Penultimate effects in the copolymerization of maleic anhydride with electron-rich styrene derivatives

Article abstract of DOI:10.1002/pola.28178

The (controlled) free-radical copolymerization of maleic anhydride and styrene or derivatives thereof is often thought to provide nearly perfect alternating copolymers. Here, the RAFT copolymerization of electron-rich styrene derivatives with maleic anhydride is reported. This copolymerization shows distinct penultimate effects, resulting in polymers with increased incorporation of styrene monomers, that is, where a tendency toward periodic (S-S-MA) copolymers exists. This work could be a first step towards periodic copolymers based on maleic anhydride and styrene derivatives.

Full text of DOI:10.1002/pola.28178

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Penultimate Effects in the Copolymerization of Maleic Anhydride with
Electron-Rich Styrene Derivatives
€
Niels ten Brummelhuis, Marvin Bjorn Stutz
€
Department of Chemistry, Humboldt-Universitat zu Berlin, Brook-Taylor-Str. 2, Berlin, 12489, Germany
Correspondence to: N. ten Brummelhuis; (E-mail: niels.ten.brummelhuis@hu-berlin.de)
Received 7 March 2016; accepted 21 May 2016; published online 00 Month 2016
DOI: 10.1002/pola.28178
ABSTRACT: The (controlled) free-radical copolymerization of
maleic anhydride and styrene or derivatives thereof is often
thought to provide nearly perfect alternating copolymers. Here,
the RAFT copolymerization of electron-rich styrene derivatives
with maleic anhydride is reported. This copolymerization
shows distinct penultimate effects, resulting in polymers with
increased incorporation of styrene monomers, that is, where a
tendency toward periodic (S-S-MA) copolymers exists. This
work could be a first step towards periodic copolymers based
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on maleic anhydride and styrene derivatives.
2016 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 00,
000–000
KEYWORDS: copolymerization; microstructure; reversible addi-
tion fragmentation chain-transfer (RAFT)
vided by the copolymerization of maleimides with bulky
INTRODUCTION The microstructure, that is, the tacticity and
monomer sequence, of polymers strongly influences their
properties. Highly iso- or syndiotactic polymers, for example,
form much more crystalline materials than do atactic poly-
mers.1 Monomer sequence is among others known to influ-
ence the glass-transition temperature, crystallinity, pKa and
pKb of polyions and the luminescent properties of copoly-
mers.2 The differences in the properties of periodic versus
random copolymers is mainly due to the alignment and dif-
ferences in the spacing of functional groups. These effects
are found for very simple periodic copolymers, primarily for
alternating copolymers, and it can reasonably be expected
that more dramatic changes in material properties can be
achieved if more complex monomer sequences are intro-
duced, a fact that is supported by the properties of many
biopolymers.3–8
nonconjugated olefins (e.g., b-pinene or D-limonene).14–17
In this work, we focus on the copolymerization of maleic
anhydride (MA) with styrene (S) derivatives, which is often
thought to yield nearly perfectly alternating copolymers. The
copolymer of styrene and maleic anhydride, a.k.a. SMA, or
close derivatives thereof, are widely used as thermoplasts, as
dispersing agents, in adhesives and many other applications.
The free-radical copolymerization of MA and S or derivatives
of these monomers proceeds in an alternating fashion
because of the electron-deficient and -rich nature of the vinyl
groups in the monomers respectively. This leads to favorable
energy levels of the reacting species as well as to the forma-
tion of charge–transfer complexes. The degree of alternation
is additionally enhanced by the inability of MA to homopoly-
merize under normal conditions.18
Whereas alternating copolymers are relatively common-
place,9–11 the creation of synthetic polymers, especially
through chain-growth polymerization methods, with complex
monomer sequences is currently highly challenging. A num-
ber of methods have been developed for the creation of
somewhat more complex periodic copolymers though, such
as the (regioselective) ring-opening metathesis polymeriza-
tion of substituted cyclooctene monomers12 and the cyclopo-
lymerization of trifunctional monomers,13 but these methods
still rely on the tedious synthesis of suitable monomers. A
more modular path towards ABA-periodic copolymers is pro-
EXPERIMENTAL
Materials
4-Vinylaniline (Fluorochem, 97%), 4-cyano-4-(phenylcarbono-
thioylthio)pentanoic acid (ABCR, 97%), 4-acetoxystyrene
(ABCR, 98%), di-tert-butyl dicarbonate (Iris Biotec, 99.2%),
acetic anhydride (Roth, 99%), hexanoyl chloride (Fluka, 98%),
methyl iodide (Sigma Aldrich, 99.5%), potassium hydroxide
(Fluka, 85%), sodium acetate (Acros Organics, 99%), sodium
chloride (Baker, 99.5%), sodium bicarbonate (Roth, 99%),
Additional Supporting Information may be found in the online version of this article.
