Synthesis and characterization of chiral poly(alkyl isocyanates) by coordination polymerization using a chiral half-titanocene complex

Article abstract of DOI:10.1002/pola.27669

A new chiral half-titanocene complex, [CpTiCl₂(O-(S)-2-Bu)], is synthesized and characterized by ¹H and ¹³C NMR spectroscopy. This complex is employed for the coordination polymerization of n-butyl and n-hexyl- isocyanate

Full text of DOI:10.1002/pola.27669

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Synthesis and Characterization of Chiral Poly(alkyl isocyanates) by
Coordination Polymerization Using a Chiral Half-Titanocene Complex
Ioannis Choinopoulos,1,2 Spyros Koinis,2 Marinos Pitsikalis1
1Industrial Chemistry Laboratory, Department of Chemistry, University of Athens, 15771 Panepistimiopolis Zografou, Athens,
Greece
2Laboratory of Inorganic Chemistry, Department of Chemistry, University of Athens, 15771 Panepistimiopolis, Zografou, Athens,
Greece
Correspondence to: M. Pitsikalis (E-mail: pitsikalis@chem.uoa.gr) or I. Choinopoulos (E-mail: ichoinop@chem.uoa.gr)
Received 22 January 2015; accepted 27 February 2015; published online 23 April 2015
DOI: 10.1002/pola.27669
ABSTRACT: A new chiral half-titanocene complex, [CpTiCl2(O-
(S)22-Bu)], is synthesized and characterized by 1H and 13C
NMR spectroscopy. This complex is employed for the coordi-
nation polymerization of n-butyl and n-hexyl- isocyanate lead-
ing to chiral polymers, as revealed by their CD spectra. Only
the left-handed helix is produced, due to the chiral (S)22-
butoxy group, which is bound to the polymer chain end. The
polymerization of 3-(triethoxysilyl)propyl isocyanate produces
less soluble polymers. On the other hand, phenyl isocyanate
reacts slowly with the complex leading quantitatively and
selectively to triphenyl isocyanurate. 2-Ethylhexyl isocyanate is
slowly and selectively cyclotrimerized in the presence of the
half-titanocene complex. However, a statistical copolymer of 2-
ethylhexyl isocyanate and hexyl isocyanate is produced. The
reaction of benzyl isocyanate with the complex leads to a mix-
ture of low molecular weight polymer and cyclotrimer. The
polymers are characterized using SEC, NMR, and CD spectros-
copy and their thermal properties are investigated by TGA/DSC
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analysis.
2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2015, 53, 2141–2151
KEYWORDS: chiral; circular dichroism; differential scanning calo-
rimetry (DSC); half-titanocene; isocyanates; thermogravimetric
analysis (TGA)
tion or an excess of one of these conformations: (a) copoly-
merization of an optically inactive monomer with an
isocyanate enantiomer, (b) termination of the polymerization
with a chiral molecule, (c) polymerization employing a chiral
initiator, and (d) dissolution of an optically inactive polyiso-
cyanate in an optically active solvent.4 As far as half-
titanocenes are concerned, optically active oligomers1 and
chiral poly(hexyl isocyanate)5 have been synthesized by chi-
ral half-titanocene initiators.
INTRODUCTION Half-titanocene alkoxides are known to be
efficient initiators for the polymerization of isocyanates.1
These complexes have been employed for the synthesis of
more complex macromolecular architectures containing poly-
isocyanate blocks, such as diblock copolymers, triblock ter-
polymers, graft copolymers, and miktoarm star copolymers.2
Poly(hexyl isocyanate) has also been grafted from carbon
nanotubes by an anchored half-titanocene complex.3
It is widely known that polyisocyanates possess a dynamic
helical conformation both in solution and in the solid state.
Due to the resonance delocalization of the nitrogen and car-
bonyl electrons and the steric hindrance induced by the
steric hindrance of the monomer’s side group, polyisocya-
nates adopt an extended 8/3 helical conformation instead of
a restricted coplanar conformation.4 In the absence of a chi-
ral component, an equal mixture of right- and left-handed
helical segments is synthesized. So, the racemic polymer
does not exhibit any optical properties.
Poly(isocyanates) are chemically unstable polymers and
undergo degradation under several reaction conditions,
both in solution and in the solid state. It is believed that
degradation begins at the -NH end of the polymer chain.
