements and Challenges.

J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 228−242.

4) Liénard, R.; De Winter, J.; Coulembier, O. Cyclic Polymers:

Advances in Their Synthesis, Properties, and Biomedical Applications.

J. Polym. Sci. 2020, 58, 1481 −1502.

5) Kricheldorf, H. R. Cyclic Polymers: Synthetic Strategies and

Physical Properties. J. Polym. Sci., Part A: Polym. Chem. 2010, 48,

51−284.

6) Dodgson, K.; Sympson, D.; Semlyen, J. A. Studies of Cyclic and

and W Imido Alkylidene Chemistry. Chem. Rev. 2009, 109, 3211−

226.

23) Vougioukalakis, G. C.; Grubbs, R. H. Ruthenium-Based

Heterocyclic Carbene-Coordinated Olefin Metathesis. Chem. Rev.

010, 110, 1746−1787.

24) Ogba, O. M.; Warner, N. C.; O ’Leary, D. J.; Grubbs, R. H.

Recent Advances in Ruthenium-Based Olefin Metathesis. Chem. Soc.

Rev. 2018, 47, 4510−4544.

27) Hilf, S.; Kilbinger, A. F. M. Functional End Groups for

25) Bielawski, C. W.; Grubbs, R. H. Living Ring-Opening

Metathesis Polymerization. Prog. Polym. Sci. 2007 , 32 , 1 −29.

26) Leitgeb, A.; Wappel, J.; Slugovc, C. The ROMP Toolbox

Upgraded. Polymer 2010, 51, 2927−2946.

Polymers Prepared Using Ring-Opening Metathesis Polymerization.

Nat. Chem. 2009, 1, 537−546.

Linear Poly(Dimethyl Siloxanes): 2. Preparative Gel Permeation

Chromatography. Polymer 1978, 19, 1285−1289.

7) Kammiyada, H.; Ouchi, M.; Sawamoto, M. A Study on Physical

Properties of Cyclic Poly(Vinyl Ether)s Synthesized via Ring-

Expansion Cationic Polymerization. Macromolecules 2017, 50, 841−

(28) Arduengo, A. J.; Harlow, R. L.; Kline, M. A Stable Crystalline

Carbene Anthony. J. Am. Chem. Soc. 1991, 113, 361−363.

Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Imidazolylidenes,

(29) Arduengo, A. J.; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;

Imidazolinylidenes and Imidazolidines. Tetrahedron 1999, 55, 14523−

848.

30) Bielawski, C. W.; Grubbs, R. H. Highly Efficient Ring-Opening

4534.

(

8) Wang, J.; Li, Z.; Pérez, R. A.; Muller, A. J.; Zhang, B.; Grayson, S.

M.; Hu, W. Comparing Crystallization Rates between Linear and

Cyclic Poly(Epsilon-Caprolactones) via Fast-Scan Chip-Calorimeter

Measurements. Polymer 2015, 63, 34−40.

Metathesis Polymerization (ROMP) Using New Ruthenium Catalysts

Containing N-Heterocyclic Carbene Ligands. Angew. Chem., Int. Ed.

2000, 39, 2903−2906.

(

9) McKenna, G. B.; Hadziioannou, G.; Lutz, P.; Hild, G.; Strazielle,

(31) Love, J. A.; Sanford, M. S.; Day, M. W.; Grubbs, R. H.

Synthesis, Structure, and Activity of Enhanced Initiators for Olefin

Metathesis. J. Am. Chem. Soc. 2003, 125, 10103−10109.

C.; Straupe, C.; Rempp, P.; Kovacs, A. J. Dilute Solution

Characterization of Cyclic Polystyrene Molecules and Their Zero-

Shear Viscosity in the Melt. Macromolecules 1987, 20, 498 −512.

(32) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.;

Scholl, M.; Choi, T. L.; Ding, S.; Day, M. W.; Grubbs, R. H. Synthesis

and Activity of Ruthenium Alkylidene Complexes Coordinated with

(10) Golba, B.; Benetti, E. M.; De Geest, B. G. Biomaterials

Applications of Cyclic Polymers. Biomaterials 2021, 267, 120468.

318

J. Am. Chem. Soc. 2021, 143, 7314−7319

Journal of the American Chemical Society

(52) Dias, E. L.; Nguyen, S. B. T.; Grubbs, R. H. Well-Defined

Communication

Phosphine and N-Heterocyclic Carbene Ligands. J. Am. Chem. Soc.

Ruthenium Olefin Metathesis Catalysts: Mechanism and Activity. J.

Am. Chem. Soc. 1997, 119, 3887−3897.

33) Choi, T. L.; Grubbs, R. H. Controlled Living Ring-Opening-

003, 125, 2546−2558.

Metathesis Polymerization by a Fast-Initiating Ruthenium Catalyst.

