Macrocyclic Side-Chain Monomers for Photoinduced ATRP: Synthesis and Properties versus Long-Chain Linear Isomers

Article abstract of DOI:10.1021/acs.macromol.8b01509

The photoinduced atom transfer radical polymerization of traditionally challenging hydrophobic monomers (lauryl C₁₂, stearyl C₁₈, and docosyl C₂₂-acrylate) and novel cyclic isomers (C₁₂, C₁₆, and C₁₇) is reported. By the judicious selection of commercially available solvents and catalytic systems, polymers with a high degree of control over dispersity (D ≈ 1.1) and end-group fidelity were obtained over a wide range of molecular weights. This level of control provides access to unique hydrophobic materials (random, block, and homopolymers) with significant differences in properties observed due to the isomeric form of the hydrocarbon side chain (macrocyclic versus linear).

Full text of DOI:10.1021/acs.macromol.8b01509

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Macrocyclic Side-Chain Monomers for Photoinduced ATRP:
Synthesis and Properties versus Long-Chain Linear Isomers
Stephanie M. Barbon,† Manon Rolland,† Athina Anastasaki,† Nghia P. Truong,§ Morgan W. Schulze,†
Christopher M. Bates,†,‡ and Craig J. Hawker*,†,‡
†Materials Research Laboratory and ‡Materials Department, University of California, Santa Barbara, Santa Barbara, California 93016,
United States
§ARC Centre of Excellence in Convergent Bio-Nano Science & Technology, Monash Institute of Pharmaceutical Sciences, Monash
University, Clayton, VIC 3800, Australia
S
* Supporting Information
ABSTRACT: The photoinduced atom transfer radical polymer-
ization of traditionally challenging hydrophobic monomers (lauryl
C12, stearyl C18, and docosyl C22-acrylate) and novel cyclic isomers
(C12, C16, and C17) is reported. By the judicious selection of
commercially available solvents and catalytic systems, polymers with a
high degree of control over dispersity (Đ ≈ 1.1) and end-group
fidelity were obtained over a wide range of molecular weights. This
level of control provides access to unique hydrophobic materials
(random, block, and homopolymers) with significant differences in
properties observed due to the isomeric form of the hydrocarbon side chain (macrocyclic versus linear).
absorbency,37 surfactant swelling,38 low-temperature flow
INTRODUCTION
■
modifiers of oils,39 and particle stabilization.40 Block
Controlled radical polymerization methods including atom
copolymers of these renewable plant-based monomers have
transfer radical polymerization (ATRP),1,2 reversible atom
also shown interesting self-assembly behavior36,41,42 and found
transfer fragmentation chain-transfer (RAFT),3,4 and nitroxide-
use as thermoplastic elastomers43 and polymeric surfactants.44
mediated polymerization (NMP)5,6 have impacted polymer
For these applications, one major area of concern is the
science broadly, allowing for unprecedented control over
crystallinity of the linear side chains36,45 with many
polymer structure, molecular weight, and end-group fidelity.7,8
More recently, visible light has been used as an external
applications requiring high hydrophobicity and amorphous
character. To address this challenge, we examined macrocyclic
stimulus, imparting additional spatial and temporal control
isomers of these long chain linear systems which would be
over polymer growth by mediating the equilibrium between
expected to have much lower crystallinity while retaining their
active and dormant species. Significantly, a variety of different
very hydrophobic character (Figure 1).46 Interestingly, cyclic
controlled radical polymerization techniques9−11 (ATRP,12−18
metal-free ATRP,19,20 and RAFT21−27) and a range of
analogues of long-chain (>6 carbon) hydrocarbon monomers
monomer families,28 such as vinyl ketones, vinyl acetates,
have not been well studied with the only report being the
polymerization of cyclododecyl acrylate (C12) under traditional
and semifluorinated derivatives, have been shown to be
free radical conditions.47 Given the importance of this broad
amenable to photocontrol.29
class of building blocks, the ability to prepare well-defined
materials from long-chain, hydrophobic monomers would
significantly enable both fundamental and applied studies.
Herein, the design, synthesis, and characterization of a
In contrast to these advances, the controlled polymerization
of long-chain, hydrophobic monomers, such as lauryl (C12),
stearyl (octadecyl, C18), and docosyl (behenyl, C22) acrylates,
is challenging due to an incompatibility with traditional ATRP
variety of hydrophobic monomers including linear (C6−C22)
catalysts/ligands and, in particular, low-ppm catalytic sys-
tems.30 While seemingly simple, only limited examples of
controlled radical polymerization of these monomers have
been demonstrated, often in expensive solvents, with the
majority of these reports focusing on low degrees of
polymerization.31−35 End-group fidelity at high conversion is
also a major issue, restricting diblock copolymer formation to
multistep schemes involving purification of macroinitiators,
rather than in-situ chain extensions.35,36 These challenges
conflict with the proven utility of long-chain (C12−C22),
hydrophobic monomers for a variety of applications, such as oil
and cyclic (C6−C17) acrylates, and their polymerization by
photoinduced ATRP is described (Figure 2). Low dispersities,
good molecular weight control, and excellent chain-end fidelity
are obtained leading to facile, in-situ block copolymer
synthesis. The resulting copolymers show unique thermal
and phase separation behavior, illustrating the potential for
Received: July 16, 2018
Revised: August 14, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.8b01509
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Figure 1. 3D representation of the structural differences between cyclic (C17) and linear (C18) acrylate-based monomers.
Figure 2. Hydrophobic acrylates used in this study.
The organic fractions were combined, dried over MgSO4, gravity
using linear versus cyclic side chains to tune physical
properties.
filtered, and concentrated in vacuo to give cycloheptadecanol. Yield =
1
4.86 g (95%). H NMR (599.8 MHz, CDCl3): δ 3.70 (p, J = 6 Hz,
1H, HO−CH), 1.31−1.55 (m, 32H, alkyl CH2). 13C NMR (100.6
MHz, CDCl3): δ 71.0, 35.9, 28.0, 27.8, 27.7, 27.6, 27.6, 27.5, 23.9.
