Carbocationic polymerization of isoprene using cumyl initiators: Progress in understanding side reactions

Article abstract of DOI:10.1039/c5ra08557h

The cationic polymerization of isoprene using cumyl chloride/B(C₆F₅)₃ and cumyl ether/TiCl₄ systems was investigated in dichloromethane or in dichloromethane/methylcyclohexane mixtures varying the polymerization conditions. Polymerizations were performed in all cases in the presence of a large excess of a proton trap (2,6-di-tert-butyl pyridine, dtBP) compared to initiator in order to suppress any protic side reactions. As a consequence, no polymerization went to completion. Independently of the reaction conditions, trans-1,4-oligomers were exclusively obtained with mainly an olefinic terminal group. It was highlighted that an important loss of double bonds yielding saturated parts was observed, even in the absence of protons, assuming that a great amount of double bond loss generally observed in isoprene cationic polymerization could be due to intramolecular cyclization reactions. Nevertheless, under particular conditions (low temperature and/or low polarity medium), branching and cross-linking reactions were also found responsible for double bond loss.

Full text of DOI:10.1039/c5ra08557h

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GRAPHICAL ABSTRACT
1

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ARTICLE TYPE
Carbocationic polymerization of isoprene using cumyl initiators:
progress in understanding side reactions
Samira Ouardad,a Anne-Laure Wirotiusa, Sergei Kostjukb, François Ganachaudc and Frédéric Peruch*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The cationic polymerization of isoprene using cumyl chloride/B(C6F5)3 and cumyl ether/TiCl4 systems
was investigated in dichloromethane or in dichloromethane/methylcyclohexane mixtures varying the
polymerization conditions. Polymerizations were performed in all cases in the presence of a large excess
of a proton trap (2,6-di-tert-butyl pyridine, dtBP) compared to initiator in order to suppress any protic side
10 reaction. As a consequence, no polymerization went to completion. Independently of the reaction
conditions, trans-1,4-oligomers were exclusively obtained with mainly an olefinic terminal group. It was
highlighted that an important loss of double bonds yielding to saturated parts was observed, even in
absence of protons, assuming that a great amount of double bond loss generally observed in isoprene
cationic polymerization could be due to intramolecular cyclization reactions. Nevertheless, in particular
15 conditions (low temperature and/or low polarity medium), branching and cross-linking reactions were
also found responsible for double bonds’ loss.
aqueous media7 while in organic media a loss of double bonds
was observed whatever the initiating system used.5-9
Introduction
In this paper, the investigation of cationic polymerization of
isoprene was conducted in order to better understand the
50 elementary processes involved and more precisely the side
reactions responsible for double bonds’ loss. All the former
studied systems, more particularly when allylic initiators were
associated to boron Lewis acid, led to a loss of information
regarding side reactions. Indeed, it was difficult to evaluate the
55 extent of protic initiation, as a proton trap was added in default
with regard to the initiator, or of the intermolecular branching
reactions. Moreover, B(C6F5)3 was shown to form strong complex
with allylic alcohol initiators at low temperature, inhibiting their
ionization and thus the polymerization. To this end, initiating
60 systems inducing rapid ionization and initiation rates were
needed. Thus, as the well-known (di-)cumyl initiators associated
to several Lewis acids proved to be efficient initiating systems for
the carbocationic polymerization of isobutene and its
copolymerization with isoprene,10-15 they were used for isoprene
65 homopolymerization in this study.