(53) Liao, L.; Liu, J.; Dreaden, E. C.; Morton, S. W.; Shopsowitz, K.

E.; Hammond, P. T.; Johnson, J. A. A Convergent Synthetic Platform

for Single-Nanoparticle Combination Cancer Therapy: Ratiometric

Loading and Controlled Release of Cisplatin, Doxorubicin, and

Camptothecin. J. Am. Chem. Soc. 2014, 136, 5896−5899.

Angew. Chem., Int. Ed. 2003, 42, 1743−1746.

(34) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Synthesis and

Activity of a New Generation of Ruthenium-Based Olefin Metathesis

Catalysts Coordinated with 1,3-Dimesityl-4,5-Dihydroimidazol-2-

Ylidene Ligands. Org. Lett. 1999, 1, 953−956.

(54) Nguyen, H. V. T.; Chen, Q.; Paletta, J. T.; Harvey, P.; Jiang, Y.;

Zhang, H.; Boska, M. D.; Ottaviani, M. F.; Jasanoff, A.; Rajca, A.;

Johnson, J. A. Nitroxide-Based Macromolecular Contrast Agents with

Unprecedented Transverse Relaxivity and Stability for Magnetic

Resonance Imaging of Tumors. ACS Cent. Sci. 2017 , 3 , 800 −811.

(35) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H.

Increased Ring Closing Metathesis Activity of Ruthenium-Based

Oiefin Metathesis Catalysts Coordinated with Imidazolin-2-Ylidene

Ligands. Tetrahedron Lett. 1999, 40, 2247−2250.

(55) Xia, Y.; Kornfield, J. A.; Grubbs, R. H. Efficient Synthesis of

(36) Roland, C. D.; Li, H.; Abboud, K. A.; Wagener, K. B.; Veige, A.

Narrowly Dispersed Brush Polymers via Living Ring-Opening

Metathesis Polymerization of Macromonomers. Macromolecules

S. Cyclic Polymers from Alkynes. Nat. Chem. 2016, 8, 791−796.

(37) Nadif, S. S.; Kubo, T.; Gonsales, S. A.; VenkatRamani, S.;

(

009, 42, 3761−3766.

Ghiviriga, I.; Sumerlin, B. S.; Veige, A. S. Introducing “Ynene ”

Metathesis: Ring-Expansion Metathesis Polymerization Leads to

Highly Cis and Syndiotactic Cyclic Polymers of Norbornene. J. Am.

Chem. Soc. 2016, 138, 6408−6411.

56) Piunova, V. A.; Miyake, G. M.; Daeffler, C. S.; Weitekamp, R.

A.; Grubbs, R. H. Highly Ordered Dielectric Mirrors via the Self-

Assembly of Dendronized Block Copolymers. J. Am. Chem. Soc. 2013,

Sumerlin, B. S.; Veige, A. S. Highly Tactic Cyclic Polynorbornene:

35, 15609−15616.

57) Zhang, K.; Lackey, M. A.; Wu, Y.; Tew, G. N. Universal Cyclic

Polymer Templates. J. Am. Chem. Soc. 2011, 133, 6906−6909.

58) Boydston, A. J.; Holcombe, T. W.; Unruh, D. A.; Fréchet, J. M.

(38) Gonsales, S. A.; Kubo, T.; Flint, M. K.; Abboud, K. A.;

Stereoselective Ring Expansion Metathesis Polymerization of

Norbornene Catalyzed by a New Tethered Tungsten-Alkylidene

Catalyst. J. Am. Chem. Soc. 2016, 138, 4996−4999.

(

J.; Grubbs, R. H. A Direct Route to Cyclic Organic Nanostructures via

Ring-Expansion Metathesis Polymerization of a Dendronized Macro-

monomer. J. Am. Chem. Soc. 2009, 131, 5388−5389.

40) Miao, Z.; Pal, D.; Niu, W.; Kubo, T.; Sumerlin, B. S.; Veige, A.

39) Miao, Z.; Kubo, T.; Pal, D.; Sumerlin, B. S.; Veige, A. S. PH-

Responsive Water-Soluble Cyclic Polymer. Macromolecules 2019, 52,

260−6265.

(59) Zhang, K.; Zha, Y.; Peng, B.; Chen, Y.; Tew, G. N. Metallo-

60) Chang, Y. A.; Waymouth, R. M. Recent Progress on the

Supramolecular Cyclic Polymers. J. Am. Chem. Soc. 2013, 135,

5994−15997.

S. Cyclic Poly(4-Methyl-1-Pentene): Efficient Catalytic Synthesis of a

Transparent Cyclic Polymer. Macromolecules 2020, 53, 7774−7782.

Synthesis of Cyclic Polymers via Ring-Expansion Strategies. J. Polym.

(41) Pal, D.; Miao, Z.; Garrison, J. B.; Veige, A. S.; Sumerlin, B. S.

