Tuning the activity of alternative Ru-based initiators for ring-opening metathesis polymerization of norbornene and norbornadiene by the substituent in 4-CH2R-piperidine

Article abstract of DOI:10.1016/j.molcata.2014.01.012

The novel [RuCl₂(PPh₃)₂(4-CH ₂R-pip)] complexes, with R = H (complex 1), Ph (2) or OH (3), were synthesized and applied as initiators for ROMP of norbornene (NBE) and norbornadiene (NBD) under different reaction times, temperatures and monomer concentrations. There is a clear difference in the homopolymer yields in the order 1 > 2 > 3 at [monomer]/[Ru] molar ratio of 5000, at 25 C for 5-60 min. Difference in the yields tends to disappear at 50 C, with quantitative yields for 15-30 min with any type of initiator. Results from copolymers obtained at RT for 60 min from fixed amounts of NBE with four different amounts of NBD suggest that the type of initiator also affects the reactions, with more insertion of NBD with 1. The occurrence of cross-linking enhanced as the NBD loading increased, evidenced by decrease in the M_c and increase in the T_g values, besides the influence of the type of initiator.

Full text of DOI:10.1016/j.molcata.2014.01.012

Journal of Molecular Catalysis A: Chemical 385 (2014) 46–53
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Tuning the activity of alternative Ru-based initiators for ring-opening
metathesis polymerization of norbornene and norbornadiene by the
substituent in 4-CH2R-piperidine
Henrique K. Chavesa, Camila P. Ferraza, Valdemiro P. Carvalho Jrb,
Benedito S. Lima-Netoa,∗
a Instituto de Química de São Carlos, Universidade de São Paulo, CP 780, CEP 13560-970, São Carlos, SP, Brazil
b Departamento de Física, Química e Biologia, Faculdade de Ciências e Tecnologia, Universidade Estadual Paulista, CEP 19060-900, Presidente Prudente, SP,
Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The novel [RuCl2(PPh3)2(4-CH2R-pip)] complexes, with R = H (complex 1), Ph (2) or OH (3), were synthe-
sized and applied as initiators for ROMP of norbornene (NBE) and norbornadiene (NBD) under different
reaction times, temperatures and monomer concentrations. There is a clear difference in the homopoly-
mer yields in the order 1 > 2 > 3 at [monomer]/[Ru] molar ratio of 5000, at 25 ◦C for 5–60 min. Difference
in the yields tends to disappear at 50 ◦C, with quantitative yields for 15–30 min with any type of initia-
tor. Results from copolymers obtained at RT for 60 min from fixed amounts of NBE with four different
amounts of NBD suggest that the type of initiator also affects the reactions, with more insertion of NBD
with 1. The occurrence of cross-linking enhanced as the NBD loading increased, evidenced by decrease
in the Mc and increase in the Tg values, besides the influence of the type of initiator.
Received 20 November 2013
Received in revised form 6 January 2014
Accepted 14 January 2014
Available online 24 January 2014
Keywords:
ROMP
Ruthenium
Piperidine
Norbornene
Norbornadiene
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
search for novel active catalytic species, which can be readily gen-
erated in situ at low cost from either organometallic or Werner
Olefin metathesis (OM) is an efficient accessible synthetic tool
for tailoring organic molecules and producing polymers with reten-
tion of the olefinic unsaturation in the resulting compounds [1–4].
New approaches using this type of reaction have been con-
tinuously tested worldwide [5–7] owing to the development of
well-defined transition metal-based initiator complexes. This is a
consequence of the recognition that the mechanism of catalytic
reaction occurs via metallocarbene species [5–7].
In particular, Ru-based Grubbs-type initiators have been used
for various OM applications with very good activity even in the
presence of many organic functional groups, with reasonable sta-
bility in the presence of air and moisture [5–7]. This behavior can be
associated with the chemical nature of the ruthenium ion combined
with the presence of appropriate ancillary ligands in the coordina-
tion sphere.
type initiators, is still a stimulating challenge [8–14]. This could be
an alternative for the well-defined Ru-based initiators.