The isoprene (IP) cationic polymerization is studied since more
than 70 years with various catalytic systems1 but it was only
20 recently that it was suggested that similar elementary processes
proceeded in the Natural rubber (NR) biosynthesis, a cis-1,4-
polyisoprene, produced by Hevea Brasiliensis crops.2, 3 Up to the
end of the 60s, Friedel-Crafts catalysts, Lewis acids and/or other
organometallic compounds were mainly used as initiating
25 systems for the cationic polymerization of isoprene, whereas
from the 70s protic promoters were preferred. Furthermore, many
polymerization conditions were investigated, varying the solvent,
the temperature and the monomer concentration. Whatever the
conditions, trans-1,4-polyisoprenes with unusual microstructures,
30 more specifically cyclized sequences leading to double bond loss,
were obtained. Recent works showed that dimethylallyl
derivatives (DMAX), homologues of the DMAPP natural
initiator, were ionizable by Lewis acids (LA) and the primary
allylic carbocation was able to initiate isoprene polymerization.4
35 Using IP/DMABr/TiCl4 system, transfer reactions were
highlighted via the observation of a partial decomposition of the
DMABr carbocation moiety into isoprene, which mimic the
transfer reaction of an active allylic chain end into an unsaturated
chain end.5 This point was later proved by Puskas et al., Kostjuk
Experimental Part
Materials
Isoprene (Aldrich, 99 %) was cryo-distilled twice over CaH2 and
then over di-n-butyl-magnesium, dichloromethane (CH2Cl2,
70 Aldrich, 99.5%) and methylcyclohexane (Mecyclo, Aldrich,
99%) were distilled respectively from CaH2 and sodium cubes
under an inert atmosphere prior to use. 2,6-di-tert-butylpyridine
(dtBP) was dried over KOH pellets before distillation. B(C6F5)3
(TCI, 97%) and TiCl4 were used as received. The purity of
40 et al. and Ouardad et al. using, DMAOH/TiCl4 or BF3.Et2O,6 1-
7
(4-methoxyphenyl)ethanol/B(C6F5)3
and 2-cyclohexylidene
8
9
ethanol/B(C6F5)3 or DMAOH/B(C6F5)3 as initiating systems
respectively, in organic media. Furthermore, 1-(4-
methoxyphenyl)ethanol/B(C6F5)3 initiating system allowed the
45 synthesis of trans-1,4-polyisoprenes without double bond loss in
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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1
B(C6F5)3 was checked by H or 19F NMR. Dioxane (DO) and
ethyl acetate (EA) were cryo-distilled over CaH2. α-
methylstyrene (α-MS, 99%, Aldrich) and Amberlist 15 were used
as received.
are monoisotopic; i.e. they include the most abundant isotopes of
the elements present in the corresponding ion.
Results and discussion
5
Initiator synthesis
60 CumylCl/B(C6F5)3 as initiating system
In several papers,7-9 thanks to Maldi-TOF MS analysis, the
presence of intensive transfer reaction during the cationic
polymerization of isoprene was evidenced, even at low
Cumyl chloride (CumylCl) was synthesized by the reaction of α-
MS with dry HCl gas over a period of 5h. The reaction is total
and excess of HCl gas is removed by flushing with argon. Cumyl
ether (CumylOCH3) was synthesized by the reaction of α-MS
conversion. As
a consequence, polyisoprene chains with
10 with methanol in presence of Amberlist 15 over a period of 24h.
The desired product, obtained with a yield of 80%, was isolated
by a column using a mixture of dichloromethane/methanol (v/v
99.5/0.5). The purity of cumyl initiators was checked by 1H
NMR. 1H NMR: α-MS: 2.22 ppm (-CH3) , 5.15 ppm (-CH2), 5.44
15 ppm (-CH2), 7.39 ppm and 7.54 ppm (aromatic protons, Ar);
CumylCl: 1.88 ppm (-CH3) , 7.17 ppm (p-Ar), 7.24 ppm (m-Ar),
and 7.54 ppm (o-Ar); CumylOCH3: 1.37 ppm (-CH3) , 2.91 ppm
(-OCH3), 7.11 ppm (p-Ar), 7.21 ppm (m-Ar), and 7.28 ppm (o-
Ar) (Figure S1, ESI).
65 unsaturated chain-ends were mainly obtained accompanied by a
proton release which was later responsible of protic initiation
and/or double bond protonation entailing double bond loss via
cyclization and/or branching reactions. In this study, in order to
understand the elementary processes involved in IP cationic
70 polymerization, the side reactions due to protons were suppressed
by adding a proton trap, namely dtBP. Here however, in contrast
9
to our previous works,8,
an excess of dtBP was used
(initiator/dtBP 1/2) to trap all protons coming from transfer
reactions (maximum one per initiator molecule) and water traces.