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pivaloyl chloride (Alfa Aesar, 98%), hydrochloric acid (Baker,
37–38%), and triethylamine (ABCR, 99%) were used as
received.
UV-Vis-Spectroscopy
UV-Vis measurements were performed using an Eon micro-
plate spectrophotometer from BioTek (Bad Friedrichshall,
Germany) with Polystyrene 96 microwell plates with flat
bottoms from Greiner Bio-One GmbH (Frickenhausen,
Germany).
Azobisisobutyronitrile (AIBN) was recrystallized from metha-
nol before use. Maleic anhydride was sublimated at 70 8C
under reduced pressure. N,N-Dimethylformamide (DMF,
VWR, 99.9%) was used as received. Dry THF was obtained
from a Pure Micro Solv, PS-Micro solvent purification system
(Innovative Technology, Oldham, United Kingdom). 1,4-Diox-
ane (Ferac, 99%) was passed through an Allox plug to
remove the radical inhibitor. All other organic solvents were
distilled before use.
UPLC-ESI-MS
UPLC-ESI-MS measurements were performed on an Acquity-
UPLC H-Class CM Core system with a C18 reversed-phase
column (Acquity UPLC BEH C18 1.7 mm, 2.1 mm 3 50 mm)
at 40 8C, an Acquity-UPLC PDA detector and an Acquity-
UPLC QDA detector from Waters (Eschborn, Germany). The
measurements took place over the course of 4 min with an
acetonitrile-water gradient with an increasing water content
of 30 to 90 vol %.
Characterization
NMR Spectroscopy
NMR spectra were recorded on a Bruker Avance III-500
1
Spectrometer (Rheinstetten, Germany). H-NMR spectra were
Elemental Analysis
Elemental analysis was performed on a CHNS 932 machine
from LECO (St. Joseph).
recorded at 500 MHz while 13C-NMR spectra were measured
1
at 126 MHz. Additional H-NMR spectra were measured on a
Bruker Avance III-300 Spectrometer (Rheinstetten, Germany)
at 300 MHz.
Monomer Synthesis
4-Methoxystyrene (MOS)
All chemical shifts (d) were measured relative to proton
residual signals of the deuterated solvents [CDCl3 (Roth,
99.8%), CD3OD (Deutero, 99.8%) or DMSO-d6 (Deutero,
99.8%)]. Assignment of the peaks is shown in the Supporting
Information.
4-Acetoxystyrene (7.0 ml, 45.5 mmol, 1.0 eq.) and potassium
hydroxide (7.688 g, 137.0 mmol, 3.0 eq.) were dispersed in
37 ml of water and were stirred for 3 h. A solution of methyl
iodide (4.25 ml, 68.3 mmol, 1.5 eq.) in 90 ml of acetone was
added dropwise over the period of an hour while the tem-
perature was kept constant at rt using a water bath. The
resulting yellow suspension was stirred at rt for 2 days after
which the volatiles were removed in vacuo. The remaining
aqueous phase was extracted four times with chloroform,
the combined organic phases were filtered over cotton wool
and the solvent was removed in vacuo. The crude product
was purified by column chromatography using 100:6 vol/vol
cyclohexane: ethyl acetate. The product was obtained as a
colorless oil (5.856 g, 43.7 mmol, 96% yield).
Size Exclusion Chromatography
SEC measurements using THF as the eluent were performed
on a WEG Dr. Bures system (WEG Dr. Bures GmbH & Co. KG,
€
Dallgow-Doberitz, Germany) equipped with a UV-detector
(Knauer UV 2500) operating at k 5 255 nm. Measurements
were performed at 60 8C with a flow rate of 1 ml/min using
PS columns (50–1000 Å).
SEC measurements in N,N-dimethylacetamide (DMAc) were
performed on
a Tosoh EcoSEC HLC-8320GPC system
1H-NMR (500 MHz, CDCl3, d, ppm): 7.37–7.34 (m, 2 H),
6.89–6.85 (m, 2H), 6.66 (dd, 1H), 5.62 (d, 1 H), 5.13 (d, 1H),
3.81 (s, 3 H).
equipped with UV- and RI-detectors. RI data is reported.