FT-IR (ATR cm−1): 3309, 2921, 2853, 1458, 1351, 1291, 1086, 1012,
715. HRMS (GC-MS) exact mass calcd for C17H34O [M−H2O]+:
236.2504; found: 236.2499.
EXPERIMENTAL SECTION
■
All reactions were performed under an argon atmosphere unless
otherwise noted and commercially obtained reagents were used as
received unless otherwise noted. All reactions were performed in vials,
unless otherwise noted. CuBr2, 2,2,2-trifluoroethanol, octadecyl
acrylate, and hexyl acrylate were purchased from Sigma-Aldrich.
Tris[2-(dimethylamino)ethyl]amine (Me6Tren), acryloyl chloride,
cyclododecyl alcohol, and bis(cyclohexyl)methanol were purchased
from Alfa Aesar. Lauryl acrylate, cyclohexyl acrylate, behenyl acrylate,
and 5-cyclohexadecen-1-one were purchased from TCI Chemicals.
Civetone was obtained from Perfumer Supply House.
Reduction of 5-Cyclohexadecen-1-one to Cyclohexadeca-
nol. Palladium on carbon (0.42 g, 0.42 mmol Pd) was added to a dry
500 mL round-bottom flask. Isopropanol (200 mL) was added,
followed by 5-cyclohexadecen-1-one (10.00 g, 42.3 mmol) and acetic
acid (5.08 g, 84.6 mmol, 4.84 mL). With stirring, sodium borohydride
(9.60 g, 254 mmol) was added, and rapid bubbling occurred. The
reaction mixture was stirred for 14 h open to the atmosphere. Excess
NaBH4 was quenched with 0.1 M HCl, with addition continuing until
no further gas evolved. The pH was then increased to 10 with 1 M
KOH; the solution was filtered through Celite and transferred to a
separatory funnel. The reaction mixture was extracted three times
with dichloromethane (100 mL). The organic fractions were
combined, dried over MgSO4, gravity filtered, and concentrated in
vacuo to give cyclohexadecanol. Yield = 9.56 g (94%). 1H NMR
(599.8 MHz, CDCl3): δ 3.73 (p, J = 6 Hz, 1H, HO−CH), 1.53−1.57
(m, 2H, alkyl CH2), 1.44−1.48 (m, 2H, alkyl CH2), 1.31−1.37 (m,
26H, alkyl CH2). 13C NMR (100.6 MHz, CDCl3): δ 70.7, 35.2, 27.2,
27.0, 26.8, 26.8, 26.7, 23.4. FT-IR (ATR cm−1): 3301, 2923, 2853,
1459, 1348, 1299, 1026, 996, 709. HRMS (GC-MS) exact mass calcd
for C16H32O [M−H2O]+: 222.2348; found: 222.2346.
General Procedure for the Preparation of Monomers. Under
argon, the appropriate alcohol (0.016 mol) was dissolved in dry
dichloromethane (50 mL) containing triethylamine (2.43 g, 3.34 mL,
0.024 mol) in a 100 mL Schlenk flask. This solution was cooled for 25
min in an ice bath. Acryloyl chloride (2.21 g, 1.94 mL, 0.024 mol) was
then added slowly to the cooled alcohol solution, which gradually
turns a light yellow color. The mixture was stirred in an ice bath for an
additional 4 h. The solution was transferred to a separatory funnel,
washed with 1 M KOH (100 mL), 1 M HCl (100 mL), and brine
solution (100 mL), dried over MgSO4, gravity filtered, and
concentrated in vacuo. The resulting residue was purified by column
chromatography (1:1 CH2Cl2:hexanes, silica) to yield the monomer
as a colorless liquid.
Nuclear magnetic resonance (NMR) spectra were recorded on a
1
Varian 600 MHz instrument. All H and 13C{1H} NMR experiments
are reported in δ units, parts per million (ppm), and were measured
relative to the signal for residual chloroform (7.26 ppm) or deuterated
chloroform (77.16 ppm), respectively, in the deuterated solvent
unless otherwise stated. Fourier transform infrared (FTIR) spectra
were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer.
Size exclusion chromatography (SEC) was performed on a Waters
Acquity APC System, with Acquity UPLC PDA and ACQUITY
UPLC refractive index detectors. Matrix-assisted laser desorption
ionization time-of-flight mass spectrometry (MALDI-ToF-MS) was
conducted using a Bruker Microflex LRF MALDI TOF mass
spectrometer equipped with a 60 Hz nitrogen laser at 337 nm.
Solutions in tetrahydrofuran of dithranol as a matrix (saturated
solution, 10 μL) and sodium bromide as cationization agent (1.0 mg/
mL, 2 μL) were mixed with the sample (1.0 mg/mL, 10 μL), and 0.7
μL of the mixture was applied to the target plate. Differential scanning
calorimetry (DSC) measurements were performed using a Q2000
DSC V24.11 instrument from −160 to 100 °C at a heating and
cooling rate of 10 °C/min (N2 flow of 50 mL/min). The
polymerization light source (UV: λmax ≈ 360 nm) was a commercial
nail curing lamp (Thermal Spa, obtained online from Amazon)
equipped with 3 × 16 W bulbs.
Reduction of Civetone to Cycloheptadecanol. Palladium on
carbon (0.20 g, 0.10 mmol Pd) was added to a dry 100 mL round-
bottom flask. Isopropanol (50 mL) was added, followed by civetone
(5.00 g, 20 mmol) and acetic acid (2.40 g, 40 mmol, 2.28 mL). With
stirring, sodium borohydride (4.53 g, 120 mmol) was added, and
rapid bubbling occurred. The reaction mixture was stirred for 14 h
open to the atmosphere. Excess NaBH4 was quenched with 0.1 M
HCl, with addition continuing until no further gas evolved. The pH
was then increased to 10 with 1 M KOH; the solution was filtered
through Celite and transferred to a separatory funnel. The reaction
mixture was extracted three times with dichloromethane (100 mL).