75 To avoid possible complexation of highly oxophilic B(C6F5)3
with the selected cumyl initiator (as encountered with the 2-
cyclohexylidene ethanol at low temperature), cumyl chloride
(cumylCl) was chosen in place of cumylether (cumylOCH3). On
the basis of this new initiating system, the influence of the
80 isoprene cationic polymerization conditions were investigated at
20°C (cumylCl/B(C6F5)3/dtBP 1/2/2).
20 Polymerization procedure
The polymerization reactions were carried out in glass Schlenk
tubes equipped with a stopcock in an argon atmosphere at 20°C.
As an example of a typical procedure in solution, polymerization
was initiated by adding LA (0.8 mmol) dissolved in 0.5 mL of
25 CH2Cl2 to a mixture consisting of a cumyl initiator (0.4 mmol),
isoprene (30 mmol, 3 mL), and CH2Cl2 for a total volume of 25
mL. After a predetermined time, ∼1 mL aliquots were withdrawn
and poured into a methanol excess. The precipitated polymer was
separated from the solution by centrifugation and dried in
30 vacuum. Monomer conversions were determined by gravimetry.
As expected, polymerizations were characterized by a fast
initiation rate. Propagation was quite fast among the first hours to
then almost stop after few hours (graph A, Figure 1). Whatever
85 the polymerization conditions, low conversions (< 30%) were
reached and only oligomers characterized by low dispersities
were obtained. Nevertheless, it can be noticed that working at
high IP concentration (green vs blue curves, graph A, Figure 1)
and lower medium polarity (pink vs blue curves, graph A, Figure
90 1) lead to lower yield at the plateau but no significant effect was
noticed regarding molar mass or dispersity (graph B, Figure 1).
The observed rate deceleration was probably due to the large
amount of added dtBP that trap all the protons generated during
transfer reactions, transforming these latter into termination
95 reactions by inhibiting protic side reactions (initiation or DB
protonation). Control polymerization reactions performed in the
absence of the proton trap showed much higher conversion with
PI characterized by low molar masses, quite low dispersities and
low DB contents.
Instrumentation
Molar masses were determined by Size Exclusion
Chromatography (SEC) at 40°C in THF as the eluent (1 mL/min),
trichlorobenzene as a flow marker, using both refractometric (RI)
35 and UV detectors (Varian). Analyses were performed using a
three-column set of TSK gel TOSOH (G4000, G3000, G2000
with pore sizes of 20, 75, and 200 ꢀ respectively, connected in
series) calibrated with polystyrene standards. 1H NMR (400
MHz), 13C NMR (100 MHz) and 19F NMR (160 MHz) spectra
40 were recorded in CDCl3 or CD2Cl2 at 25°C on a Bruker Avance
400 spectrometer calibrated relative to the solvent peak in
reference to tetramethylsilane and CFCl3 standards. Maldi-TOF
MS was performed on a Voyager mass spectrometer (AB Sciex).
The instrument is equipped with a pulsed N2 laser (337 nm) and a
45 time-delayed extracted ion source. Spectra were recorded in the
positive-ion mode using the reflectron and with an accelerating
voltage of 20kV. Samples were dissolved in THF at 10 mg.mL-1.
The DCTB matrix (trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-
propenylidene]malononitrile) solution was prepared by dissolving
50 10 mg in 1 mL of THF. A solution of cationation agent (AgTFA
or CuCl, 10 mg.mL-1) was also prepared. The solutions were
combined in a 10:1:1 volume ratio of matrix to sample to
cationation agent. 1-2 μL of the obtained solution was deposited
onto the sample target and vacuum dried. The instrument was
55 calibrated externally using poly(ethylene glycol) standards at the
appropriate molar mass. All mass-to-charge ratios (m/z) quoted
100 For more insight on the microstructures, NMR analyses were
1
performed (Figure 2). Thanks to the H NMR, it was observed
that even in presence of dtBP, the oligoisoprenes were not fully
unsaturated. The double bonds’ loss was only due in this case to
the active chain ends which reacted with the oligoisoprenes
105 double bonds as well as with the IP monomer. It could be thus
concluded that the loss of double bonds observed in previous
works,8, 9 where dtBP was added in lack compared to the initiator,
was not only due to protic side reactions but also to some other
processes. No significant double bond conservation could be
110 obtained in the various studied conditions (graph A, Figure 1) as
at the very beginning of the polymerization, 40% of the expected
double bonds were already consumed.

