Macromolecules
Article
Polymer samples were withdrawn from the reaction mixtures, diluted
with THF, filtered using Acrodisc 0.2 μm PTFE filters, and injected
into the SEC system without any further purification. Prior to spectral
analysis, the β-hydroxy ether-containing polymers were carefully
purified of unreacted alcohol and catalyst either by dialysis against
acetone using Spectra/Por dialysis membrane with MWCO = 2000 Da
(wet in 0.05% aqueous NaN3) or by precipitation in either hexane or
diethyl ether. Infrared (IR) spectra of thin films of the polymers cast
on NaCl plates from chloroform solutions (ca. 20 mg/mL) were
collected on a Thermo Scientific Nicolet iS10 FT-IR spectrometer.
Synthetic Procedures. Synthesis of 4-Vinylphenol. 4-Acetoxy-
styrene (10.00 g, 60.12 mmol) was diluted with THF (100 mL) in a
round-bottom flask equipped with a magnetic stir bar and then chilled
in an ice bath. NaOH (6.02 g, 150 mmol, 2.5 equiv) was dissolved in
30 mL of water and then added dropwise over 5 min to the vigorously
stirred solution of 4-acetoxystyrene. After 4 h, 100 mL of 1.5 M HCl
chilled in an ice bath was added dropwise over 15 min to the cold,
yellow reaction mixture, which was then further diluted with 200 mL
of cold water. The mixture was extracted with diethyl ether (2 × 200
mL) using a separatory funnel. The organic phase was collected and
dried with MgSO4, and the majority of the solvent was removed under
vacuum by rotary evaporation at 25 °C. Ethanol (100 mL, anhydrous)
was then added to the product solution, and the majority of the
solvent was again evaporated to remove all remaining THF and acetic
acid. Note: some ethanol (ca. 10 mL) is allowed to remain to prevent
self-initiated cationic polymerization of the monomer.26 The yield, as
determined by NMR spectroscopy, was quantitative (7.22 g), and the
product was immediately used in the next step without any further
(4VPGE) is another interesting epoxide-containing styrenic
monomer that has been neglected owing to the lack of easy and
efficient synthetic procedures. Unlike 4VPO, the epoxide
moiety of 4VPGE is not stabilized by an adjacent aryl ring
and is known to be more susceptible to ring-opening reactions
with nucleophiles under basic or neutral conditions and less
reactive under acidic conditions.21,22 Given the similarity in
structure, and therefore reactivity, of 4VPGE to GMA and
especially bisphenol A diglycidyl ether, two epoxy−resin
precursors with well-understood chemistries that are already
widely used on an industrial scale, this glycidyl-containing
monomer may be more valuable than 4VPO as a non-
degradable substitute in some cases.
Herein, it is demonstrated that 4VPGE can be synthesized
from 4-acetoxystyrene using a two-step method in much higher
yield than has been previously reported in a one-step
procedure.23,24 To the best of our knowledge, this reactive
monomer has not been polymerized under CRP conditions,
and we report on the utility of RAFT polymerization to prepare
well-defined homopolymers and block copolymers derived
from 4VPGE. Additionally, it is shown that poly4VPGE, despite
being less reactive than poly4VPO toward nucleophiles under
acidic conditions, can also be efficiently modified with alcohols
using BF3 as a Lewis acid catalyst and is in some instances
actually superior to poly4VPO as a precursor to well-defined β-
hydroxy ether-functionalized polymers.
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purification or storage. H NMR (500 MHz, DMSO-d6, δ [ppm]):
9.52 (s, 1H), 7.31−7.25 (m, 2H), 6.76−6.70 (m, 2H), 6.61 (dd, J =
17.7, 10.9 Hz, 1H), 5.58 (dd, J = 17.7, 0.9 Hz, 1H), 5.04 (dd, J = 10.9,
1.0 Hz, 1H).
EXPERIMENTAL SECTION
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Materials. The chain transfer agent (CTA), butyl 1-phenylethyl
trithiocarbonate (BPT), was synthesized according to the literature.25
Styrene (99%, stabilized with 10−15 ppm 4-tert-butylcatechol, Alfa
Aesar) was purified by passing the neat liquid through a short column
filled with basic alumina (Brockmann grade I, 58 Å, Alfa Aesar). All
other reagents and solvents were used as received: 4-acetoxystyrene
(96%, stabilized with hydroquinone monomethyl ether, Aldrich),
NaOH (95+%, pellets, Fisher), aqueous HCl (11.6 M, Fisher),
epichlorohydrin (99%, ACROS), hydroquinone (99%, Alfa Aesar),
MgSO4 (97+%, anhydrous powder, Fisher), azobiscyclohexane-
carbonitrile (VAZO88, 98%, Aldrich), phenyl glycidyl ether (PGE,
99%, Aldrich), allyl alcohol (98+%, Alfa Aesar), phenol (99+%,
Fisher), propargyl alcohol (99%, ACROS), 1-butanol (BuOH, 99.4+%,
Baker), cyclohexanol (99%, Alfa Aesar), benzyl alcohol (99.8%,
Aldrich), tert-butanol (t-BuOH, 99+%, Alfa Aesar), L-(−)-menthol
(99.5%, Acros), 4-nitrobenzyl alcohol (NBA, 99%, Acros), BF3−Et2O
(98+%, Alfa Aesar), anisole (99%, ACROS), acetone (99+%, Fisher),
CH2Cl2 (99+%, Fisher), diethyl ether (99+%, Fisher), ethanol (99+%,
anhydrous, Koptec), hexanes (99%, Fisher), and tetrahydrofuran
(THF; 99%, VWR). The deuterated solvents, DMSO-d6 (99.9% D),
CD2Cl2 (99.9%), and CDCl3 (99.8% D), were purchased from
Cambridge Isotope Laboratories; either the solvent peak or a small
amount of added tetramethylsilane (TMS) was used as a chemical shift
reference.