Our research group has developed [RuCl2(phosphine)x(amine)y]
type complexes for ring-opening metathesis polymerization
(ROMP) and ROM-copolymerization (ROMCP) [15–21]. The elec-
tronic combination of ␲-acceptor phosphine with ␴-donor amine
is a convenient modular approach to tune the reactivity of such
initiators when varying the amine electronic nature, in addition
to generating an appropriate steric hindrance balance. These are
long-term air-stable complexes because of their phosphine nature
and they present high reactivity within few minutes at room tem-
perature. The resulting polymers show moderate PDI values from
runs at [monomer]/[Ru] molar ratio of 5000–15,000. It is worth
mentioning that this type of compound utilizes cheap friendly lig-
ands, which could be suitable for preparative scale. In particular,
successful results were obtained combining PPh3 with piperidine
or 3,5-dimethyl-piperidine [19–21], and this fact was an alert to
explore other functionalized piperidines.
The understanding of the types of initiators is very important
for planning modifications in the search for quality improvements
and the discovery of new starting compounds [1–7]. Besides the
importance of carbene moiety to well-defined OM initiators, the
Therefore, as part of a continuous interest in the development
of alternative initiators for ROMP for preparative scale, this article
reports the preparation of [RuCl2(PPh3)2(4-CH2R-pip)] type com-
plexes, where the R substituent is H (for complex 1), Ph (for 2) and
OH (for 3). Ethyldiazoacetate (EDA) was used as the starting source
∗
Corresponding author. Tel.: +55 16 33739953; fax: +55 16 33739976.
E-mail address: benedito@iqsc.usp.br (B.S. Lima-Neto).
1381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2014.01.012

H.K. Chaves et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 46–53
47
of carbene. The study aims to evaluate how minor changes in the
amine ligand can influence the metal reactivity for ROMP of NBE
and NBD under different conditions of reaction time, temperature
and monomer concentration.
The occurrence of ROMCP with novel initiators was also stud-
ied when the reactions were started with a fixed quantity of
NBE (5000 equiv./[Ru]) and different amounts of NBD (500, 1000,
1500 and 2000 equiv./[Ru]). Reactions with different NBD loading
were performed to evaluate the occurrence of crosslinking in the
resulting material and the influence in the thermal–mechanical
properties.
the case of 2, and 6.16 mmol in the case of 3. Then NBD was
added to result in a [NBD]/[Ru] = n (means, [NBD]n): 0.63 mmol
(1), 0.57 mmol (2), and 0.61 mmol (3) for n = 500; 1.26 mmol (1),
1.15 mmol (2), and 1.23 mmol (3) for n = 1000; 1.88 mmol (1),
1.72 mmol (2), and 1.85 mmol (3) for n = 1500; and 2.51 mmol (1),
2.29 mmol (2), and 2.46 mmol (3) for n = 2000. The polymerization
reaction was initiated by adding 1.1 ␮mol of an initiator (1, 2 or
3), followed by the addition of 2 ␮L of EDA. After 60 min, methanol
(c.a. 5 mL) was added and the precipitated material was filtered,
washed with methanol, and dried in vacuum oven at 27 ◦C until a
constant weight was achieved.
For NMR measurements, the isolated polymers were re-
precipitated with methanol from CHCl3 solutions, and then
dissolved in CDCl3.
2. Experimental
The films for DMA measurements were prepared in Teflon molds
(disks of 65 mm) via solution-cast from saturated CHCl3 solutions.