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The NMR (1H and 13C) analyses showed that in all conditions,
As
oligoisoprenes were mainly trans-1,4- with saturated segments.
A/ conversion and DB% = f(t)
B/ Mn and dispersity = f(t)
30
20
10
0
100
75
50
25
0
1500
3
2.5
2
1000
500
0
1.5
1
0
24
48
72
96
120
144
0
24
48
72
96
120
144
t (h)
t (h)
IP/CumylCl/B(C6F5)3 75/1/2
[IP] = 2M
IP/CumylCl/B(C6F5)3 75/1/2
[IP] = 4M
IP/CumylCl/B(C6F5)3 75/1/2
[IP] = 2M
CH2Cl2
CH2Cl2
CH2Cl2/Methylcyclohexane 50/50
5
Fig. 1 Cationic polymerization of isoprene at 20°C in different conditions; DB % determined by 1H NMR (100 % if no DB loss), Mn and dispersity
determined by SEC (PS calibration, THF, 40°C, 1 mL/min).
an example, in 1H NMR spectra of oligoisoprene obtained in
dichloromethane using the system IP/CumylCl/B(C6F5)3/dtBP 20 by signals at 38.67 ppm and at 28.38 ppm, respectively. In
few “head-to-head” and “tail-to-tail” enchainments characterized
10 75/1/2/2 (Figure 2; corresponding to the blue curves in Figure 1),
it can be seen that the oligoisoprenes were mainly made of 1,4-
unit (-CH2-CCH3=CH-CH2- at 5.04 ppm), traces of 1,2- and 3,4-
units were detected (-CH=CH2 at 4.92 ppm et -CH=CH2 at 5.71
addition to signals of the various IP units, signals corresponding
to the cumyl head unit were also detected; at 7 ppm and 7.5 ppm
(aromatic protons) and at 1.2 ppm (methyl group) on H NMR
1
spectra, and at 28.98 ppm (methyl groups), 38.11 ppm (C6H5-
ppm) and saturated units (broad band between 0.5 ppm and 1 25 C(CH3)2-) and 125.39 ppm, 126.14 ppm and 127.88 ppm
15 ppm, Figure 2). Regarding the 1,4- units, 13C NMR confirmed the
exclusive presence of trans configuration (-CH2-CCH3=CH-CH2-
at 15.85 ppm) mainly made up by “head-to-tail” enchainments (-
CH2-CCH3=CH-CH2-CH2-CCH3=CH-CH2- at 39.87 ppm) and of
(aromatic carbons) on 13C NMR spectra. Absence of
dimethylallyl head signals ((CH3)2=C-CH2- at 17.52 ppm and
(CH3)2=C-CH2- at 25.55 ppm), proved the absence of protic
a
a
a
a
1
a
2
*
5
2
3
C
H
C
H
C
H
C
C
C
H
C
H
c
3
3
3
3
3
c
3
e
3
4
4
H
C
c
H
C
H
C
H
C
H
C
H
C
2
2
2
2
2
2
H
C
C
H
b
C
C
C
H
C
C
H
C
C
H
b
C
H
2
f
1
f
c
'
c
3
3
c
'
2
*
2
*
2
2
f
'
d
b
b
n
2
*
5
5
5
5
2
C
H
H
b
C
3
a
H
C
C
H
2
3
1
H
C
2
d
a
4
4
d
p
m
ε
a2
1
H NMR
2
*
2
5
13C NMR
a
a
a
1
a
a
3
C
H
C
H
C
H
C
C
C
H
C
H
c
3
3
3
3
c
3
e
3
4
4
Partie aliphat
Aliphatic part
H
C
c
H
C
H
C
H
C
H
C
H
C
2
2
2
2
2
2
H
c2
q-q
C
C
H
b
C
C
C
H
C
C
C
C
H
b
C
H
2
a1
f
H
1
f
c
'
c
2
*
2
*
c
'
3
3
2
2
f
'
d
b
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2
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C
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b
C
3
c2
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a
H
C
C
H
1
2
3
H
C
c2’
t-q
2
d
a
4
4
d
p
m
a1
a2
ε
1-c2
f1
1-a2
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Cis
Saturated
units
c2;c2‘
a4
e4
Partie oléfi
Olefinic part
1
ε
1
b2
b2*
a3
f2
b2
1
d3; d4
d5
b3
1
b3
d3
30
Fig. 2 1H and 13C NMR spectra of oligoisoprenes obtained with the system IP/cumylCl/B(C6F5)3/dtBP 75/1/2/2 in CH2Cl2 at 20°C, t = 24h (corresponding
to blue curves in Figure 1).