In order to prevent decomposition, termination of the
polymerization reaction with other means than HCl/meth-
anol are used.6 It is also known that polyisocyanates are
thermally unstable. The thermal degradation of poly(alkyl
isocyanates), the decomposition products and the reaction
mechanism have been investigated. The principal decom-
position product is the cyclic trimer (trialkyl isocyanurate)
in all cases. Intramolecular cyclization is proposed as the
The synthesis of chiral polymers is of paramount importance,
due to their special applications. There are four general
ways to obtain optically active polyisocyanates, that is either
the pure left-handed or the right-handed helical conforma-
dominant
mechanism
of decomposition
of
these
polymers.7
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In this study, we report the successful synthesis of well-
defined, chiral poly(alkyl isocyanates) using a novel half-
titanocene complex. The mechanism of the polymerization
reaction was investigated by kinetic experiments. The poly-
mers were characterized by size exclusion chromatography
(SEC), 1H NMR and 13C NMR spectroscopy. The thermal
properties of the materials were examined by thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC). The optical properties were measured by circular
dichroism (CD) spectroscopy. The effect of the monomer
structure on the performance of the initiator was
investigated.
(about 20 min), the solution was stirred for 3 h and was
then filtered through a dry Por.4 sintered glass funnel in an
inert atmosphere. The filtrate was collected in a 50-mL
Schlenk flask and the solvent was removed in vacuo. The yel-
low product was dried under vacuum and stored in a dry-
box. Yield: 0.2400 g (93.4%). 1H NMR (300 MHz, CDCl3, d):
1.01 CH3CH2CHCH3(t,3H); 1.38 CH3CH2CHCH3 (d, 3H); 1.52–
1.80 CH3CH2CHCH3 (m, 2H); 4.76 CH3CH2CHCH3 (m, 1H);
6.72 C5H5 (s, 5H). 13C NMR (75 MHz, CDCl3, d): 10.07
CH3CH2CHCH3; 22.72 CH3CH2CHCH3; 32.06 CH3CH2CHCH3;
93.11 CH3CH2CHCH3; 118.82 C5H5.
Typical Procedure for Polymer Synthesis
In a 50-mL Schlenk flask, [CpTiCl2(O-(S)-2-Bu)] (0.0600 g,
0.233 mmol) was dissolved in toluene (0.5 mL). To the yel-
low solution, the monomer (2.0 mL) was added. After the
appropriate reaction time, dichloromethane (2 mL) was
added and the viscous solution was cooled to 08C. Subse-
quently, methanol/aq.HCl (2 mL) was added. The solution
became faint yellow immediately and methanol (20 mL) was
added. The mixture was concentrated under vacuum (2–
3 mL) in order to remove most of the dichloromethane. The
polymer was precipitated in methanol (140 mL). After 24 h
at 2208C, the liquid was decanted and the purification pro-
cess was repeated twice. The white polymer was dried under
EXPERIMENTAL
Materials
CpTiCl3 (Aldrich, 97%), methanol, hexane, tetrahydrofuran
and (S)-2-butanol (Aldrich, 99%) were used as received. Phe-
nyl isocyanate, PhIC, (Aldrich, 98%) was dried over P2O5
overnight and distilled under reduced pressure. All other
monomers (Aldrich) were dried over CaH2 overnight and
distilled under reduced pressure. Toluene was dried over Na
and distilled.
Characterization
1H and 13C NMR spectra were acquired by a Varian Unity
Plus 300 NMR spectrometer in chloroform-d at 258C. DSC
experiments were performed with a 2910 modulated DSC
instrument from TA Instruments. The samples were heated
or cooled at a rate of 108C/min. The samples were annealed
at 1208C for 10 min and the measurements were obtained at
the second heating run. TGA were performed using a TA
Instruments TGA Q50 model under nitrogen at a heating
rate of 108C/min. SEC experiments were conducted at 408C
with a modular instrument consisting of a Waters model
510 pump, a Waters model U6K sample injector, a Waters
model 410 differential refractometer, a Waters model 486
UV spectrophotometer and a set of four l-Styragel columns
with a continuous porosity range from 106 to 103 Å. The col-
umns were housed in an oven thermostatted at 408C. Tetra-
hydrofuran was the carrier solvent at a flow rate of 1 mL/
min. The system was calibrated with seven polystyrene (PS)
standards with molecular weights in the range of 4000–
1,000,000. CD spectra were measured in hexane or tetrahy-
drofuran solutions using a Jasco J-815 CD spectrometer.
vacuum for 24
gravimetrically.
h
and the yield was determined
Synthesis of 2-Ethylhexyl Isocyanurate
In a 50-mL Schlenk flask, [CpTiCl2(O-(S)-2-Bu)] (0.0600 g,
0.233 mmol) was dissolved in toluene (0.5 mL). To the yel-
low solution, 2-ethylhexyl isocyanate, EHIC, (2.0 mL, 11.20
mmol) was added. The solution was stirred for 10 days. A
sample from the reaction mixture was taken for NMR mea-
surement. Yield (by NMR): ;18%.