Cyclododecyl Acrylate. Prepared from 2.95 g (16.0 mmol) of
cyclododecanol. Yield = 2.78 g (73%). 1H NMR (599.8 MHz,
CDCl3): δ 6.37 (d, J = 17 Hz, 1H, vinyl CH), 6.10 (dd, J = 17, 10 Hz,
1H, vinyl CH) 5.78 (d, J = 10 Hz, 1H, vinyl CH), 5.07−5.11 (m, 1H,
tertiary CH), 1.71−1.77 (m, 2H, alkyl CH2), 1.52−1.57 (m, 2H, alkyl
CH2), 1.32−1.45 (m, 18H, alkyl CH2). 13C NMR (100.6 MHz,
CDCl3): δ 166.1, 130.1, 129.3, 72.6, 29.2, 24.3, 24.0, 23.5, 23.4, 21.0.
B
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FT-IR (ATR cm−1): 2934, 2921, 2850, 1721, 1404, 1296, 1273, 1197,
1047, 984, 960. HRMS (ESI) exact mass calcd for C15H26O2 [M−
Na]+: 261.1830; found: 261.1830.
(599.8 MHz, CDCl3): δ 4.63 (1H), 2.59 (1H), 1.01−2.07 (24H).
13C{1H} NMR (150.8 MHz, CDCl3): δ 174.4, 81.2, 41.6, 38.7, 30.1,
27.9, 26.6. SEC: Mn = 2600 g/mol, Đ = 1.11. FT-IR (ATR cm−1):
2921, 2851, 1725, 1448, 1248, 1160, 1092, 965, 907.
Bis(cyclohexyl) Acrylate. Prepared from 3.15 g (16.0 mmol) of
1
dicyclohexylmethanol. Yield = 2.73 g (68%). H NMR (599.8 MHz,
Poly(cyclohexadecyl acrylate). Synthesized from 0.5 g (1.7
CDCl3): δ 6.40 (d, J = 17 Hz, 1H, vinyl CH), 6.13 (dd, J = 17, 10 Hz,
1H, vinyl CH) 5.81 (d, J = 10 Hz, 1H, vinyl CH), 4.71 (t, J = 6 Hz,
1H, tertiary CH), 1.60−1.74 (m, 12H, alkyl CH2), 0.87−1.26 (m,
10H, alkyl CH2). 13C NMR (100.6 MHz, CDCl3): δ 166.5, 130.3,
129.0, 81.7, 38.6, 30.0, 27.6, 26.5, 26.4, 26.2. FT-IR (ATR cm−1):
2921, 2915, 2852, 1723, 1443, 1402, 1295, 1269, 1188, 1042, 984,
966. HRMS (ESI) exact mass calcd for C16H26O2 [M−Na]+:
273.1830; found: 273.1820.
Cyclohexadecyl Acrylate. Prepared from 10.17 g (42.3 mmol) of
cyclohexadecanol. Yield = 7.97 g (64%). 1H NMR (599.8 MHz,
CDCl3): δ 6.37 (dd, J = 17.3, 1.5 Hz, 1H, vinyl CH), 6.10 (dd, J =
17.3, 10.4 Hz, 1H, vinyl CH) 5.78 (dd, J = 10.4, 1.5 Hz, 1H, vinyl
CH), 4.99 (p, J = 6.1 Hz, 1H, tertiary CH), 1.58−1.67 (m, 4H, alkyl
CH2), 1.32−1.39 (m, 26H, alkyl CH2). 13C NMR (100.6 MHz,
CDCl3): δ 166.1, 130.2, 129.3, 74.0, 31.9, 27.1, 27.0, 26.8, 26.8, 26.8,
26.7, 23.3. FT-IR (ATR cm−1): 2925, 2857, 1721, 1619, 1461, 1404,
1293, 1271, 1194, 1045, 984, 962, 809. HRMS (GC-MS) exact mass
calcd for C19H34O2[M−C3H4O2]+: 222.2348; found: 222.2358.
Cycloheptadecyl Acrylate. Prepared from 1.60 g (63.4 mmol) of
cycloheptadecanol. Yield = 1.38 g (71%). 1H NMR (599.8 MHz,
CDCl3): δ 6.38 (d, J = 17 Hz, 1H, vinyl CH), 6.10 (dd, J = 17, 11 Hz,
1H, vinyl CH) 5.79 (d, J = 11 Hz, 1H, vinyl CH), 4.93−4.97 (m, 1H,
tertiary CH), 1.58−1.66 (m, 4H, alkyl CH2), 1.32−1.37 (m, 28H,
alkyl CH2). 13C NMR (100.6 MHz, CDCl3): δ 166.1, 130.2, 129.3,
74.2, 32.5, 27.9, 27.8, 27.7, 27.6, 27.5, 27.4, 23.8. FT-IR (ATR cm−1):
2924, 2856, 1722, 1621, 1461, 1404, 1295, 1272, 1196, 1046, 985,
810. HRMS (ESI) exact mass calcd for C20H36O3 [M−Na]+:
331.2613; found: 331.2628.
General Polymerization Procedure. A stock solution of
copper(II) bromide (2.2 mg, 0.0098 mmol) and Me6Tren (13.9
mg, 16.1 μL, 0.060 mmol) was prepared in 1 mL of trifluoroethanol.