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initiation thanks to the efficiency of proton trapping by dtBP.
For more insight on side reactions, Maldi-TOF MS
characterization was performed (Figure 3 and Figure S2 in ESI).
NMR, Friedel-Craft reactions can be also discarded. Finally, on
HSQC NMR spectrum (Figure S3, ESI), two sets of three peaks
were detected in the aromatic region, one of high intensity
5
Results obtained for all the investigated conditions were similar. 40 corresponding to the cumyl fragment head and a second less
1
At low molar mass only one family (F1) was detected
corresponding to polyisoprene (n*C5H8) initiated by a cumyl
cation (C9H11+) and bearing terminal unsaturation (C5H7) due to
termination by β-H elimination (transfer reaction process). At
intense one (hidden in the H NMR spectrum) corresponding to
α-MS incorporated in the PI chain. This assumption that the
presence of F2, chain bearing two cumyl fragments, is due to the
incorporation of α-MS (coming from a partial decomposition of
10 higher molar mass, for all the investigated conditions, a second 45 the initiator) is in good accordance with a previous work that
family (F2) is detected in addition to the first one (F1). The F2
family corresponds also to polyisoprene chains but here bearing
two cumyl fragments. The first family, F1 (C9H11-PI-C5H7), is in
larger amount than the second one, F2, present only at higher
highlighted
a
partial
decomposition
of
1-(4-
methoxyphenyl)ethanol.7 When experiments were performed in
100% dichloromethane, two others families, F3 and F4, were
observed, both of them bearing a cumyl fragment and a hydroxy
15 molar mass. The F2 family could thus be explained by an 50 or methoxy chain-end respectively. The formation of F3 could be
intermolecular branching between two chains initiated by a cumyl
cation (path (a), Scheme 1), by a Friedel-Crafts (FC) reaction of
an active chain bearing a cumyl head with any chain (path (b),
Scheme 1) or by a partial decomposition of cumyl chloride
due to the presence of adventitious water, whereas F4 could be
formed during precipitation in methanol at the end of the
polymerization. As these chain-ends are most probably present in
very low amounts, they were not observed by NMR. In addition,
20 initiator after ionization leading to α–methylstyrene which is then 55 no chain initiated by protons was observed which is in good
incorporated as a co-monomer (path (c), Scheme 1).
correlation with NMR results. On the contrary, for experiments
performed in the absence of the proton trap, chains were mainly
initiated by the protons released during transfer reactions.
From this first set of experiments, it can be concluded that
Previous results on the IP cationic polymerization at 20°C
using 2-cyclohexylidene ethanol/B(C6F5)3 as initiating systems
showed the absence of branching reactions at low conversion
25 based on absence of family bearing two 2-ethylcyclohexylidene 60 transfer reactions are in these conditions the major termination
fragments.8 In the present study, as conversions did not exceed
25%, the intermolecular branching as the side reaction
responsible of the F2 family formation can be reasonably
discarded. Besides, the presence of a FC reaction would induce
30 the formation of p- or o-substituted aromatic group inside a
polyisoprene chain. Therefore, in addition to the peaks of the
cumyl chain head one or different new sets of aromatic peaks
(depending on the aromatic position attack and degree of
process limiting the conversion and that the intramolecular
cyclization is responsible of the loss of DB (absence of DB
protonation and interchain branching reaction).