Synthesis of Triphenyl Isocyanurate
In a 50-mL Schlenk flask, [CpTiCl2(O-(S)-2-Bu)] (0.0600 g,
0.233 mmol) was dissolved in toluene (0.5 mL). To the yel-
low solution, PhIC (2.0 mL, 18.40 mmol) was added leading
to the formation of an orange solution. After 2 h, the solution
became red. After 46 h, the solution darkened and a solid
was visible. Seven days later all liquids were evaporated in
vacuo and dichloromethane was added under stirring, until
the solid was dissolved. After 24 h at 2208C, a white solid
precipitated from the dichloromethane solution and filtered
through a Por.4 sintered glass funnel. It was washed with
cold dichloromethane. Yield: 2.0819 g (95%).
All synthetic procedures were carried out using standard
high-vacuum, Schlenk or glovebox techniques, unless stated
differently.
Synthesis of Poly(benzyl isocyanate), PBzIC, and
Tribenzyl Isocyanurate
Synthesis of [CpTiCl2(O-(S)-2-Bu)]
In a 50-mL Schlenk flask, CpTiCl3 (0.2193 g, 0.100 mmol)
was added and dissolved in toluene (7.50 mL). In another
flask, (S)-2-butanol (92.3 lL, 0.100 mmol) and triethylamine
(139.4 lL, 0.100 mmol) were dissolved in toluene (3.50 mL).
The solution of (S)-2-butanol was added dropwise via a
pressure equalizing addition funnel to the solution of the
metal complex under vigorous stirring. A white precipitate of
Et3NH1Cl– was formed. After the addition was completed
In a 50-mL Schlenk flask, [CpTiCl2(O-(S)-2-Bu)] (0.0600 g,
0.233 mmol) was dissolved in toluene (0.5 mL). Benzyl iso-
cyanate, BzIC, (2.0 mL, 19.05 mmol) was added to the yellow
1
solution. After 3 h, a sample for H NMR analysis was taken
to determine the quantity of the polymer and the cyclo-
trimer. Dichloromethane (2 mL) was then added and the vis-
cous solution was cooled to 08C, followed by the addition of
methanol/aq.HCl (2 mL). The solution became faint yellow at
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once and more methanol (20 mL) was added. The mixture
was evaporated under vacuum (2–3 mL) in order to remove
most of the dichloromethane. The content of the Schlenk
flask was then transferred to a 400-mL beaker containing
methanol (140 mL). The purification process was conducted
as stated previously. Yield (by NMR): ;40% polymer, ;9%
cyclotrimer.
SCHEME 1 Synthesis of [CpTiCl2(O-(S)22-Bu)].
Synthesis of 3-(Triethoxysilyl)propyl isocyanate,
PTEOSiPIC
In a 50-mL Schlenk flask, [CpTiCl2(O-(S)-2-Bu)] (0.0600 g,
0.233 mmol) was dissolved in toluene (0.5 mL). 3-(Triethox-
ysilyl)propyl isocyanate, TEOSiPIC, (2.0 mL, 8.07 mmol) was
added to the yellow solution. After 7 h, a sample for 1H
NMR analysis was taken. Dichloromethane (2 mL) was added
and the viscous solution was cooled to 08C, followed by the
addition of methanol/aq.HCl (2 mL). The solution became
faint yellow immediately and an extra quantity of methanol
(20 mL) was added. The mixture was evaporated under vac-
uum (2–3 mL) in order to remove most of the dichlorome-
thane. The content of the Schlenk flask was transferred to a
400-mL beaker containing methanol (140 mL). The white
solid was filtered through a Por.4 sintered glass funnel. The
white polymer was dried under vacuum for 24 h. The poly-
mer was partially soluble in toluene, tetrahydrofuran, chloro-
form. Yield (by NMR): ;75%.
of the Schlenk flask was then poured in a 400-mL beaker
containing methanol (140 mL). After 24 h at 2208C, the liq-
uid was decanted and the purification process was repeated
twice. The white polymer was dried under vacuum for 24 h.
Yield: 0.8160 g (21%).
Kinetic Study of the Polymerization of HIC
Four vials containing a magnet, a 0.525 M solution of [CpTi-
Cl2(O-(S)-2-Bu)] in toluene (0.30 mL) and HIC (1.50 mL)
were stirred at 208C for specific time intervals. Samples
1
were taken to monitor the reaction progress by H NMR and
SEC experiments.
RESULTS AND DISCUSSION
Synthesis and Characterization of [CpTiCl2(O-(S)-2-Bu)]
The goal of this study was the synthesis of chiral polyisocya-
nates. In order to achieve this, a chiral alkoxo half-titanocene
complex was employed.