Measurements were performed at 50 8C with a flow rate of
1 ml/min using PS columns (30–1000 Å).
Commercial polystyrene standards were used for calibration.
N-(4-Vinylphenyl)Acetamide (VAAc)
VAAc was synthesized according to a slightly adapted litera-
ture procedure19
:
MALDI-TOF-MS
MALDI-TOF MS measurements were performed on an Auto-
flex III Smartbeam system from Bruker (Billerica). A solution
containing 7 mg of a-cyano-4-hydroxycinnamic acid or 7 mg
4-Vinylaniline (3.4 ml, 29.0 mmol, 1 eq.) was dissolved in a
2 M aqueous solution of hydrochloric acid (32 ml) under
argon and cooled to 0 8C. A solution of 37 g of sodium ace-
tate in 116 ml water was added as well as acetic anhydride
(37 ml, 390 mmol, 13.4 eq.). The mixture was allowed to
slowly heat to rt and was stirred for 62 h. The slightly yel-
low precipitate was collected by filtration and washed with
distilled water. The solid was redissolved in 400 ml of ethyl
acetate and filtered through a silica plug. The solvent was
removed in vacuo, yielding the product as a slightly yellow
solid. (3.905 g, 24.2 mmol, 83% yield).
of 2,5-dihydroxybenzoic acid in
1 ml of a 50:50:0.1
acetonitrile-water-trifluoroacetic acid solution was used as a
matrix solution. 2 mg of the polymers were dissolved in
1 ml of a 50:50:0.1 acetonitrile-water-trifluoroacetic acid
solution. 1 ml of the polymer solution was mixed with 1.5 ml
of the matrix solution on the substrate. 1 ml of this solution
was taken and diluted with another 1.5 ml of the matrix solu-
tion. The samples were dried at room temperature. The
measurements were performed in positive mode.
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1H-NMR (300 MHz, CDCl3, d, ppm): 7.48–7.35 (m, 4 H), 7.20
(s, 1 H), 6.67 (dd, 1 H), 5.67 (d, 1 H), 5.19 (d, 1 H), 2.18 (s,
3 H).
organic phase was washed twice with a 10 wt % sodium
bicarbonate solution and subsequently with distilled water.
The combined aqueous solutions were extracted three times
with dichloromethane after which the organic phases were
combined and filtered over cotton wool. The solvent was
removed in vacuo and the product was further purified by
column chromatography using 100:12 vol/vol cyclohexane:
ethyl acetate as eluent. The product was obtained as a white
solid (0.682 g, 3.11 mmol, 82% yield).
N-(4-Vinylphenyl)Hexanamide (VAHex)
5.0 ml of 4-vinylaniline (42.7 mmol, 1.0 eq.) was added to a
solution of 9.8 ml of triethylamine (70.5 mmol, 1.5 eq.) in
100 ml ethyl acetate under argon at 0 8C. Hexanoyl chloride
(7.7 ml, 59.4 mmol, 1.4 eq.) was added dropwise. After com-
plete addition the solution was diluted with an additional
30 ml of ethyl acetate and stirred at rt for 24 h. The solution
was washed three times with a 2:1 mixture of brine and 1 M
sodium hydroxide solutions. The organic phase was filtered
over cotton wool and the solvent is removed in vacuo to
yield the product as a white solid (8.108 g, 37.3 mmol,
88%).
1H-NMR (500 MHz, CDCl3, d, ppm): 7.35–7.31 (m, 4 H), 6.65
(dd, 1 H), 6.48 (s, 1 H), 5.65 (d, 1 H), 5.16 (d, 1 H), 1.52 (s,
9 H).
Polymer Synthesis
Maleic anhydride and the styrene derivative (total ꢀ40 eq.)
were dissolved in stock solutions of the chain-transfer agent
(4-cyano-4-(phenylcarbonothioylthio)pentanoic acid, 1 eq.)