In a 20 mL scintillation vial, hexyl acrylate (0.50 g, 3.21 mmol) was
dissolved in a mixture of 0.4 mL of toluene and 0.1 mL of the Cu(II)/
trifluoroethanol stock solution. Ethyl α-bromoisobutyrate (EBiB, 9.8
mg, 0.05 mmol) was then added, the vial was capped with a septum,
and the solution was degassed with argon for 5 min. With stirring, the
polymerization mixture was then irradiated (λ ≈ 360 nm) for 8 h in a
commercial UV nail lamp system. The resulting polymer was purified
via dissolution in a minimal amount of toluene and precipitation in
cold methanol three times.
Poly(n-hexyl acrylate). Synthesized from 0.5 g (3.2 mmol) of n-
hexyl acrylate and 9.8 mg (0.050 mmol) of EBiB in 0.5 mL of the
solvent mixture. Yield = 0.36 g (72%). 1H NMR (599.8 MHz,
CDCl3): δ 4.01 (2H), 1.88−2.35 (2H), 1.45−1.61 (3H), 1.31 (6H),
0.90 (3H). SEC: Mn = 6400 g/mol, Đ = 1.08.
mmol) of cyclohexadecyl acrylate and 3.9 mg (0.020 mmol) of EBiB
in 0.5 mL of the solvent mixture. Yield = 0.42 g (84%). H NMR
(599.8 MHz, CDCl3): δ 4.82 (1H), 2.23−2.29 (1H), 1.55−1.89
(7H), 1.32 (27H). 13C{1H} NMR (150.8 MHz, CDCl3): δ 174.0,
73.9, 41.9, 41.4, 31.8, 27.2, 26.8, 26.7, 26.7 23.5, 23.4. SEC: Mn =
5800 g/mol, Đ = 1.21. FT-IR (ATR cm−1): 2924, 2856, 1728, 1461,
1348, 1256, 1161, 1034.
1
Poly(cycloheptadecyl acrylate). Synthesized from 0.5 g (1.6
mmol) cycloheptadecyl acrylate, and 9.8 mg (0.050 mmol) EBiB in
0.5 mL of the solvent mixture. Yield = 0.39 g (78%). 1H NMR (599.8
MHz, CDCl3): δ 4.78 (1H), 1.80−2.21 (1H), 1.11−1.60 (34H).
13C{1H} NMR (150.8 MHz, CDCl3): δ 173.8, 74.2, 41.9, 41.4, 32.5,
28.0, 27.7, 27.6, 27.3, 23.9. SEC: Mn = 3400 g/mol, Đ = 1.14. FT-IR
(ATR cm−1): 2922, 2855, 1728, 1459, 1348, 1253, 1160, 1033, 728.
Poly(n-octadecyl acrylate). Synthesized from 0.5 g (1.5 mmol)
of n-octadecyl acrylate and 9.7 mg (0.050 mmol) of EBiB in 0.5 mL of
1
the solvent mixture. Yield = 0.35 g (71%). H NMR (599.8 MHz,
CDCl3): δ 4.01 (2H), 2.27−2.34 (1H), 1.89−2.02 (1H), 1.60 (3H),
1.26−1.30 (30 H), 0.88 (t, 3H). SEC: Mn = 3500 g/mol, Đ = 1.12.
Poly(docosyl acrylate). Synthesized from 1.0 g (2.6 mmol) of
docosyl acrylate and 7.8 mg (0.040 mmol) of EBiB in 1.1 mL of the
solvent mixture. Yield = 0.79 g (79%). 1H NMR (599.8 MHz,
CDCl3): δ 4.00 (2H), 1.59 (3H), 1.25−1.30 (40 H), 0.88 (t, J = 7 Hz,
3H). SEC: Mn = 9800 g/mol, Đ = 1.13.
General Procedure for Diblock Copolymer Formation. A
stock solution of copper(II) bromide (4.2 mg, 0.019 mmol) and
Me6Tren (25.6 mg, 29.7 μL, 0.11 mmol) was prepared in 0.4 mL of
trifluoroethanol. In a 20 mL scintillation vial, cyclododecyl acrylate
(0.50 g, 2.09 mmol) was dissolved in a mixture of 0.4 mL of toluene
and 0.1 mL of the trifluoroethanol stock solution. EBiB (4.5 mg, 0.02
mmol) was then added, the vial was capped with a septum, and the
solution was degassed with argon for 5 min. With stirring, the
polymerization mixture was then irradiated with ≈360 nm light for 8
h in a commercial UV nail lamp system. A second vial, charged with
0.1 mL of the trifluoroethanol stock solution, 0.3 mL of
trifluoroethanol, 1.6 mL of toluene, and tert-butyl acrylate (2.32 g,
18.1 mmol), was then degassed with argon for 5 min, and the
contents were transferred by syringe to the polymerization vial. This
mixture was returned to the UV nail lamp and irradiated with ≈360
nm light with stirring for an additional 8 h. The resulting polymer was
purified via dissolution in a minimal amount of toluene and
precipitation in cold methanol three times.
RESULTS AND DISCUSSION
■
Poly(cyclohexyl acrylate). Synthesized from 1.0 g (6.5 mmol) of
cyclohexyl acrylate and 19.6 mg (0.10 mmol) of EBiB in 1.0 mL of the
solvent mixture. Yield = 0.81 g (81%). 1H NMR (599.8 MHz,
CDCl3): δ 4.69 (1H), 2.24−2.31 (1H), 1.23−1.82 (12H), 1.11 (d,
0.15 H). SEC: Mn = 7400 g/mol, Đ = 1.07.
Poly(lauryl acrylate). Synthesized from 2.0 g (8.3 mmol) of
lauryl acrylate and 39.0 mg (0.20 mmol) of EBiB in 2.0 mL of the
solvent mixture. Yield = 1.68 g (84%). 1H NMR (599.8 MHz,
CDCl3): δ 4.01 (2H), 2.19−2.32 (2H), 1.01−1.91 (21H), 0.88 (t,
3H). SEC: Mn = 6600 g/mol, Đ = 1.05.