Synthesis of Poly(2-ethylhexyl isocyanate-co-hexyl
isocyanate), PEHIC-co-PHIC
In a 50-mL Schlenk flask, [CpTiCl2(O-(S)-2-Bu)] (0.0600 g,
0.233 mmol) was dissolved in toluene (0.5 mL). EHIC
(2.0 mL, 11.43 mmol) was subsequently added. After 21 h,
the solution viscosity and color remained unchanged and
hexyl isocyanate, HIC, (2.0 mL, 13.81 mmol) was added.
After 5 h, the solution became more viscous and the color
was orange. After 24 h, the stirring stopped, dichlorome-
thane (2 mL) was added and the viscous solution was cooled
to 08C, followed by the addition of methanol/aq.HCl (2 mL).
The solution became faint yellow and methanol (20 mL) was
added. The mixture was evaporated under vacuum (2–3 mL)
in order to remove most of the dichloromethane. The con-
tent of the Schlenk flask was then transferred to a 400-mL
beaker containing methanol (140 mL). After 24 h at 2208C,
the liquid was decanted and the purification process was
repeated twice. The white polymer was dried under vacuum
for 24 h. Yield: 1.150 g (35%).
The complex [CpTiCl2(O-(S)-2-Bu)] was synthesized by modi-
fying already known procedures for similar compounds.1
The chirality of the complex derives from the alkoxy ligand
(Scheme 1). The stereogenic carbon atom should be as close
as possible to the oxygen atom, to maximize chiral induction
to the polymer chain. There should also be as limited stereo-
chemical hindrance as possible so as to achieve the maxi-
mum initiation rate. For these reasons, (S)-2-butanol was
employed.
1
The complex was characterized by H and 13C NMR spectros-
copy. All peaks were assigned as expected in both 1H and
13C NMR spectra, except for those of the cyclopentadienyl
ring. Only one peak appears both in 1H NMR and 13C NMR
spectra, although more were expected (Fig. 1). This phenom-
enon is common in (half) titanocene complexes and is attrib-
uted to lability processes of the substituents that render all
the ring atoms equivalent.8,9 The complex is moisture sensi-
tive and (S)-2-butanol is observed in the 13C NMR spectrum
(-CH-OH, 64.35 ppm).
Synthesis of a Copolymer of BzIC and HIC
In a 50-mL Schlenk flask, [CpTiCl2(O-(S)-2-Bu)] (0.0600 g,
0.233 mmol) was dissolved in toluene (0.5 mL). BzIC
(2.0 mL, 19.05 mmol) was then added to the yellow solution.
After 21 h, HIC (2.0 mL, 13.81 mmol) was added. After 24 h,
the solution became very viscous; dichloromethane (2 mL)
was added and the viscous solution was cooled to 08C, fol-
lowed by the addition of methanol/aq.HCl (2 mL). The solu-
tion became faint yellow and methanol (20 mL) was added.
The mixture was evaporated under vacuum (2–3 mL) in
order to remove most of the dichloromethane. The content
Polymerization of Isocyanates
It should be emphasized that the (S)-2-butoxy ligand is cova-
lently bound to the end of the polymer chain. Thus, after the
second monomer unit is incorporated in the polymer chain,
this chiral alkoxy group is already far from the metal atom
and does not contribute to the chirality of the metal centre
anymore. Nevertheless, this does not imply that the polymer-
ization does not proceed in a chiral manner. On the contrary,
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FIGURE 2 1H NMR spectrum of PHIC bearing (S)22-butoxy end
group.
centrations. During the propagation process, the solution
becomes very viscous and the polymerization is terminated
with methanol/aq.HCl before completion. If the reaction is
not terminated at the appropriate time (yield), the product
has a broad molecular weight distribution, due to the inho-
mogeneity of the reaction mixture. The termination reaction
must be carried out at 08C, due to the reaction of methanol
with the remaining monomer to give a carbamate product.10
This alcohololysis reaction is greatly exothermic and is, prob-
ably, catalyzed by the titanium complex or HCl. A sudden
rise in the temperature results in polymer degradation and
formation of the undesired cyclotrimer, trihexyl isocyanurate.
The incorporation of the alkoxy group at the end of the poly-
mer chain is easily demonstrated in the 1H NMR spectrum
by the assignment of the multiplet peak at 3.25 ppm to the -
CH- proton (h) (Fig. 2). The molecular weight of the polymer
can be determined by this peak (3.25 ppm) and the peak of
the polymer chain at 3.70 ppm (-CH2- (f)).
FIGURE 1 1H (a) and 13C (b) NMR spectra of [CpTiCl2(O-(S)22-
Bu)] (*(S)22-butanol).
the end of the growing polymer chain, which is bound to the
metal, is chiral due to the developing single-handed helix
from the other chain end. Therefore, the whole polymeriza-
tion procedure evolves in a chiral manner. In other words,
the (S)-2-butoxy chain end induces a left-handed helical
polymer chain, which induces the chirality to the metal
centre (Scheme 2). This detail is emphasized in order to dis-
criminate this complex and the polymerization process from
others, where the chiral substituent is bound (or supposed
to be bound) to the metal centre during the entire polymer-
ization process and is not incorporated into the polymer
chain.