and AIBN (0.1 eq.) and some extra solvent (1,4-dioxane or
DMF) according to tables shown in the Supporting Informa-
1H-NMR (500 MHz, CD3OD, d, ppm): 7.52 (d, 2 H), 7.37 (d,
2 H), 6.69 (dd, 1 H), 5.70 (d, 1 H), 5.15 (d, 1 H), 2.36 (t,
2 H), 1.70 (m, 2 H), 1.25-1.4 (m, 4 H), 0.94 (t, 3 H). 13C-APT-
NMR (125 MHz, CD3OD, d, ppm): 174.7 (C, 1 C), 139.6 (C,
1 C), 137.6 (CH, 1 C), 134.9 (C, 1 C), 127.6 (CH, 2 C), 121.1
(CH, 2 C), 113.0 (CH2, 1 C), 38.0 (CH2, 1 C), 32.6 (CH2, 1 C),
26.6 (CH2, 1 C), 23.5 (CH2, 1 C), 14.3 (CH3, 1 C). ESI-MS
(m/z): calcd. for C14H20NO1, 218.154, found, 218.22
[M 1 H]1. Elemental analysis calcd. for C14H19NO: C, 77.38;
H, 8.81; N, 6.45. Found C, 77.357; H, 9.107; N, 6.421.
1
tion. A sample was taken and characterized by H-NMR spec-
troscopy to determine the exact ratio of monomers. The
solutions were degassed by three freeze-pump-thaw cycles
after which the flasks were backfilled with argon and heated
to 65 8C.
Conversion of the monomers was monitored by drawing ali-
1
quots at regular intervals and measuring H-NMR spectra for
these samples. Spectra were recorded diluted in CDCl3 or
CD3OD. One of the signals where the monomer and polymer
signals overlap (a signal from the group in para-position to
the vinyl group of the styrene derivative) was used as an
internal standard by which the decrease in signal intensity
of the vinyl signals of MA and the styrene derivative can be
determined which is assumed to be due solely to the poly-
merization. Often two signals are found for MA: the anhy-
dride and the hydrolyzed product, the sum of which is used
to determine the conversion of MA.
N-(4-Vinylphenyl)Pivalamide (VAPiv)
VAPiv was prepared following a slightly modified literature
procedure19
:
3.6 ml of 4-vinylaniline (30 mmol, 1.0 eq.) was added to a
solution of 4.6 ml of triethylamine (33.0 mmol, 1.1 eq.) in
70 ml ethyl acetate under argon at 0 8C. Pivaloyl chloride
(4.4 ml, 36.0 mmol, 1.2 eq.) was added dropwise. After com-
plete addition the solution was diluted with an additional
30 ml of ethyl acetate and stirred at rt for 24 h. The solution
was washed three times with a 2:1 mixture of brine and sat-
urated sodium bicarbonate solutions (3 3 50 ml). The
organic phase was filtered over cotton wool and the solvent
was removed in vacuo. The product was further purified by
column chromatography using 4:1 vol/vol cyclohexane: ethyl
acetate as the eluent. The product was obtained as a white
solid (5.690 g, 28.0 mmol, 93% yield).
For each series of copolymerizations one polymer (from a
5:3 mixture of the styrene derivative and MA) was purified
after polymerization at higher conversions. The polymers
were purified by dialysis in acetone followed by freeze-
drying from 1,4-dioxane.
1H-NMR and SEC elugrams of the purified polymers are
shown in the Supporting Information.
1H-NMR (300 MHz, CDCl3, d, ppm): 7.52-7.48 (m, 2 H), 7.39–
7.35 (m, 2 H), 7.31 (s, 1 H), 6.67 (dd, 1 H), 5.68 (d, 1 H),
5.19 (d, 1 H), 1.32 (s, 9 H).
Job’s Plot
Stock solutions with a concentration of 0.1 M were prepared
for all monomers in both 1,4-dioxane and in DMF. The stock
solutions were mixed in varying ratios in a 96-wells plate.
Three (or in some cases six) solutions were prepared for
each ratio to exclude errors in the mixing. The plates were
shaken for 10 sec and UV-Vis spectra were subsequently
recorded (range: 300–500 nm, Dk 5 1 nm)
Boc-Protected 4-Vinylaniline (VABoc)
VABoc was prepared following a slightly modified literature
procedure20
:
4-Vinylaniline (0.44 ml, 3.77 mmol, 1.0 eq.) and di-tert-butyl
dicarbonate (0.990 g, 4.54 mmol, 1.2 eq.) were dissolved in
10 ml dry THF and the solution was refluxed under argon
overnight. The solvent was removed in vacuo and the crude
product was redissolved in dichloromethane (20 ml). The
UV-Vis spectra were also recorded for the stock solutions.