To increase the range of well-defined hydrophobic polymers,
two structural libraries of monomers with side chains of varied
carbon number configured in a linear or macrocyclic form were
examined under photoinduced ATRP conditions. Hexyl
acrylate (L-6), lauryl acrylate (L-12), octadecyl acrylate (L-
18), and docosyl acrylate (L-22) with 6, 12, 18, and 22
carbons in the side chain, respectively, were chosen as linear
monomers, while cyclohexyl acrylate (C-6), cyclododecyl
acrylate (C-12), bis(cyclohexyl acrylate) (C-13), cyclohex-
adecyl acrylate (C-16), and cycloheptadecyl acrylate (C-17)
with 6−17 carbons in the side chain were selected as
macrocyclic analogues. To illustrate the synthetic utility
inherent in these novel macrocyclic side chains, readily
available macrocyclic ketones were selected as starting
materials, and highly efficient chemistry was developed to
convert these systems to the desired acrylate derivatives. For
example, C-17 was designed starting from civetone, a
commercially available fragrance additive. As shown in Scheme
Poly(cyclododecyl acrylate). Synthesized from 0.3 g (1.3 mmol)
of cyclododecyl acrylate and 5.6 mg (0.029 mmol) of EBiB in 0.3 mL
1
of the solvent mixture. Yield = 0.23 g (77%). H NMR (599.8 MHz,
CDCl3): δ 4.95 (1H), 2.25 (1H), 1.36−1.84 (24H). 13C{1H} NMR
(150.8 MHz, CDCl3): δ 174.1, 72.6, 41.5, 29.2, 24.5, 23.5, 21.1. SEC:
Mn = 4600 g/mol, Đ = 1.14. FT-IR (ATR cm−1): 2921, 2852, 1725,
1447, 1250, 1161, 1094, 907.
Poly(bis(cyclohexyl) acrylate). Synthesized from 0.7 g (2.8
mmol) of biscyclohexyl acrylate and 13.6 mg (0.070 mmol) of EBiB
1
in 0.7 mL of the solvent mixture. Yield = 0.60 g (85%). H NMR
C
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1, room temperature reduction and hydrogenation of civetone,
1, was achieved in a one-pot process to give the desired
Scheme 2. Polymerization Scheme for Hydrophobic Linear
and Cyclic Monomers, Including Hexyl (L-6), Lauryl (L-
12), Octadecyl (L-18), Docosyl (L-22), Cyclohexyl (C-6),
Cyclododecyl (C-12), Bis(cyclohexyl) (C-13),
Cyclohexadecyl (C-16), and Cycloheptadecyl Acrylate (C-
17)
Scheme 1. Synthesis of Cycloheptadecyl Acrylate, C-17,
Monomer from Civetone, 1
cycloheptadecyl alcohol, 2. As with the other commercially
available cyclic alcohols, such as cyclododecyl alcohol which is
a precursor for nylon-6-12, esterification with acryloyl chloride
leads to the acrylate derivative in excellent yields (Scheme 1
and Figures S1−S10). This synthetic pathway is amenable to
other similar starting materials with C-16 being synthesized in
the same manner. These macrocyclic isomers of traditional
long-chain acrylate monomers have similar hydrophobicity but
are expected to have significantly different crystallization
properties due to the inability of the side chains to
interdigitate.
As a simple and versatile polymerization protocol, our efforts
were directed to photoinduced ATRP14,17 with the initial
challenge being to identify polymerization conditions that
allow for a wide range of molecular weights to be synthesized
with good control and high end-group fidelity. A critical feature
is selection of a solvent system which solubilizes both the very
hydrophobic monomers and resulting polymers as well as the
hydrophilic copper/ligand catalyst (Cu(II)/Me6Tren). Tradi-
tional solvents like DMSO were not effective and in
accordance with previous studies led to poor molecular weight
control with dispersities of >2.0.30 Toluene was shown to have
increased utility with the monomers and resulting polymers
soluble; however, the copper/ligand catalyst was sparingly
soluble, leading to only a moderate decrease in dispersity. To
address these opposing solubility requirements, a range of
cosolvents with toluene such as isopropanol, methanol, and
DMSO were therefore investigated, with little or no control
being observed. In direct contrast, trifluoroethanol (TFE)48−50
showed promise as a cosolvent, with a 4:1 ratio of toluene:TFE
solubilizing all components of the reaction mixture. Signifi-
cantly, good control and high reproducibility for the
polymerization of all hydrophobic monomers were achieved
with a CuBr2 and Me6Tren catalytic system using ethyl α-
bromoisobutyrate (EBiB) as the initiator (Scheme 2).
The controlled photopolymerization of hydrophobic mono-
mers was initially studied with the library of linear, long-chain
C6, C12, C18, and C22 derivatives: L-6, L-12, L-18, and L-22.
Significantly, polymerization kinetics for lauryl acrylate at a low
theoretical DP of 20 showed a linear increase of ln[M]0/[M]t
with time, confirming the controlled nature of the process in
4:1 toluene/TFE (see Figure S13). Building from this
observation, a major requirement for these systems is to
maintain solubility of the growing polymer chains with
increasing molecular weight. It was therefore important to
show that polymers with a wide range of molar masses could
be synthesized with a high degree of control. To illustrate this,
a series of poly(lauryl acrylate) polymers were synthesized,
ranging in theoretical molar masses from 2000 to 100000 g/
mol. For low molecular weight polymers, good agreement is
observed between the targeted and observed degree of
polymerization by H NMR spectroscopy (see Table 1). At
1
higher molecular weights, end-group analysis by NMR
spectroscopy is not reliable; however, SEC clearly indicates
well-defined increases in molecular weight, and symmetrical,
low-dispersity peaks are evident (see Figure 3). The disparity
between molecular weight measured by SEC and targeted
molecular weight is attributed to the inherent hydrodynamic
differences of the linear polystyrene calibrants and our polymer
samples. In each case, good molecular weight control was
maintained, with all dispersities ≤1.11 (see Figure 3 and Table
1, entries 3−8). This is in contrast to other reported conditions
for the polymerization of lauryl acrylate, where degrees of
polymerization over 50 (Mn ≈ 12000) could not be targeted,
and at lower DPs, increased dispersity was observed.51
Extension of these conditions to other linear side-chain
monomers proved successful with homopolymers of all
monomers reaching >98% conversion within 8 h, with low
dispersities (see Table 1 and Figures S14−S19). Matrix-
assisted laser-desorption ionization (MALDI) was utilized to
examine the end-group fidelity for these different hydrophobic
polymers. With all linear side-chain systems, high end-group
fidelity was maintained, with the bromine chain end clearly
observed in molecular ion peaks, suggesting minimal
termination or side reactions. Additionally, the m/z for each
system confirmed the presence of the ethyl isobutyrate
initiating group on the α-chain end (see Figures S20−S22).