The chirality of the polymer chain is verified by the CD spec-
trum. The polymer exhibits a Cotton effect at 255 nm, due to
the n-p* transitions of the amide chromophore, which is neg-
ative. This leads to the conclusion that the helix is left-
handed (M). At shorter wavelengths (205 nm) an exciton
couplet is shown, due to the arrangement of the chiral amide
linkages along the main chain [Fig. 3(a)]. The stability of the
helical structure in solution in relation with the temperature
was also studied. The structure changes progressively (from
rigid rod to coil) upon increasing temperature in agreement
with the literature.11 This behavior is reversible in heating
and cooling cycles. This phenomenon is not readily observed
[Fig. 3(b)] in a low molecular weight polymer (Mw 5 7000),
meaning that the helical structure remains intact at least up
to 558C (Fig. 3).
Hexyl Isocyanate
The organotitanium mediated polymerization of isocyanates
is fully reversible, that is, there is equilibrium between poly-
merization and depolymerization.1 Therefore, in order to
suppress the depolymerization reaction and achieve high
yields the polymerization has to be conducted at high con-
The thermal behavior of the solid polymer upon heating was
studied by DSC and TGA. Two samples with (a) Mw,a 5 9900
and (b) Mw,b 5 8500 were measured (Table 1). Weak transi-
tions were observed for each sample (Fig. 4). The first tran-
sition (T1,a 5 –106.18C, T1,b 5 –96.08C) is attributed to the
mobility of the n-hexyl side groups of the polymer. It has
SCHEME 2 The metal centre becomes chiral due to the devel-
oping left-handed helix during propagation.
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FIGURE 4 DSC thermogram of PHIC (Table 1, sample 1).
known that the persistence length of the helical structure of
polyisocyanates is quite big, meaning that our polymer chain
maintains its left-handedness in full length.12b
FIGURE 3 CD spectra of PHIC Mw 5 7000. (a) 208C and (b) 20,
35, 45, and 558C. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
been proposed that the alkyl chains, which form a sheath
around the stiff main chain, become less rigid and this
sheath softens.12a The second transition (Tg,a 5 –41.48C,
Tg.b 5 –40.88C) is referred to as the glass transition tempera-
ture. There is a variety of reported Tg values in the literature,
from 215 to 2508C.2,12a As expected, the polymer with
lower molecular weight has lower transition temperature.
Conformational changes in the polymer backbone, which
have been reported2,12a in the 40–758C range were not
observed in our samples. This indicates that the helical
structure remains intact up to 758C. Another indication that
supports this conclusion is the weakness of the transitions,
which is attributed to the rigidity of the polymer due to its
chemical structure and its low molecular weight. It is well
TABLE 1 Thermal Transitions of PHICs of Different Mw by DSC
I (Mw/Mn)a
T1 (8C)
Tg (8C)
a
Sample
Mw
1
2
9900
8500
1.17
1.11
2106.1
296.0
241.4
241.8
FIGURE 5 (a) Derivative weight loss with temperature for PHIC
and (b) TGA curve of PHIC Mw 5 10100.
a
By SEC in tetrahydrofuran at 408C.
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TABLE 2 Kinetic Study Data of HIC Polymerization
Yield (%)a
Mw
I (Mw/Mn)b
b
Time (s)
2700
5400
9000
10,200
12.5
27
3000
5200
9300
9700
1.07
1.13
1.12
1.17
50
58
a
Calculated by 1H NMR spectra.
By SEC in THF in 408C.
b
From the TGA [Fig. 5(a)], PHIC begins to degrade at 141.78C,
displays maximum rate in mass loss at 231.18C and reaches
complete degradation at 254.68C. The two shoulders at 174.
and 209.18C reveal the presence of a complicated degrada-
tion mechanism. It has been reported that thermal degrada-
tion proceeds either through formation of the amide anion,
or through an intermolecular mechanism. In both cases, the
cyclotrimer is the main product and monomer is produced
in smaller quantities.13 The thermogram [Fig. 5(b)] is quite
different from others. One difference is the absence of a fast
mass loss (10%) at the beginning of the process, which
appears to be characteristic for all poly(alkyl isocya-
nates),14–16 and is attributed to the loss of monomer.15
Another difference is the temperature where decomposition
is completed (254.58C), since it has been reported that the
decomposition is completed with the evaporation of the
cyclotrimer, which occurs above 3008C. These remarks lead
to the assumption that there is a different, more complex,
mechanism for the decomposition of the polymers produced
by the present complex. These differences may be attributed
to the different end groups and/or the pure left-handed heli-
cal structure. These observations apply also to PBIC pro-
duced by the same complex, as will be discussed later (Fig.