2.1. General remarks
All manipulations were performed under argon or nitro-
gen atmosphere. HPLC-grade CHCl3 was used throughout. Other
solvents were of analytical grade and were distilled from the appro-
priate drying agents prior to use. Other commercially available
reagents were used without further purification. RuCl3·xH2O, nor-
bornene (NBE), norbornadiene (NBD), 4-methylpiperidine (4-CH3-
pip), 4-benzylpiperidine (4-CH2Ph-pip), 4-piperidinemethanol
(4-CH2(OH)-pip), and ethyldiazoacetate (EDA), from Aldrich were
used as acquired. The [RuCl2(PPh3)3] complex was prepared fol-
2.4. Equipment
Elemental analyses were performed in a Perkin-Elmer CHN 2400
in the Elemental Analysis Laboratory at the Institute of Chemistry –
USP. ESR measurements from solid sample were carried out at 77 K,
using a Bruker ESR 300C apparatus (X-band) equipped with a TE102
cavity and HP 52152A frequency counter. FTIR measurements were
performed in CsI pellets on a Bomem FTIR MB 102. The NMR (1H;
lowing the literature and its purity was verified by satisfactory
1
1
13C{ H}; 31P{ H}) spectra were obtained in CDCl3 at 25.0 0.1 ◦C
using Bruker AC-200 and Bruker AVANCE-III spectrometers. The
obtained chemical shifts were reported in ppm relative to TMS or
85% H3PO4. SEC analyses were carried out on a Shimadzu Promi-
nence LC system equipped with a LC-20AD pump, a DGU-20A5
degasser, a CBM-20A communication module, a CTO-20A oven at
27 ◦C, and a RID-10A detector connected to three PL gel columns
(5 ␮m MIXED-C: 30 cm, Ø = 7.5 mm). Retention time was calibrated
with standard monodispersed polystyrene using HPLC-grade CHCl3
as eluent. PDI is Mw/Mn. The storage modulus (Eꢀ) and loss tan-
gent (tan ı) were recorded as a function of the temperature using a
Netzsch Instruments DMA 242C, with a heating rate of 3.0 ◦C/min,
working in the tensile mode at a fixed frequency of 1 Hz. Liquid N2
was used to cool the sample and provide inert atmosphere for the
analyses.
1
elemental analysis and spectroscopic examination (31P{ H} and 1
H
NMR; FTIR) [22–24]. The room temperature (RT) was 24 1 ◦C.
2.2. Synthesis of the [RuCl2(PPh3)2(4-CH2R-pip)] complexes (1, 2
and 3)
The amine (4-CH3-pip, 4-CH2Ph-pip or 4-CH2(OH)-pip;
0.34 mmol) was added to a solution of [RuCl2(PPh3)3] (0.26 mmol;
0.25 g) in acetone (40 mL). The resulting dark green solution was
stirred for 2 h at RT. A green precipitate was formed, filtered,
washed with methanol and ethyl ether, and then dried in vacuum.
Complex
1
(R = H): 75% yield. Analytical data for
RuCl2P2NC42H43 are 63.40C, 5.45H, and 1.76% N; found 63.59C,
5.47H, and 1.88% N. FTIR in CsI: 322 cm−1 for ꢀ(Ru–Cl); 3228 cm−1
1
for ꢀ(N H). 31P{ H} NMR in CDCl3: 62.7 ppm (s).
Complex
2
(R = Ph): 58% yield. Analytical data for
3. Results and discussion
RuCl2P2NC48H47 are 66.13C, 5.43H, and 1.61% N; found 66.41C,
5.37H, and 1.72% N. FTIR in CsI: 320 cm−1 for ꢀ(Ru–Cl); 3257 cm−1
1
for ꢀ(N H). 31P{ H} NMR in CDCl3: 62.7 ppm (s).
3.1. Synthesis and characterization of the new complexes
Complex
3
(R = OH): 67% yield. Analytical data for
The isolated compounds presented CHN elemental analyses in
agreement with five-coordinated [RuCl2(PPh3)2(4-CH2R-pip)] type
complex compositions, with R = H (complex 1), Ph (complex 2), or
OH (complex 3).
RuCl2P2NC42H43O are 62.15C, 5.34H, and 1.73% N; found 62.19C,
5.26H, and 1.83% N. FTIR in CsI: 323 for cm−1 for ꢀ(Ru–Cl);
1
3230 cm−1 ꢀ(N H). 31P{ H} NMR in CDCl3: 62.7 ppm (s).
2.3. Polymerization reactions
Typical amine stretching bands in the FTIR spectra characterized
the amines in the coordination metal sphere. The ꢀ(N H) bands
were shifted toward lower wavenumbers (3220–3260 cm−1) when
compared to the ꢀ(N H) bands in the uncoordinated piperidine
ligands (c.a. 3280 cm−1), which are consistent with the coordina-
tion of the 4-CH2R-piperidine molecules. The FTIR spectra showed
only one ꢀ(Ru–Cl) band at c.a. 320 cm−1 in each case, suggesting
that the two Cl− ligands are arranged in the trans positions, as in
[RuCl2(phosphine)3] with phosphine = PPh3 or PPh2Bz [19,22–26].