This high degree of end-group fidelity and compatibility
with different side-chain lengths prompted an examination of
chain extension and formation of in-situ block copolymers
(Table S1). tert-Butyl acrylate was selected as a functional
comonomer, which after deprotection would lead to novel
diblock copolymers composed of disparate hydrophobic and
hydrophilic blocks. From an initial short block of poly(lauryl
acrylate) (DP 9, Mn(NMR) = 2400 g/mol, 99% conversion),
tert-butyl acrylate was added directly into the reaction mixture
with additional catalyst and solvent; the resulting polymer
obtained an overall molar mass of ≈12000 g/mol (NMR) and
a degree of polymerization for the tert-butyl acrylate block of
≈80, in agreement with expected block lengths. The SEC trace
(Figure 4, top) shows efficient chain extension, with the final
D
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Table 1. Homopolymers of Hydrophobic Monomers Synthesized by Photoinduced ATRP (All Polymers Reached Conversions
of >98%)
bc
,
ac
,
a
c
b
polymer
Mn(SEC)
Mn(NMR)
DP(NMR)
13
DP(theor)
Mn (theor)
Đ(SEC)
poly(L-6)
1600
6400
2000
4000
6600
12600
29400
50700
2300
4900
11400
2200
13
64
9
21
42
104
250
417
6
2200
10200
2400
1.14
1.08
1.11
1.07
1.05
1.07
1.08
1.07
1.09
1.08
1.09
1.13
poly(L-12)
2200
5000
8
20
5200
10300
25200
60300
100400
2100
10300
25200
25300
poly(L-18)
poly(L-22)
2000
6
31
77
66
9800
a
Due to the overlap between initiator signals from the initiator and the side-chain in the 1H NMR spectrum, these values could only be calculated
b
c
reliably for low molar mass polymers. Relative to polystyrene standards. In units of g/mol.
Figure 3. SEC traces of poly(lauryl acrylate) homopolymers
synthesized with a range of molecular weights. Theoretical DP values
are 9 (red), 21 (orange), 42 (yellow), 104 (green), 250 (blue), and
417 (purple).
diblock copolymer having a low dispersity of 1.06. To prove
that the scope of this system extends to polymers of higher
molar mass, in-situ chain extension was also attempted from
poly(octadecyl acrylate) (Mn(SEC) = 6100 g/mol). Character-
ization by SEC (Figure 4, bottom) again shows efficient chain
extension with low dispersity well-defined diblock copolymers
being obtained by this method (Mn(SEC) = 10600 g/mol, Đ =
1.13). These examples clearly illustrate the potential of
photoinduced ATRP under these optimized polymerization
conditions for in-situ block copolymerization using very
hydrophobic building blocks.
Figure 4. Diblock copolymers of poly(lauryl acrylate-block-tert-butyl
acrylate), n ≈ 8, m ≈ 80 (top) and poly(octadecyl acrylate-block-tert-
butyl acrylate), p ≈ 30, q ≈ 80 (bottom).
With the advances described above for long-chain linear
hydrophobic monomers, our attention was then directed to
macrocyclic isomers. It is noteworthy that this monomer class
is surprisingly understudied, despite the potential for markedly
different properties when compared to their linear analogues.46
In this work, the polymerization of a broad spectrum of cyclic
monomers C-6, C-12, C-13, C-16, and C-17 was examined.
Initial observations proved to warrant study with the
polymerization of bis(cyclohexyl) acrylate proceeding much
slower than linear analogues, often requiring 18 h to reach full
conversion, though good dispersities were still achieved, likely
due to the steric bulk of the monomer (Đ ≤ 1.11, Table 2 and
Figures S23, S24). In contrast, the rates of polymerization of
cycloheptadecyl acrylate, cyclohexadecyl acrylate, cyclododecyl
acrylate, and cyclohexyl acrylate are similar to that of lauryl
acrylate, with full conversion reached within 8 h and with good
dispersities (Table 2 and Figures S25−S30) and a linear
increase of ln[M]0/[M]t with time (Figure S31), using the
conditions described above (Figures S32−S39). As an
illustrative example, cyclododecyl acrylate shows a linear
evolution of molar mass with conversion. Aliquots were
taken from the reaction mixture every hour, and SEC analysis
of the crude reaction mixture shows a linear dependence of Mn
on conversion, with consistently low dispersity (Đ ≈1.10,
Figure 5).