11).
FIGURE 7 Temporal evolution of Mw. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
was conducted. From the data in Table 2 and the graphs in
Figures 6 and 7, the linear function between yield, a, and
time and between molecular weight, Mw, and time as well
becomes apparent. The good control of the molecular charac-
teristics of the product is confirmed by the low molecular
weight distribution values, which increase slightly over time
(Fig. 8). When a large degree of polymerization is reached,
the polymer is insoluble in the small amount of solvent
used. The mixture becomes heterogeneous and, thus, the
molecular weight distribution is increased dramatically
(yield 72%, I 5 1.21).
In order to clarify the mechanism for the polymerization of
HIC with the complex [CpTiCl2(O-(S)-2-Bu)], a kinetic study
FIGURE 8 SEC chromatograms in THF (408C) of PHIC kinetic
study (curves from right to left: 2700, 5400, 9000, and 10,200 s).
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
FIGURE 6 Temporal evolution of yield (a) determined by 1H
NMR. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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FIGURE 9 1H NMR spectrum of PBIC bearing (S)22-butoxy end
group.
Butyl Isocyanate
The polymerization of BIC was also studied, employing the
same half-titanocene chiral complex. The calculated molecu-
lar weight of PBIC from the 1H NMR spectrum, integrating
the peaks of the end group (f) and the side group (d), was
Mn 5 1660. Size exclusion chromatography provides
Mw 5 2960, Mn 5 2650, I 5 1.12. The difference is due to
instrument calibration with polystyrene standards and the
difference in hydrodynamic volumes between PBIC and poly-
styrene. A correction factor of 0.58 due to the difference in
hydrodynamic volume is given in the literature. In this sam-
ple, applying the correction factor Mn 5 1540, which is in
good agreement with the value calculated from the 1H NMR
spectrum (Fig. 9). The calculated molecular weight from the
yield of the reaction, Mn 5 1868, is also in good agreement
with the experimental data.
FIGURE 11 (a) Derivative weight loss with temperature for
PBIC and (b) TGA curve of PBIC.
impossible to observe the expected Cotton effect at
;205 nm, due to the arrangement of the chiral amide link-
ages along the main chain (Fig. 10). In the CD spectrum, the
synthesis of the left-handed helix was verified by the nega-
tive Cotton effect at 257 nm, which is due to the n-p* transi-
tions of the amide chromophore group. The variable
temperature study of the structure by CD revealed that the
helical structure remains intact up to 458C, as in the case of
low molecular weight PHIC.
PBIC is insoluble in hexane, in contrast to poly(hexyl isocya-
nate). For this reason, tetrahydrofuran was used to prepare
the samples for CD measurements. However, the observation
area became narrower (>220 nm) and therefore, it was
TGA analysis (Fig. 11) reveals that the polymer starts to
degrade at 136.48C, has maximum mass loss rate at 207.68C
and reaches complete degradation at 223.58C. The bulkier
side group of PHIC provides the polymer more protection
from thermal degradation than PBIC, which has a smaller
side group. For this reason, the degradation of PBIC begins
(136.48C) and is completed (223.58C) at lower temperatures
than that of PHIC (141.7 and 254.68C, respectively). The
behavior is similar to that of PHIC, which has been discussed
previously.
2-Ethylhexyl Isocyanate
EHIC was employed in order to study the monomer’s steric
hindrance effect on the activity of the half-titanocene chiral
FIGURE 10 CD spectra of PBIC at 208C.
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FIGURE 12 13C NMR spectrum of the cyclotrimerization product of triphenyl isocyanate (triphenyl isocyanurate).
complex. A lower polymerization rate of this monomer was
expected. However, even after several days of reaction no
polymer was detected. In fact, only the cyclotrimer was
formed. It can, thus, be concluded that bulky monomers can-
not be homopolymerized by this complex. Nevertheless, they
can be selectively cyclotrimerized. Even though cyclotrimeri-
zation was not the goal of this research, isocyanurates are
still useful compounds with industrial applications, such as
pesticides (chlorinated isocyanurates), crosslinking agents
(triallyl isocyanurate), electronics (triglycidyl isocyanurate),
food preservatives [(2,3-dibromopropyl) isocyanurate], etc.