CumylOCH3/TiCl4 as initiating system
65 CumylOCH3/TiCl4 and more generally cumylX/TiCl4, initiating
systems have been widely used for living cationic polymerization
of isobutene16, 17 or cationic copolymerization of isoprene with
isobutene allowing higher incorporation of isoprene unit with a
lower loss of DB15. Therefore, this system was also studied with
70 the aim to disfavour the transfer reaction and to obtain PI with
1
substitution) should be observed in H and 13C NMR. Since just
35 one set of aromatic peaks, corresponding to the monosubstituted
1
cumyl fragment at the chain head was observed in H and 13C
Scheme 1 Possible pathways for the formation of chains bearing two cumyl fragments.

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Fig. 3 Maldi-TOF MS spectra of oligoisoprenes obtained by the system IP/CumylCl/B(C6F5)3/dtBP at 20°C.
5
higher mass and DB content than those obtained with
cumylCl/B(C6F5)3 initiating system. Besides, cationic
polymerization of isobutene was shown to be free of transfer 20 reactions were still present but retarded as higher conversions
When polymerization were conducted in
CH2Cl2/methylcyclohexane mixture, even at -80°C, transfer
a
50/50
reactions when performed below -60°C, whereas they became
more and more important with temperature above -60°C.18
10 Polymerizations were then mainly performed at -80°C. Again, 2
equivalents of dtBP with regard to cumylOCH3 initiator were
were obtained compared to reactions performed at 20°C (Run 1
vs Run 2, Table 1). Temperature decrease was also beneficial to
molar masses as they increased from 520 g/mol to 3680 g/mol at
20°C and -80°C respectively after 24h, with a dispersity increase
used in order to trap all protons. Polymerization conditions were 25 but without gel formation. Finally, lowering the temperature
varied (temperature, M/I ratio, monomer concentration and
solvent polarity). Moreover, since a total gelation was observed
15 after 30 min at -80 °C in plain CH2Cl2, experiments were
performed in solvent mixture. Main results are summarized in
Table 1.
induced a slight higher DB content. A decrease of the medium
polarity at -80 °C (Run 3 vs Run 4, Table 1) by increasing the
amount of methylcyclohexane in the solvent mixture (50/50 vs
25/75), lowered the yield, the molar mass and the dispersity
30 without noticeable DB content change. Increasing M/I ratio at
Table 1 Isoprene cationic polymerization with cumylOCH3/TiCl4 as the initiating systema
b
Run CH2Cl2/Mecyclo IP/CumylOCH3
[IP]
(M)
T
(°C)
t
Yield
(%)
23
29
27
46
29
48
15
Mn
Đb
DBc
(%)
45
53
60
55
55
58
57
57
65
66
(h)
0.5
24
0.5
24
0.5
24
0.5
24
(g/mol)
560
520
1.2
1.1
1.7
1.9
2.1
3.5
2.0
2.1
3.9
8.7
1
2
3
4
5
50/50
50/50
50/50
25/75
25/75
75/1
75/1
1.2
1.2
2.4
2.4
4.8
20
2180
3680
3040
7180
1330
2090
3770d
3640d
-80
-80
-80
-80
150/1
150/1
299/1
30
36
100
23h
48h
a IP/cumylOCH3/TiCl4/dtBP = IP/cumylOCH3/2/2. b molar mass and dispersity of the soluble fraction determined by SEC (PS calibration, THF, 40 °C, 1
mL/min). c DB% determined by 1H NMR (100 % if no loss). d observation of gel formation.

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volume total constant and thus increasing monomer concentration
increase of the degree of branching and the amount of gel content
(Run 4 vs Run 5, Table 1) led to a large multimodal molar mass 15 with decreasing temperatures.19-21
distribution and high gel content (Figure S4, ESI). Therefore,
high monomer concentration combined to apolar medium and
low temperature seemed to be the worst condition for isoprene
cationic polymerization. In these conditions, intermolecular
In a second set of experiments, we tried to overcome gel
5
formation observed in the worst conditions defined previously.