In addition, SEC characterization displayed narrow peaks
over a wide range of molar masses (from 2000 to 100000 g/
mol, see Figure 6) with good correlation between experimental
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Table 2. Homopolymers of Hydrophobic Monomers Synthesized by Photoinduced ATRP (All Polymers Reached Conversions
of >98%)
b
a
a
c
b
polymer
Mn(SEC)
Mn(NMR)
DP(NMR)
12
DP(theor)
Mn (theor)
Đ(SEC)
poly(C-6)
1500
7400
1200
2600
4600
8000
18100
29500
850
2600
5800
930
3400
8000
2000
13
65
8
21
42
105
252
420
8
40
85
6
32
81
2200
10200
2100
1.13
1.07
1.13
1.14
1.14
1.09
1.09
1.10
1.09
1.11
1.18
1.13
1.14
1.15
poly(C-12)
2200
4400
8
18
5200
10200
25200
60300
100300
2200
10200
25200
2100
poly(C-13)
poly(C-16)
poly(C-17)
2100
7
10100
25300
a
1
Due to the overlap between initiator signals from the initiator and the side chain in the H NMR spectrum, these values could only reliably be
b
c
calculated for low molar mass polymers. Relative to polystyrene standards. In units of g/mol.
for all cyclic side-chain monomers corresponding to the
expected bromine end group and ethyl isobutyrate initiating
group (Figure 7, Figures S40 and S41). It should be noted that
for all samples, including linear side chains, MALDI spectra
collected at higher laser power (55%) revealed a small second
set of peaks corresponding to chain−chain coupling. These
peaks increased in intensity with increased laser power (see
Figure S42) and were observed in similar proportion at low
and high conversions, further reinforcing their artifact nature
(see Figure S43).
To initially monitor block copolymer formation, a short
block of cycloheptadecyl acrylate (DP(NMR) 6, Mn(NMR) ≈
2000 g/mol) was polymerized to >98% conversion and then
chain extended in situ with tert-butyl acrylate. The resulting
polymer had a dispersity of 1.18 by SEC and an overall
Mn(NMR) of 12600 g/mol, illustrating high fidelity for chain
extension and the ability to prepare block copolymers with
significantly different block sizes (DP = 6 and 82) (Figure 8,
top). In-situ chain extension to give blocks of comparable
molecular weight was then demonstrated from an initial
cyclododecyl acrylate block (DP ≈ 42) with chain extension
proceeding as expected (tert-butyl acrylate, DP ≈ 80), resulting
in a diblock copolymer with low dispersity and controlled
molecular weight (Figure 8, bottom). As shown in Figures
S44−S46, other block ratios and monomer units can be used
to prepare a wide range of hydrophobic block copolymers
using in-situ procedures.
Figure 5. Kinetic data for the homopolymerization of cyclododecyl
acrylate, C-12, showing a linear evolution of molar mass with
conversion and low dispersities.
(1H NMR analysis) and theoretical values for low DP samples
(Table 2, entries 3 and 4).
Further structural confirmation was obtained from MALDI-
TOF analysis which showed one major set of molecular ions
A major driver for these very hydrophobic systems is the
thermal properties of the macrocyclic isomers and comparison
with the long chain, linear materials. Initially, the homopol-
ymers of each monomer with a common molecular weight of
≈10000 g/mol were examined. As expected, a significant
difference in thermal properties between the linear, long-chain
derivatives and their macrocyclic analogues is evident. This is
exemplified in the differential scanning calorimetry thermo-
grams for polymers based on side chains containing 12 or 13
carbon atoms (L-12, C-12, and C-13, see Figures S47−S49).
Poly(lauryl acrylate), with a linear 12-carbon side chain,
exhibits a melt transition (Tm) at −5.0 °C due to the
interdigitation of the side chains. In contrast, poly-
(cyclododecyl acrylate) shows only a glass transition temper-
ature, Tg of −20 °C, with no observable melting transition
Figure 6. SEC traces of poly(cyclododecyl acrylate) homopolymers
synthesized over a range of molecular weights. Theoretical n values
are 8 (red), 21 (orange), 42 (yellow), 105 (green), 252 (blue), and
420 (purple).
F
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Figure 7. MALDI-ToF-MS analysis for (a) poly(C-12) and (b) poly(C-17). Magnified regions show the expected monomer spacing and Br
termination.
while poly(bis(cyclohexyl) acrylate) exhibited a significantly
increased glass transition of 28 °C. This difference in thermal
behavior is even more pronounced for longer alkyl side chains
with the linear 18-membered side chain of poly(octadecyl
acrylate), resulting in an increase in the melting transition to
47 °C and the 17-membered macrocyclic side chain of
poly(cycloheptadecyl acrylate) showing a significantly lower
glass transition of −25 °C. The variation in crystallization
behavior and glass transition temperature is fully in agreement
with tendency of the linear side chains to interdigitate and the
cyclic side chains to pack inefficiently. All thermal transitions of
known polymers are consistent with literature values (see
Table 3).52,53
This important difference between long-chain, linear and
macrocyclic acrylate monomers with the same or similar
number of carbon atoms was further illustrated by examining
Table 3. Thermal Properties of Polymers
no. of C’s in monomer side
Tm(lit.)
(°C)
polymer
chain
Tg(lit.) (°C)
poly(L-6)
6
6
−66 (−57)53
poly(C-6)
poly(L-12)
poly(C-12)
poly(C-13)
poly(C-16)
poly(C-17)
poly(L-18)
poly(L-22)
9 (19)53
12
12
13
16
17
18
22
−5 (−3)53
−20
28
−19
−25
Figure 8. Diblock copolymers of poly(cycloheptadecyl acrylate-block-
tert-butyl acrylate), n ≈ 6, m ≈ 78 (top) and poly(cyclododecyl
acrylate-block-tert-butyl acrylate), p ≈ 42, q ≈ 78 (bottom).
47 (41)52
65 (71)54
G
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the diblock copolymer consisting of a ∼ 1:1 molar ratio of
octadecyl acrylate (L-18) and cycloheptadecyl acrylate (C-17).
This copolymer was synthesized according to the procedure
described above, with in-situ growth of the octadecyl acrylate
block from poly(cycloheptadecyl acrylate), and the resulting
polymer was purified by precipitation, leading to a well-defined
copolymer, Mn ≈ 10000 and Đ = 1.28 (Figure S50).