The synthetic procedures for some of these compounds
might be simplified using this or similar catalysts with bulky
isocyanates.
peak at 3.8–5.5 ppm, which is attributed to the polymer. The
peak at 4.51 ppm is assigned to the monomer and the peak
at 5.05 ppm is attributed to the cyclotrimer.17 The peaks of
the aromatic rings of each compound overlap and cannot be
assigned separately. The polymerization reaction is much
slower compared to alkyl isocyanates, as expected. The main
feature of this particular polymerization reaction is the fact
that, unlike the polymerization of BIC and HIC, cyclotrimer is
produced as a byproduct during the polymerization reaction.
The polymer has very low molecular weight, probably due to
cyclotrimerization reactions, which are essentially termina-
tion reactions. This is the only monomer which exhibits both
polymerization and cyclotrimerization reactions simultane-
ously. This behavior can be attributed to the intermediate
flexibility of BzIC compared with PhIC and EHIC.
Phenyl Isocyanate
Taking into consideration the results from all the monomers
discussed previously, it can be concluded that the reaction
with the chiral complex depends almost exclusively on the
stereochemical properties of the monomer and not on its
electronic properties.
PhIC is a bulky monomer, which, however, has different elec-
tronic effects from those of alkyl isocyanates due to the aro-
matic ring. When phenyl isocyanate reacts with the complex
no polymer is produced. After 7 days of reaction, triphenyl
isocyanurate is recognized as the only product by 13C NMR
spectroscopy (Fig. 12).16 This result indicates that the steric
factors prevail over any electronic effect. Therefore, phenyl
isocyanate behaves in a similar manner as EHIC during the
reaction with the chiral half-titanocene.
3-(Triethoxysilyl)propyl isocyanate
This compound is commonly used as a linker molecule and
not a monomer. PTEOSiPIC has been reported to be prepared
by anionic polymerization.18 The size exclusion chromato-
gram of our product is bimodal with a broad distribution. In
the 1H NMR spectrum of a sample of the polymerization
solution (Fig. 14), the characteristic peak of the polymer
appears in the 3.40–3.70 ppm region (e).17 Although, during
the polymerization reaction the polymer is soluble, after the
Benzyl Isocyanate
BzIC is also a bulky monomer. The aromatic ring affects the
electronic properties of the molecule compared to those of
alkyl isocyanates. The 1H NMR spectrum (Fig. 13) of a sam-
ple taken from the reaction solution of BzIC has a broad
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FIGURE 13 1H NMR spectrum of the polymerization reaction mixture.
termination with acidic methanol, it is insoluble in common
solvents. The polymer is cross-linked due to rapid hydrolysis
of the triethoxysilyl group and the formation of siloxane link-
ages. As was revealed in the IR spectrum (Si-O-Si
1050 cm21), the cross-linking is quite extensive, in agree-
ment with literature data.17 Taking into account this initial
FIGURE 14 1H NMR spectrum of the polymerization reaction mixture.
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FIGURE 15 13C NMR spectrum of PHIC-co-PEHIC, *peaks attributed to EHIC units.
result, other efficient procedures for the termination of the
polymerization without involving cross-linking reactions are
tested.
without the formation of trimer. Even though a detailed
mechanistic analysis is not available, it is assumed that, in
the coordination polymerization of isocyanates, cyclotrimeri-
zation is a termination reaction.
Cyclotrimerization Versus Polymerization in Anionic and
Coordination Polymerization
Copolymers
The polymerization of isocyanates has been studied exten-
sively using anionic initiators.19 Backbiting reactions of the
active chain end take place during the polymerization lead-
ing to the formation of inactive trimers of the isocyanate
monomer. Cyclotrimerization is not a termination reaction in
anionic polymerization, because the anion is still active at
the polymer chain end. However, this side reaction leads to
low polymer yields and broad molecular weight distribu-
tions. In order to prevent cyclotrimerization, the polymeriza-
tion has to be conducted at low temperatures in the
presence of several additives, such as crown ethers or
NaBPh4.13,20
Due to steric effects of the monomer, the homopolymeriza-
tion of EHIC is impossible under the same reaction condi-
tions as for the other alkyl isocyanates. However, efforts to
copolymerize EHIC and HIC were rather successful. In the
13C NMR spectrum (Fig. 15), the peaks attributed to PHIC
are dominant but there are also several smaller and wider
peaks (10.50, 30.04, and 38.61 ppm) attributed to EHIC
units incorporated in the polymer chain. Therefore, a statisti-
cal copolymerization of the two monomers occurred. The
intensity of the peaks attributed to PEHIC indicate that there
were very few units of EHIC introduced in the polymer
chain, even though the feed composition of the monomers
(mol) was approximately 1:1. Therefore, the reactivity ratio
for HIC is much higher than that of EHIC. The copolymer
had relatively narrow molecular weight distribution
(Mw 5 16700, I 5 1.11) revealing good control of the copoly-
merization reaction.