The absorption attributed to charge–transfer complexes was
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model is applicable when only the last monomer in the
growing chain influences the addition of monomers. Penulti-
mate models can be used to describe systems where both
the last and the second-to-last (penultimate) monomer in
the chain determine the rate of addition of subsequent
monomers. A number of different methods have been devel-
oped to determine reactivity ratios, especially for the termi-
nal model. Of these methods the linear-least-square (LLS)
method, Joshi–Joshi (J–J),21 Fineman–Ross (F–R),22 inverted
Fineman-Ross (inv. F–R, where the definition of Monomer 1
and 2 is reversed), Yezrielev–Brokhina–Roskin (Y–B–R),23
24
25
€ }
€ }
Kelen–Tudos (K–T), extended Kelen–Tudos (ext. K–T), Tid-
well–Mortimer (T–M),26 and Mao–Huglin (M–H)27 methods
were used here. Though all of these methods are based on
the original copolymerization equations by Mayo and
Lewis,28 some differences apply. All except the last three are
only suited for very low conversions since they only consider
the immediate polymer composition. Additionally the weight-
ing of data points is different.
SCHEME 1 Periodic RAFT copolymerization of maleic anhy-
dride with Boc-protected 4-vinylaniline
found by subtracting the weighted (based on the ratio of the
two monomers) absorption found for the pure monomers
from the measured absorption.
The reactivity ratios that were found using the terminal
model for the copolymerization of MA and VABoc were often
significantly negative for either r1 or r2 (Table 1, the various
styrene derivatives in this manuscript are defined as Mono-
mer 1, MA as Monomer 2), which is physically impossible. In
addition, the found reactivity ratios do not accurately predict
the composition of the polymers that is found experimen-
tally. Both of these facts indicate that this copolymerization
is not accurately described by the terminal model.
RESULTS AND DISCUSSION
RAFT Copolymerization of Maleic Anhydride and
Boc-Protected 4-Vinylaniline
To our great surprise such a strictly alternating copolymer
was not formed when maleic anhydride (MA) is copolymerized
with Boc-protected 4-vinylaniline (VABoc) in a reversible
addition-fragmentation chain-transfer (RAFT) copolymeriza-
tion in 1,4-dioxane. For an alternating copolymerization a 1:1
ratio of the monomers is expected in the polymer. Instead, MA
and VABoc are incorporated into the polymer in a ratio of
about 2:3. Since this ratio is found almost independently of the
feed ratio of the monomers (also see Supporting Information)
this strongly suggests that this finding is not due to the pres-
ence of access VABoc but rather due to a tendency towards a
periodic copolymerization, where, besides the typical alternat-
ing sequence of VABoc and MA also ABA-sequences [(VABoc-
MA–VABoc)n] are incorporated into the polymer (Scheme 1).
The penultimate model assumes that the last two monomers,
rather than only the last one, in a growing chain determine
the rate of addition of the following monomers. This results
in four chain-ends that need to be considered and accord-
ingly four reactivity ratios (r11, r12, r21, and r22). Fortunately
the inability of MA to homopolymerize (except under very
extreme conditions) simplifies the problem by allowing the
presumption that two of these reactivity ratios (r12 and r22
are 0. The remaining reactivity ratios (r11 and r21) can, for
)
€ }
example, be determined by a modified Kelen-Tudos proce-
dure or by a linear least square method. In both cases the
values found for r11 are close to 0, whereas r21 is deter-
mined to be around 0.6 [0.67 (K–T) or 0.53 (LLS)]. The fact
that r11 ꢀ 0 means that k111 ꢁ k112, meaning that homopo-
lymerization of VABoc onto a chain-end consisting of two
VABoc moieties (as terminal and penultimate moieties) is
negligible. r21 values around 0.6 mean that k211 is only
slightly lower than k212, indicating that the addition of MA is
slightly preferred for a chain with VABoc terminal and with
The RAFT copolymerization of MA and VABoc in 1,4-dioxane
was performed using different feed ratios. The conversion of
both monomers was monitored by taking aliquots and the
composition of the polymer was determined from the mono-
1
mer conversion as determined by H-NMR spectroscopy. The
composition of the formed polymers at low conversion
(ꢀ20%) was used to determine reactivity ratios using both
the terminal model and the penultimate model. The terminal
TABLE 1 Reactivity Ratios Found for the Copolymerization of MA and VABoc Using Various Methods Based on the Terminal
Model
LLS
J–J
F–R
Inv. F–R
Y–B–R
K–T
Ext. K–T
T–M
M–H
r1
0.14
0.16
0.12
0.34
0.01
0.00
0.13
0.14
20.01
20.14
0.00
0.05
0.00
0.00
0.17
r2
20.06
20.01
20.12
20.02
20.15
20.02
20.07
20.01
20.12
20.02
20.01
20.00
r1.r2
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TABLE 2 Chemical Shift (ppm) of Vinyl Signals in the
Monomers that were Used in this Work
Monomer
d1 (ppm)
d2 (ppm)
d3 (ppm)
S
5.22
5.19
5.19
5.19
5.16
5.13
5.74
5.67
5.67
5.68
5.65
5.61
6.65
6.66
6.66
6.66
6.65
6.66
VAAc
VAHex
VAPiv
VABoc
MOS
Styrene (S) is shown as a reference.
