X. Zhou et al. / Polymer 59 (2015) 42e48
43
The incorporation of semi-rigid alternating copolymer dyads
into hypercrosslinked polymer particles offers potentially attractive
possibilities of reducing collapse of pores during the hyper-
crosslinking chemistry, while at the same time delivering specific
functionalities into the particles from carefully chosen substituents
on the alternating comonomer pairs. In this paper, we enchained Tg
enhancing comonomers, 4-methyl substituted stilbene or styrene
and N-substituted maleimide, into Davankov-type hyper-
crosslinked systems in order to increase the rigidity of the polymer
backbones. We report here the synthesis of hypercrosslinked
polymers containing various levels of semi-rigid N-substituted
maleimideesubstituted stilbene or styrene alternating sequences.
We investigated the effect of these semi-rigid alternating se-
quences on the polymer backbones using thermal analysis of pre-
cursor particles before the hypercrosslinking reactions. Surface area
and porosity of the hypercrosslinked polymers were explored and
correlated to the rigidity of polymer backbones. The results will
provide guidance for the synthesis of hypercrosslinked polymers
containing high levels of functional groups in the hypercrosslinked
polymers from the enchainment of specifically functionalized
alternating comonomer pairs.
Scheme 1. Synthesis of (E)-4-methyl stilbene.
methyltoluene(diethylphosphonate), 4-methylbenzyl chloride
(15.0 g, 107 mmol) and triethyl phosphite (70.9 g, 427 mmol) were
stirred in a 250 ml flask and heated to 160 ꢁC for 24 h. The remaining
triethyl phosphite was removed by vacuum distillation at 80 ꢁC
under 0.2 mmHg to afford a colorless liquid (27.1 g, 100%). 4-
Methyltoluene(diethylphosphonate) (27.1 g, 112 mmol) and benz-
aldehyde (11.9 g, 112 mmol) in dry THF (60 ml) were stirred in a
250 ml round bottom flask cooled in an ice bath. tBuOK (1.0 M inTHF,
120 ml) was then added drop wise. The solution formed was stirred
at roomtemperature for an additional 24h afterwhich it was poured
into water (500 ml). The product was precipitated from the solution,
filtered and washed with water and vacuum dried overnight to yield
(E)-4-methyl stilbene (white crystalline solid, 18.7 g, 86%). 1H NMR
2. Experimental section
2.1. Materials
(CDCl3, 500 MHz) d ppm: 7.52e7.08 (m,11H), 2.37 (s, 3H) (Figure S1).
4-Methylbenzyl chloride (Aldrich, 98%), triethyl phosphite
(Aldrich, 98%), benzaldehyde (Aldrich, ꢀ99%), tBuOK (Aldrich, 1.0 M
solution in THF), maleic anhydride (Aldrich, 99%), toluene (Fisher,
certified ACS), m-toluidine (Aldrich, 99%), acetic anhydride (Aldrich,
98%), sodium acetate (Aldrich, anhydrous), methanol (Fisher, HPLC
grade), sodium 1,2-dichloroethane (Aldrich, ꢀ99%), FeCl3 (Aldrich,
97%), poly(vinyl alcohol) (Aldrich, 87e89% hydrolyzed), sodium
chloride (Fisher, certified ACS crystalline) were used as purchased.
AIBN was recrystallized from methanol.
Melting point: 117e118 ꢁC (lit [30]: 118e120 ꢁC).
2.4. N-(3-Methylphenyl)maleimide (3MPMI)
N-(3-Methylphenyl)maleimide was synthesized from a two-step
reaction (Scheme 2). The second step of the reaction used a modified
literature method [31]. To synthesize N-(3-methylphenyl)maleamic
acid, maleic anhydride (10.0 g, 102 mmol) and toluene (8 ml) were
stirring in a 100 ml flask. m-Toluidine (11.1 g, 102 mmol) was added
drop wise. During the addition,15 ml toluene was added to maintain
a good slurry consistency, after which the mixture was refluxed for
15 min. The product precipitated from solution was filtered warm
and vacuum dried for 24 h to afford a light yellow solid (19.63 g,
2.2. Instrumental characterization
Proton nuclear magnetic resonance (1H NMR) was utilized to
confirm the chemical composition of the monomers and non-
crosslinked polymers. CDCl3-d and DMSOd6 were used as the sol-
vents. 1H NMR was obtained on Varian Utility 400 MHz or JOEL
EclipsePlus 500 MHz spectrometers at 25 ꢁC. Infrared analysis,
using a Midac FTIR spectrometer, was used to confirm the incor-
poration of N-substituted maleimide. Thermogravimetric Analysis
(TGA) was employed to study the thermal stability of polymer
precursors and hypercrosslinked polymers using TA instrument
model Q5000. Samples were heated from 30 to 600 ꢁC at a heating
rate of 10 ꢁC/min under nitrogen. Thermal transitions such as glass
transition temperatures (Tgs) were obtained by Differential Scan-
ning Calorimetry (DSC) using a TA instrument model Q1000.
Samples were measured using heat/cool/heat method at heating
and cooling rate of 20 ꢁC under nitrogen. SEM micrographs were
obtained using the LEO (Zeiss) 1550 field emission scanning elec-
tron microscope. The surface areas were calculated using the
Brunauer-Emmett-Teller (BET) equation and data from nitrogen
adsorption/desorption isotherms at 77 K obtained with a TriStar II
3020 surface area analyzer (Micromeritics, Norcross, GA). Prior to
analysis, the sample were degassed at 140 ꢁC overnight.
94%). 1H NMR (DMSO-d6, 400 MHz)
d ppm: 10.27 (s, 1H), 7.43e6.87
(m, 4H), 6.42 (d, 1H), 6.26 (d, 1H), 2.23 (s, 3H). A mixture of N-(3-
methylphenyl)maleamic acid (17.6 g, 85.9 mmol), acetic anhydride
(43.8 g, 429 mmol) and anhydrous sodium acetate (3.52 g,
42.9 mmol) was heated at 100 ꢁC for 6 h. The reaction mixture was
cooled and poured in ice water. The crude product was then washed
with ice water and vacuum distilled (62 ꢁC, 0.2 mmHg) to afford N-
(3-methylphenyl)maleimide (yellow liquid, 15.1 g, 94%). The crude
product was purified bycolumn chromatography (silica gel, hexane-
ethyl acetate ¼ 5:1). The first band afforded pure product as a yellow
liquid (6.19, 41%) (lit [32] 78%) 1H NMR (CDCl3, 500 MHz)
d ppm: 7.36
(m, 1H), 7.22e7.10 (m, 5H), 2.34 (t, 3H) (Figure S2).
2.5. Hypercrosslinked copolymers
For clarity, we named the polymers in a number order (Scheme
3). The lightly crosslinked polymer precursors with only styrene as
2.3. (E)-4-Methyl stilbene (4MSTB)
(E)-4-Methyl stilbene was synthesized via the WittigeHorner
reaction using 4-methyltoluene(diethylphosphonate) and benzal-
dehyde
(Scheme
1)
[24].
To
synthesize
4-
Scheme 2. Synthesis of N-(3-methylphenyl)maleimide.