In this study, the polymerization of stereochemically nonhin-
dered monomers (HIC, BIC) proceeds without the production
of cyclotrimers. On the other hand, the reaction of bulky
monomers (PhIC, EHIC) leads exclusively to cyclotrimers.
The case of BzIC, where both cyclotrimerization and poly-
merization occur simultaneously, is unique. It seems that the
crucial parameter is the bulkiness close to the carbonyl
group of the monomer. This conclusion is further supported
by the behavior of TEOSiPIC. This is a sterically hindered
monomer but the bulky triethoxysilyl group is connected to
the nitrogen atom through a spacer, which is the propyl
group. Therefore, TEOSiPIC is polymerized to a high yield
The copolymerization of BzIC with HIC was also examined.
After reaction of the first monomer (BzIC), the second mono-
mer was introduced in the reaction mixture, without removal
of the first one. A bimodal distribution was revealed in the
size exclusion chromatogram of the final product. Formation
of the cyclotrimer during the polymerization reaction of BzIC
possibly leads to termination of polymer molecules and
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TABLE 3 Molecular Characteristics of the Homo- and Copoly-
In conclusion, the monomer structure, or in other words the
steric hindrance close to the isocyanate group influences the
progress of the reaction with the chiral half-titanocene complex,
leading either to polymerization or cyclotrimerization products.
mers Synthesized
b
Sample
Yield (%)
Mw
I (Mw/Mn)b
PHIC (1)
66
8500
1.11
1.17
1.32
-
PHIC (2)
21
9900
REFERENCES AND NOTES
PHIC (3)
71
10100
Bimodal
Insol.
16,700
4800
PBzIC
40a
75a
23
1 T. E. Patten, B. M. Novak, J. Am. Chem. Soc. 1996, 118,
1906–1916.
PTEOSiPIC
PEHIC-co-PHIC
PBzIC-b-PHIC
-
2 (a) S. Mourmouris, K. Kostakis, M. Pitsikalis, N.
Hadjichristidis, J. Polym. Sci. Part A: Polym. Chem. 2005, 43,
6503–6514; (b) A. Touris, K. Kostakis, S. Mourmouris, V.
Kotzabasakis, M. Pitsikalis, N. Hadjichristidis, Macromolecules
2008, 41, 2426–2438.
1.11
1.21
20
a
Calculated in 1H NMR spectrum.
By SEC in tetrahydrofuran at 408C.
b
3 D. Priftis, N. Petzetakis, G. Sakellariou, M. Pitsikalis, D.
Baskaran, J. W. Mays, N. Hadjichristidis, Macromolecules 2009,
42, 3340–3346.
disengagement from the metal. The formation of a copolymer
of the type PBzIC-b-P(BzIC-co-HIC) is more possible. The
homopolymerization of HIC can be ruled out taking into
account the SEC and NMR data.
4 S. Mayer, R. Zentel, Prog. Polym. Sci. 2001, 26, 1973–2013.
5 T. Satoh, R. Ihara, D. Kawato, N. Nishikawa, D. Suemasa, Y.
Kondo, K. Fuchise, R. Sakai, T. Kakuchi, Macromolecules 2012,
45, 3677–3686.
A strong evidence for the copolymer formation is the solu-
bility of the final product in chloroform, where PBzIC homo-
polymer is almost insoluble. From the 1H NMR spectrum,
the composition of the copolymer in BzIC is equal to 70%.
Further studies are in progress to elucidate the structure of
this copolymer. The molecular characteristics of the copoly-
mers along with characteristic homopolymers are given in
Table 3.
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CONCLUSIONS
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A new chiral, half-titanocene, alkoxy complex [CpTiCl2(O-(S)-
2-Bu)] was synthesized. The aforementioned complex indu-
ces asymmetric polymerization of non chiral isocyanates.
The (S)-2-butoxy group, which is ultimately covalently
bonded to the end of the polymer chain, induces the poly-
merization of only the left-handed (M) helix. The products
have relatively narrow molecular weight distributions, and
the polymerization reaction is well controlled. Sterically hin-
dered monomers, such as 2-ethylhexyl isocyanate and phenyl
isocyanate are exclusively cyclotrimerized producing the cor-
responding isocyanurates. However, statistical copolymeriza-
tion of 2-ethylhexyl isocyanate and hexyl isocyanate was
achieved, but with limited incorporation of 2-ethylhexyl iso-
cyanate units along the polymer chain. Benzyl isocyanate
was polymerized to a low molecular weight polymer, due to
termination reactions (cyclotrimerization). The product has
limited solubility in common solvents and only a copolymer
with hexyl isocyanate was dissolved in chloroform. The poly-
merization of 3-(triethoxysilyl)propyl isocyanate proceeds to
a product which is less soluble in common solvents, prob-
ably due to partial hydrolysis of the triethoxysilyl groups
during the termination process.
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