Synthesis of 4VPGE. The solution of 4-vinylphenol (7.22 g, 60.1
mmol) in ethanol (ca. 10 mL) from the previous step was further
diluted with anhydrous ethanol (50 mL) in a round-bottom flask
equipped with a magnetic stir bar, and NaOH (3.12 g, 78.0 mmol, 1.3
equiv) was added. The mixture was stirred for 30 min to completely
dissolve the NaOH, and then epichlorohydrin (14.1 mL, 180 mmol, 3
equiv) was rapidly added to the mixture using a dropping funnel. NaCl
slowly precipitated, and the mixture became cloudy. After 12 h, water
(100 mL) was added, and the mixture was extracted with hexane (2 ×
100 mL) using a separatory funnel. The organic phase was dried with
MgSO4, and the solvent was removed under vacuum by rotary
evaporation at 35 °C to yield a crude yellow oil (10.71 g), which was
composed of 4VPGE, unreacted epichlorohydrin, and an unidentified
impurity. Hydroquinone (10 mg) was added to the crude product, and
high-vacuum (0.015 Torr (2 Pa)) distillation at 83 °C rendered
4VPGE as a yellow-tinged oil with a small amount of impurity.
Alternatively, the majority of the relatively polar impurities could be
separated from the product by flash chromatography using hexane/
methylene chloride as an eluent. All traces of impurities were removed
by crystallization of 4VPGE in hexane (200 mL) at −20 °C. Final
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product yield was 5.730 g (32.52 mmol, 54%). H NMR (400 MHz,
CDCl3, δ [ppm]): 7.38−7.30 (m, 2H), 6.92−6.83 (m, 2H), 6.66 (dd, J
= 17.6, 10.9 Hz, 1H), 5.62 (d, J = 17.6 Hz, 1H), 5.14 (d, J = 10.9 Hz,
1H), 4.23 (dd, J = 11.0, 3.1 Hz, 1H), 3.95 (dd, J = 11.0, 5.7 Hz, 1H),
3.36 (ddt, J = 5.8, 4.1, 2.9 Hz, 1H), 2.91 (t, J = 4.5 Hz, 1H), 2.76 (dd, J
Analyses. The structures of all synthetic intermediates, 4VPGE,
model compounds, and β-hydroxy ether-functionalized polymers were
1
confirmed by H NMR spectroscopy on either a Bruker Avance DRX
RAFT Polymerization of 4VPGE. In the following procedure, the
targeted degree of polymerization at complete conversion of monomer
(DPn,targ = [4VPGE]0/[CTA]0) was 200 with 30 mol % radical
initiator vs the CTA. 4VPGE (0.500 g, 2.84 mmol) was weighed out in
a 5 mL glass vial equipped with a stir bar, and dry anisole (0.3 mL) was
added. A 10× stock solution of CTA and radical initiator was prepared
by dissolving BPT (38.4 mg, 0.142 mmol) and VAZO88 (10.4 mg,
0.0426 mmol) in anisole (2.0 mL), and a 1/10 portion of the stock
solution (0.2 mL) was added to the reaction vial. The vial was capped
with a rubber septum and sealed with electrical tape. The reaction
mixture was sparged for 30 min with a steady flow of nitrogen and
then immersed in an oil bath at 90 °C. Samples (ca. 0.2 mL) were
400 or JEOL ECA-500 spectrometer operating at 400 or 500 MHz,
respectively, using samples diluted in DMSO-d6, CD2Cl2, or CDCl3.
Monomer conversions were determined by periodically removing
samples from the polymerization reaction mixtures and diluting them
with DMSO-d6 or CDCl3 for NMR analysis. Apparent number-average
molecular weights (Mn) and molecular weight distribution (MWD)
dispersities (Đ = Mw/Mn) of the polymer samples were determined by
size exclusion chromatography (SEC) on a Tosoh EcoSEC HLC-8320
system equipped with a series of four columns (TSK gel guard Super
HZ-L, Super HZM-M, Super HZM-N and Super HZ2000) using THF
as the eluent with a flow rate of 0.35 mL min−1 at 40 °C. The SEC
calibration was based on linear narrow-MWD polystyrene standards.
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Macromolecules XXXX, XXX, XXX−XXX