Sci., Part A: Polym. Chem. 2017, 55, 2892−2902.

Ultra-High-Molecular-Weight Macrocyclic Bottlebrushes via Post-

Polymerization Modification of a Cyclic Polymer. Macromolecules

(61) Zhang, K.; Lackey, M. A.; Cui, J.; Tew, G. N. Gels Based on

Cyclic Polymers. J. Am. Chem. Soc. 2011, 133, 4140−4148.

(

020, 53, 9717−9724.

62) Rubio, A. M.; Freire, J. J.; Bishop, M.; Clarke, J. H. R. Θ State,

42) Von Kugelgen, S.; Bellone, D. E.; Cloke, R. R.; Perkins, W. S.;

Transition Curves, and Conformational Properties of Cyclic Chains.

Macromolecules 1995, 28, 2240−2246.

Fischer, F. R. Initiator Control of Conjugated Polymer Topology in

Ring-Opening Alkyne Metathesis Polymerization. J. Am. Chem. Soc.

(63) Geiser, D.; Hócker, H. Synthesis and Investigation of

Macrocyclic Polystyrene. Macromolecules 1980 , 13 , 653 −656.

(64) Hild, G.; Strazielle, C.; Rempp, P. Cyclic Macromolecules.

Synthesis and Characterization of Ring-Shaped Polystyrenes. Eur.

Polym. J. 1983, 19, 721−727.

43) Furstner, A.; Krause, H.; Ackermann, L.; Lehmann, C. W. N-

016, 138, 6234−6239.

Heterocyclic Carbenes Can Coexist with Alkenes and C −H Acidic

Sites. Chem. Commun. 2001, 2240, 2240.

(44) Boydston, A. J.; Xia, Y.; Kornfield, J. A.; Gorodetskaya, I. A.;

(65) Niu, W.; Gonsales, S. A.; Kubo, T.; Bentz, K. C.; Pal, D.; Savin,

D. A.; Sumerlin, B. S.; Veige, A. S. Polypropylene: Now Available

without Chain Ends. Chem. 2019, 5, 237−244.

Grubbs, R. H. Cyclic Ruthenium-Alkylidene Catalysts for Ring-

Expansion Metathesis Polymerization. J. Am. Chem. Soc. 2008, 130,

(

2775−12782.

(66) Jeong, Y.; Jin, Y.; Chang, T.; Uhlik, F.; Roovers, J. Intrinsic

Viscosity of Cyclic Polystyrene. Macromolecules 2017 , 50 , 7770 −7776.

(67) Fukatsu, M.; Kurata, M. Hydrodynamic Properties of Flexible-

Ring Macromolecules. J. Chem. Phys. 1966 , 44 , 4539 −4545.

45) Xia, Y.; Boydston, J. A.; Yao, Y.; Kornfield, A. J.; Gorodetskaya,

A. I.; Spiess, W. H.; Grubbs, R. H. Ring-Expansion Metathesis

Polymerization: Catalyst Dependent Polymerization Profiles. J. Am.

Chem. Soc. 2009, 131, 2670−2677.

(68) Bloomfield, V.; Zimm, B. H. Viscosity, Sedimentation, et

Cetera, of Ring- and Straight-Chain Polymers in Dilute Solution. J.

Chem. Phys. 1966, 44, 315−323.

(46) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. An “Endless ”

Route to Cyclic Polymers. Science 2002, 297, 2041 −2044.

(69) Douglas, J. F.; Freed, K. F. Renormalization and the Two-

(47) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. Synthesis of Cyclic

Parameter Theory. Macromolecules 1984, 17, 2344−2354.

Polybutadiene via Ring-Opening Metathesis Polymerization: The

Importance of Removing Trace Linear Contaminants. J. Am. Chem.

Soc. 2003, 125, 8424−8425.

(70) Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H.

A Standard System of Characterization for Olefin Metathesis

Catalysts. Organometallics 2006, 25, 5740−5745.

(48) Zhang, K.; Tew, G. N. Cyclic Brush Polymers by Combining

Ring-Expansion Metathesis Polymerization and the “Grafting from ”

Technique. ACS Macro Lett. 2012, 1, 574−579.

(49) Xia, Y.; Boydston, A. J.; Grubbs, R. H. Synthesis and Direct

Imaging of Ultrahigh Molecular Weight Cyclic Brush Polymers.

Angew. Chem., Int. Ed. 2011, 50, 5882−5885.

(50) Lin, Y.; Zhang, Y.; Wang, Z.; Craig, S. L. Dynamic Memory

Effects in the Mechanochemistry of Cyclic Polymers. J. Am. Chem.

Soc. 2019, 141, 10943−10947.

(51) Bruno, N. C.; Tudge, T.; Buchwald, S. L. Design and

Preparation of New Palladium Precatalysts for C − C and C − N

Cross-Coupling Reactions. Chem. Sci. 2013, 4, 916−920.

319