electron-rich, but this monomer could not be used since it
was very poorly soluble in 1,4-dioxane.
FIGURE 1 Scheme of the various steps that are significant in
the copolymerization of VABoc and MA. Dashed arrows show
less preferable pathways.
RAFT copolymerizations of the various styrene derivatives
with maleic anhydride were, unless mentioned otherwise, per-
formed in 1,4-dioxane at 65 8C using AIBN as the radical initia-
tor and 4-cyano-4-(thiobenzoylthio)pentanoic acid as the
chain-transfer agent (CTA); the same conditions used for the
copolymerization of VABoc and MA. The RAFT process pro-
vides a good control over the copolymerizations (Ð between
1.08 and 1.40 in all cases). For the copolymerization of MA
with VAPiv the evolution of the molecular weight and disper-
sity with conversion is shown in the Supporting Information.
As is typical for many controlled polymerizations Ð decreases
(from 1.34 to 1.22) with increasing conversion.
MA penultimate monomers. Nevertheless, VABoc is added to
this chain-end to a significant degree, so that two subse-
quent VABoc moieties are regularly incorporated. The rele-
vant steps in the copolymerization are shown in Figure 1.
RAFT Copolymerization of Maleic Anhydride with Other
Electron-Rich Styrene Derivatives
To investigate the reason for the increased incorporation of
this styrene derivative a number of other activated styrene
derivatives were prepared and copolymerized with MA. Dif-
ferent electron-donating substituents were introduced in the
para-position. Both the nature of the group (ether, carbamate
or amide) was varied as well as the size of the groups. The
monomers are shown in Figure 2.
For all combinations of styrene derivatives with MA copoly-
merizations were performed with different monomer feed-
ratios (ranging from 7:1 to 1:7) and the conversion of the
monomers was followed by 1H-NMR spectroscopy. Because
of the poor solubility of VAAc in 1,4-dioxane it was not pos-
sible to complete this entire series: the two copolymeriza-
tions with the highest fraction of VAAc could not be
performed, so that only four data points were attained, mak-
ing the reactivity ratios that were calculated from this data
somewhat less reliable. The same methods of calculating the
reactivity ratios as mentioned before were used. The full list
of reactivity ratios that were determined is shown in the
Supporting Information, but an excerpt showing the results
All para-substituents are electron-donating, but ethers are
most strongly so, while amide groups are weaker donors.
The carbamate group in VABoc has an intermediate effect.
This is also confirmed by the positions of the peaks corre-
sponding to the vinyl groups of the monomers in 1H-NMR
spectra, which relates to the electron density of the ring and
vinyl group. Three monomers with amide groups are chosen
with different steric properties, ranging from small (VAAc) to
bulky (VAPiv). Originally also the copolymerization of N,N-
dimethyl-4-vinylaniline was envisioned, which is even more
€ }
for the extended Kelen–Tudos, Tidwell–Mortimer, and Mao–
Huglin methods, those methods best suited for higher con-
versions for the terminal model, is shown in Table 2.
As with the copolymerization of VABoc and MA many of
these ratios are, impossibly, negative, and the other values
do not accurately describe the composition of the copoly-
mers, which again plateau at about 60% of the styrene
derivative. Only for the copolymerization of VAHex and MA
more or less decent fits are achieved, indicating that penulti-
mate effects are probably limited in this copolymerization.
The least-square method of fitting the data with the penulti-
mate model seems much more reliable. This method gives
fits that are in much better agreement with experimental
data for the copolymerizations of VAAc, VAPiv, and MOS with
FIGURE 2 Chemical structures of the various styrene deriva-
tives used in this work.