To this end, the influence of monomer concentration or of the
presence of an electron donor (ED) was investigated. These latter
branching reactions were present in contrast to polymerizations 20 were expected to stabilize the active chain end or to modulate the
8
conducted at 20°C with the 2-cyclohexylidene ethanol/B(C6F5)3
Lewis acid acidity. Dioxane and ethyl acetate were thus selected.
Results are summarized in Table 2. In all experiments, a decrease
of the polymerization rate was observed leading to oligoisoprenes
with lower dispersity but similar DB content. The gel formation
10 and CumylCl/B(C6F5)3 initiating systems. In these conditions,
polyisoprene is probably not fully soluble. For instance, with
other initiating systems, Rozentsvet et al. showed the presence of
intermolecular branching reactions at low temperature and an 25 was
Table 2 Isoprene cationic polymerization using cumylOCH3/TiCl4 initiating systema
b
Run
[IP]
(M)
ED
t
Yield
(%)
36
100
17
21
14
17
9
Mn
Đb
DBc
(%)
65
66
70
76
69
69
64
(days)
(g/mol)
3770d
3640d
970
1180
1170
1245
1770
1210
1
2
1
2
1
2
1
2
3.9
8.7
1.4
1.4
2.0
2.0
1.3
1.6
5
6
7
8
4.8
2.4
4.8
4.8
-
-
DO
EA
12
66
a IP/cumylOCH3/TiCl4/dtBP = 299/1/2/2; CH2Cl2/Mecyclo: 25/75; T = – 80 °C. b molar mass and dispersity of the soluble fraction determined by SEC
(PS calibration, THF, 40 °C, 1 mL/min). c DB% determined by 1H NMR (100 % if no DB loss). d observation of gel formation.
cumyl-a2
a2
Run 5
DtBp
Without electron donor
c2, c2’
a1
cycles
a4
a3
1-c2
1-c2
1-c2
Run 7
In presence of dioxane
a1
c2, c2’
DtBp
Run 8
a1
In presence of ethyl acetate
CH-X
CH2-X
30
Fig. 4 1H NMR spectra of polyisoprenes obtained with IP/CumylOCH3/TiCl4/dtBP/ED 299/1/2/2/1, [IP] = 4.8 M, 24 h at -80 °C in presence or absence of
an electron-donor additive.

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DOI: 10.1039/C5RA08P55a7gHe 8 of 9
1
successfully avoided either by lowering the monomer 35 and 1 ppm on H NMR spectra. In order to have more details on
concentration or by adding an electron donor. Furthermore, while
the polyisoprene microstructure was unchanged by lowering the
IP concentration, changes were observed on NMR spectra (Figure
4) in the presence of electron donors (Runs 7 and 8, Table 2).
this saturated part, PIs have been analyzed combining various
NMR techniques (1H, 13C, NOESY, TOCSY, HSQC, HMBC,
JMOD). All PIs obtained in our conditions were analyzed by the
same techniques (1H NMR in Figure S5, ESI) but also by 2D
5
Indeed, the broad peaks attributed to cyclized segments become 40 NMR (COSY, HSQC, HMBC, TOCSY, NOESY) and JMOD
more defined (0.5-1 ppm) and higher functionalization degree is
obtained (peaks between 3.2-3.7 ppm and between 3.9-4.2 ppm
10 corresponding respectively to –CH-X and –CH2-X protons). The
(Figure S6, ESI) for deeper local and spatial microstructural
analyses. TOCSY (Figure S7, ESI) and NOESY (Figure S8, ESI)
analyses led to artefacts only while combination of COSY
(Figure S9, ESI), HSQC (Figure S3, ESI), HMBC (Figure S10,
effect of ethyl acetate was more pronounced than that of dioxane.
Electron donors may thus interact with active species, directly or 45 ESI) confirmed the trans-1,4 configuration of polyisoprenes but
via association with the Lewis acid.
unfortunately no additional information was obtained. It can be
assumed that side reactions leading to double bond loss and thus
to saturated units formation did not lead to regular units,
otherwise, few specific saturated units would have been formed
As expected, Maldi-TOF MS analysis (Figure 5) showed that
15 no protic initiation occurred (exclusive initiation by a cumyl
fragment). Transfer reaction was again the major termination
process (F1 family). As with CumylCl/B(C6F5)3 initiating system, 50 and could be thus detectable by NMR analysis, considering the
a second family bearing two cumyl fragments was observed at
higher molar mass. In presence of electron donors, functionalized
20 families could also be detected, cumyl-PI-OH (F3 family) and
cumyl-PI-OCH3 (F4 family), in good correlation with NMR
results.
significant amount of double bond loss.