Interestingly, the DSC thermogram showed an observable
Tm (≈40 °C) and Tg (at −16 °C), very similar temperatures to
those of the discrete homopolymers (see Figure 9). This
Figure 10. DSC thermograms collected for homopolymers of
poly(cyclohexadecyl acrylate) (bottom), poly(docosyl acrylate)
(top), and random copolymers of these two monomers in varying
ratios. Legend indicates mol % cyclohexadecyl acrylate in the
copolymer.
Figure 9. DSC thermograms collected for homopolymers of
poly(cycloheptadecyl acrylate) (red) and poly(octadecyl acrylate)
(black) as well as a diblock copolymer of these two monomers (blue).
suggests that microphase separation of the diblock copolymer
is occurring in the bulkdespite the very similar chemical
composition/hydrophobicitylikely promoted by the con-
formation and packing differences between the side chains. A
diblock copolymer with blocks of ∼25000 g/mol was targeted
to improve separation; however, high dispersity indicated loss
of end-group fidelity at high molecular weights.
also significantly decreased from 110.1 J/g57,58 to 61.2, 44.7,
and 37.7 J/g in the 25, 40, and 50 mol % poly(cyclohexadecyl
acrylate) copolymers, indicating reduced crystallinity. Similar
behavior was observed with random copolymers of octadecyl
acrylate (C18) and cyclododecyl acrylate (C12) (Figure S51)
with all copolymers showing high hydrophobicity and
solubility in hexanes.
To illustrate the tuning of thermal properties by modifying
the architecture of the side-chain alkyl group, materials based
on docosyl (C22) acrylate were examined because they show
promise in a variety of applications, but their high tendency to
crystallize limits full utility.55 To reduce this crystallinity,
docosyl acrylate is often copolymerized with noncrystalline
monomers, such as acrylic acid56 or methyl methacryate,55
with significant incorporation of comonomer often required.
For example, crystallinity was still observed in the copolymer
of docosyl acrylate and acrylic acid at 86 mol % acrylic acid,56
significantly decreasing the hydrophobicity and, in turn,
functionality as an oil absorbant. We rationalized that
copolymerization with macrocyclic hydrophobic monomers
should significantly decrease the crystallinity of the polymer
without reducing overall hydrophobicity. A series of random
copolymers were therefore synthesized based on docosyl (L-
22) acrylate and cyclohexadecyl (C-16) acrylate (specifically,
25, 40, 50, 60, 70, and 75 mol % cyclohexadecyl acrylate) with
the observed phase transitions compared to the respective
homopolymers (Figure 10). Significantly, the thermal tran-
sitions could be tuned from the highly crystalline poly(docosyl
acrylate) homopolymer (Tm = 67 °C) to the amorphous
poly(cyclohexadecyl acrylate) homopolymer (Tg = −19 °C),
with the sharp melting transition being significantly decreased
at only 40 mol % cyclohexadecyl acrylate and completely
suppressed above 70 mol %. The enthalpy of fusion (ΔHf) was
CONCLUSIONS
■
In conclusion, we describe the controlled radical polymer-
ization of hydrophobic, macrocyclic side-chain monomers
(C12, C16, and C17) using photoinduced ATRP. These
materials are obtained from readily available starting materials;
in particular, cycloheptadecyl acrylate is prepared from
civetone, found naturally in the musk oil of civet cats and
used widely in the fragrance industry. Our selected method of
photoinduced ATRP works equally well with both macrocyclic
and linear hydrocarbon side chains, exhibiting good control
over molar mass and dispersity. A high degree of chain-end
fidelity also allows for the efficient in-situ generation of diblock
copolymers. Significant differences in thermal properties were
observed between these analogous acrylate building blocks
with incorporation of the macrocyclic derivatives into random
copolymers, minimizing the crystallinity of poly(docosyl
acrylate) (C22) and poly(octadecyl acrylate) (C18) while
maintaining the high hydrophobicity of the polymer. The
synthetic advances reported in this study will allow new
hydrophobic materials to be prepared with tunable properties
based on the architecture of the side chains, leading to
enhanced application as viscosity modifiers and oil absorbants.
H
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Activation of Thiocarbonylthio Compounds for Controlled Polymer-
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b01509.
■
S
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NMR spectra, SEC chromatograms, MALDI spectra,
and DSC thermograms (PDF)
AUTHOR INFORMATION
Corresponding Author
*(C.J.H.) E-mail: hawker@mrl.ucsb.edu.
ORCID
■
(14) Anastasaki, A.; Nikolaou, V.; Zhang, Q.; Burns, J.; Samanta, S.
R.; Waldron, C.; Haddleton, A. J.; McHale, R.; Fox, D.; Percec, V.;
Wilson, P.; Haddleton, D. M. Copper(II)/Tertiary Amine Synergy in
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of ω-Functional and α,ω-Heterofunctional Poly(acrylates). J. Am.
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Nghia P. Truong: 0000-0001-9900-2644
Christopher M. Bates: 0000-0002-1598-794X
Craig J. Hawker: 0000-0001-9951-851X
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation,
Division of Materials Research under the Materials Research
Science & Engineering Centers Program (DMR 1720256
(IRG-3)). The research reported here made use of shared
facilities of the UCSB MRSEC (NSF DMR 1720256), a
member of the Materials Research Facilities Network. S.B. and
A.A. acknowledge the California NanoSystems Institute for an
Elings Prize Fellowship in Experimental Science. A.A. is
grateful to the European Union’s Horizon 2020 research and
innovation program for a Marie Curie Global Postdoctoral
Fellowship, and S.B. is grateful to the National Science and
Engineering Research Council of Canada for the Banting
Postdoctoral Fellowship. N.P.T. acknowledges the award of a
DECRA Fellowship from the Australian Research Council
(DE180100076). We also sincerely thank Dr. Rachel Behrens
for help with polymer characterization.
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DOI: 10.1021/acs.macromol.8b01509
Macromolecules XXXX, XXX, XXX−XXX