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to both alternating copolymers and oligomers with larger
numbers of styrene derivatives were found (the spectra for
the copolymer of VAPiv and MA is shown in Figure 3, all
other MALDI spectra are shown in the Supporting Informa-
tion, though no peaks at all were found for the MALDI spec-
tra of the copolymer of VABoc and MA, possibly because the
molecular weight of this polymer was too high). Although
the 1H-NMR spectra of the polymers clearly show aromatic
signals corresponding to the Z-group of the CTA, the
dithioester Z-group is not found in most MALDI spectra, pos-
sibly because this group is labile under the ionization condi-
tions. This quantitative removal of this one end-group
significantly simplifies the assignment of the peaks. Various
species of oligomers are found: (1) oligomers with equal
amounts of MA and SD, (2) species with n MA and n 1 1 SD
and (3) n MA and n 1 2 SD residues. The first two could
form also for an alternating copolymer, but the third series
can only occur when SD dimers are incorporated in the poly-
mer chain.
FIGURE 3 MALDI-TOF MS spectrum of the copolymer of VAPiv
and MA synthesized in 1,4-dioxane. Only a selection of the
peaks are assigned, for example, only one of the counterions
is labeled (Na1, whereas in most cases also the corresponding
H1 and K1 peaks are observed). “I” represents the R-group of
the CTA: the dithioester is not found. The underlined assign-
ments indicate species with increased SD content.
13C-NMR spectra of the various polymers were also
recorded. For the copolymers of VAAc with MA and VAPiv
with MA spectra were recorded both for the polymers pre-
pared in 1,4-dioxane and in DMF, the former solvent result-
ing in distinct penultimate effects whereas in DMF
alternating sequences seem to prevail. For comparison a
homopolymer of VAPiv was also synthesized. All spectra are
shown in the Supporting Information. While the spectra of
the VAPiv homopolymer and its copolymers differ strongly,
the copolymers prepared in 1,4-dioxane and DMF are very
similar, though minor differences are present. Larger differ-
ences are apparent between the spectra of the copolymers of
VAAc and MA synthesized in different solvents, possibly
because the penultimate effect is much more pronounced in
this monomer system (Table 3). Especially the MA carbonyl-
C and aryl-C signals differ significantly. Any differences that
might be present in the backbone signals are hard to distin-
guish because of the strong broadening of these signals,
likely due to the rather rigid backbone. These differences in
MA than any of the fits with the terminal model. For VAHex
the reactivity ratios attained are in good agreement with the
values found using the terminal model. Though we will not
discuss the copolymerization of VAAc with MA in detail
because of the limited number of data points for this copoly-
merization due to limited solubility of VAAc in 1,4-dioxane.
Both VAPiv and MOS show r11 ꢀ 0 and r21 ꢀ 1, indicating
that the chance of two styrene moieties between MA moi-
eties is as high as only one.
The increased incorporation of styrene derivatives could also
be confirmed by MALDI-TOF MS, where peaks corresponding
TABLE 3 Reactivity Ratios Found for Various Copolymerizations Using Either the Terminal or the Penultimate Model
Ext. K–T T–M
M–H
Mon 1
Mon 2
r1
r2
r1
r2
r1
r2
Terminal model
VAAc
MA
20.003
20.001
20.003
20.002
r11
20.152
20.058
20.205
20.013
r21
0.986
0.046
0.134
0.081
20.010
20.023
0.019
1.417
0.255
0.310
0.175
20.002
20.005
20.004
20.011
VAHex
MA
VAPiv
MA
MOS
MA
20.008
Mon 1
Mon 2
Penultimate model
VAAc
VAHex
VAPiv
MOS
MA
0.000
0.000
6.424
0.235
1.041
1.029
MA
MA
MA
0.018
20.003
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DISCUSSION
This difference in complexation stoichiometry is likely
related to the polarity of the solvents: due to its higher
polarity DMF is better able to stabilize the charge generated
in the charge–transfer complex, therefore, allowing for higher
charge densities than in 1,4-dioxane. The formation of 2:1
MA SD complexes nevertheless does not explain the tend-
ency toward the incorporation of more SD in the copolymer.
Rather, if the complex were to polymerize as a single unit, as
has been proposed for the alternating copolymerization of
MA and S or other combinations of electron-rich and
-deficient monomers,9 a higher degree of incorporation of
MA would be preferred. Since this is unlikely based on the
reluctance of MA to homopolymerize the most likely product
would therefore be the alternating copolymer, as is usually
encountered.