As the NMR investigation was not conclusive, it was also
performed pyrolysis studies coupled to gas chromatography-mass
spectrometry technique (GC-MS) in order to identify
55 oligoisoprenes subunits. Unfortunately, again it was not possible
to find possible structures for the cyclized units. Finally,
controlled degradation techniques in the presence of ozone or
periodic acid were performed. Again the elucidation of the
saturated units was not possible as it was only detected the
60 fragments arising from the 1,4-trans units.
Further polyisoprene characterization
To this date, details on the microstructure of polyisoprenes
25 synthesized through cationic polymerization were only based on
the unsaturated part.1, 7-9, 19-21 It was thus shown using 1H and 13
C
NMRs that they were trans-1,4-PI mainly linked in a regular
fashion “head-to-tail” with few “head-to-head” and “tail-to-tail”
linkages, with some saturated structures. Nevertheless, the
30 presence of saturated parts was highlighted since the first studies
on cationic polymerization of IP. Several assumptions were given
with time, going from mono-, bi-, tri-, tetra- or poly-cycles but
without any fully convincing proofs. Assumption of the presence
of cycles was based on the presence of a broad band between 0.5
Conclusions
This article showed that, whatever the polymerization conditions,
the presence of a large excess of dtBP prevented polymerization
completion, as transfer reactions were transformed in termination
65 reactions, the released protons being trapped by dtBP. As
expected for isoprene cationic polymerization, trans-1,4-
Fig. 5 Maldi-TOF MS spectra of oligoisoprenes obtained with CumylOCH3/TiCl4 initiating system at -80 °C (IP/CumylOCH3/TiCl4/dtBP/ED =
299/1/2/2/1, [IP] = 4.8 M, t = 24 h).

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DOI: 10.1039/C5RA08557H
Page 9 of 9
RSC Advances
oligoisoprenes were produced with some saturated units. In this
13 M. Gyor, J. P. Kennedy, T. Kelen, F. Tudos, J. Macromol. Sci.
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study, no polymerization conditions could avoid the consumption
of double bond, whereas the best polymerization conditions
described so far for the conservation of the double bonds in the
cationic polymerization of isoprene are in aqueous medium.7, 22
Besides, the important double bond loss (30-60 %) observed in
this work was always detected since the very beginning of the
polymerization, involving most probably intramolecular
10 cyclization reactions, as it was present even in conditions where
protons and interchain branching were absent. Indeed, sufficient
proton trap was added to remove all free protons (coming from
transfer reactions and water traces) and avoid gelation at 20°C. It
was also shown that the cross-linking reactions observed at lower
15 temperature (gelation temperature depending on the solvent
polarity) could be suppressed by addition of an electron donor or
by lowering the isoprene concentration. Finally, further attempts
to characterize the saturated segments turned out to be difficult
because of a probable diversity of the segment units.
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20 Acknowledgements
Christelle Absalon is greatly thanked for Maldi-TOF MS
analysis. This work was supported in part by the ANR project
“SYNBIORUB” ANR-08-BLAN-0075.
Notes and references
25 a Univ. Bordeaux, CNRS, INP Bordeaux, LCPO, UMR 5629, F-33600
Pessac, France. Fax: +33 540 008 487; Tel: +33 540 002 745; E-mail:
peruch@enscbp.fr
b. Research Institute for Physical Chemical Problems of the Belarusian
State University, 220030 Minsk, Belarus
30 c Univ Lyon, CNRS, INSA-Lyon, IMP, UMR5223, F-69621 Villeurbanne,
France
† Electronic Supplementary Information (ESI) available: 1H and 13C
NMR spectra, 2D NMR spectra, Maldi-Tof spectra, SEC chromatograms.
See DOI: 10.1039/b000000x/
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