FIGURE 4 Job’s plot of the formation of charge–transfer com-
plexes between VAPiv and MA in 1,4-dioxane (triangles, from
k 5 316 nm) and in N,N-dimethylformamide (squares, from
k 5 321 nm).
A depletion effect might provide an explanation. Because
more MA than SD is incorporated into the complex the con-
centration of free MA is effectively lowered while, relatively,
the concentration of free SD is increased. Assuming for a
moment that the complex is less reactive in the polymeriza-
tion this effect would lead to an effective feed ratio with
higher amounts of SD than calculated, and therefore a more
efficient incorporation of the SD.
the 13C-NMR spectra might be another indication that the
monomer sequence differs for these polymers.
Charge–Transfer Interactions
The charge–transfer complex that is known to form between
styrene derivatives and MA29 appears to be strongly involved
in the observed penultimate effect. First indications for this
were gained from the copolymerization of VAAc and MOS
with MA in DMF. In these cases good fits were achieved with
the terminal model, which yielded reactivity ratios close to
0, indicative of a strictly alternating copolymerization.
Thus far such a depletion effect seems the most likely expla-
nation. The fact that both VABoc, VAPiv and MOS show very
similar reactivity ratios when fitted with the penultimate
model indicates that, based on this series of monomers, nei-
ther sterics nor electron-density alone can explain the penul-
timate effect found in the RAFT copolymerization of
electron-rich styrene derivatives and MA. Additionally, H-
bonding cannot play a significant role (as could be proposed
for monomer pairs that contain H-bond acceptors and
donors) since no H-bond donors are present in either MOS
or MA. Further studies are required to investigate this effect.
The charge–transfer complexes between electron-rich sty-
rene derivatives (SD) and MA was further studied by prepar-
ing Job’s plots using UV-Vis spectroscopy. The UV-Vis spectra
of solutions (in 1,4-dioxane at room temperature) containing
both styrene derivatives and MA show a new adsorption
band between 300 and 320 nm which is attributed to the
formation of charge–transfer complexes. A series of spectra
was recorded with constant total concentrations of mono-
mers and the corrected (against the absorption of the mono-
mers at this wave length) absorption at the wave length
where the complex absorbs strongest was plotted against
the fraction of the styrene derivative (fSD). For all monomers
a maximum intensity of the new band is found at fSD 5 0.33,
indicating that the highest concentration of charge–transfer
complexes is formed at this ratio, which indicates that the
complexes likely contain twice as much MA as SD. This is
only the case for the electron-rich styrenic monomers used
in this study though: styrene itself shows a maximum at
Altogether it seems clear that the electron-rich nature of the
styrene monomers and an apolar (and likely nonaromatic)
solvent are needed to achieve the penultimate effect
described in this work. This work serves as a first step
toward systems where a strict ABA-periodic copolymeriza-
tion of styrene derivatives and MA might become accessible.
Thus far only a small number of (free-radical) ABA-copoly-
merizations14–17 are known to result in ABA-periodic copoly-
mers. These copolymerizations, for example, rely on the
copolymerization of maleimides with bulky monomers con-
taining nonconjugated vinyl groups, often in solvents that
coordinate to the carbonyl-group in the maleimide monomer.
f
f
MA 5 0.5. In DMF most monomers show a single peak at
SD 5 0.5, indicating a 1:1 ratio of MA and SD in the com-
CONCLUSIONS
In conclusion, we were able to show that the RAFT copoly-
merization of electron-rich styrene derivatives with maleic
anhydride in 1,4-dioxane shows a relatively strong penulti-
mate effect which results in the increased incorporation of
the styrene derivatives as compared to the typical
plexes. Only for VAAc a second peak at fSD ꢀ 0.25 is
observed reproducibly, possibly indicating a second species
of complexes with a 3:1 ratio of MA and SD. The Job’s plots
for VAPiv and MA in 1,4-dioxane and DMF are shown in Fig-
ure 4, respectively.
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11 N. ten Brummelhuis, M. Weck, J. Polym. Sci., Part A: Polym.
Chem. 2014, 52, 1555–1559.
alternating copolymerization, that is, a certain tendency
toward the formation of periodic (ABA) sequences is wit-
nessed. Though the exact reason for this behavior has not
yet been elucidated, the formation of 2:1 charge–transfer
complexes of the monomers in apolar solvents seems to play
a significant role. Further studies are currently under way to
obtain a deeper understanding of these observations.
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We wish to thank the Chemical Industry Fund for funding
through a Liebig fellowship.
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