The 31P NMR spectrum in CDCl3 at 25 ◦C showed only one singlet
signal at 62.7 ppm in each case, suggesting equivalent phosphine
ligands. Correlating this fact with the observations from the FTIR
spectra with respect to the Cl− ligands, the complexes can be
arranged in a square pyramidal (SP) type configuration, with the
4-CH2R-piperidine ligands at the apical positions (Fig. 1). The ESR
spectra were silent, suggesting that the complexes are diamagnetic
In a typical ROMP experiment, 1.1 ␮mol of an initiator (1, 2 or
3) was dissolved in 2 mL of CHCl3 with an appropriate amount of
monomer (NBE or NBD), followed by the addition of EDA. Usually,
the solution was gelled in 1–2 min, but the reaction mixture was
stirred for an additional period at 25 or 50 ◦C in silicon oil bath
(
1 ◦C). At RT, methanol (c.a. 5 mL) was added and the polymer
was filtered, washed with methanol, and dried in vacuum oven at
27 ◦C until a constant weight was achieved. The reported yields are
average values from catalytic runs performed at least three times,
with maximum 10% error. The isolated polyNBEs were dissolved in
2 mL of CHCl3 for GPC data.
In a typical ROMCP experiment, an appropriate NBE amount was
dissolved in 2 mL of CHCl3 at RT to result in a [NBE]/[Ru] = 5000
(means, [NBE]5000): 6.30 mmol in the case of 1, 5.74 mmol in

48
H.K. Chaves et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 46–53
Fig. 1. Illustration of the possible structures of complexes 1 (R = H), 2 (R = Ph), and 3 (R = OH).
in a low spin d6 electronic configuration of RuII complexes, giving
support to the SP arrangements [23].
100
90
80
70
60
50
40
30
20
10
0
The 31P NMR measurements showed the appearance of a signal
at 45.0 ppm as a function of time with concomitant disappearance
at 62.7 ppm in each case. This fact is associated with the intra-
molecular geometrical rearrangement to the trigonal bypiramidal
(TBP) type configuration (Scheme 1, species II), probably owing
to the steric hindrance of PPh3 in the SP configuration. The PPh3
ligands are now localized in the axial axis, remaining chemically
equivalent, whereas the 4-CH2R-pip is in the equatorial planes of
the complexes. Further, the Cl− ligands remained in the equato-
rial planes, but with smaller Cl Ru Cl angles than those of the SP
arrangements [23].
2.13
2.02
2.07
2.14
2.14
2.26
2.13
1.80
2.40
2.19
2.10
5.40
4
2.89
5.80
6.60
6
The signal at 45.0 ppm was not observed when the complexes
were dissolved in PPh3 or amine solutions. Similar behavior was
also observed in previous studies on PPh3 Ru amine complexes
[15,19,20].
0
2
8
EDA volume ( L)
No other signal was observed in the 31P NMR spectra for 12 h.
This fact indicates the absence of dissociated PPh3 ligands or dimer
complexes in the solution, unlike the case of [RuCl2(PPh3)3] upon
dissolution in CHCl3 at RT [22–24].
Fig. 2. Dependence of yield on the [EDA]/[Ru] molar ratio for ROMP of NBE with 1
(᭹), 2 (᭹), and 3 (᭹); [NBE]/[Ru] = 5000 with 1.1 ␮mol of initiator in CHCl3 at 25 ◦
C
for 5 min. The numbers are the PDI values for each run.
Fig. 4 shows that quite quantitative yields were obtained for
[NBE]/[Ru] molar ratios in the range of 1000–5000 at 50 ◦C for
30 min. At 25 ◦C, c.a. 30% improvement was observed with 2 and
3 when the molar ratios increased from 1000 to 5000, whereas the
yields practically doubled when using 1 (from c.a. 35 to 65%). In this
way, the reactions are monomer dependent and the yields appear
to be susceptible to the initiator.
It is interesting to emphasize that the variations in both Ru Cl
stretches and 31P-NMR signals were not significant when changing
the R substituent groups in the 4-CH2R-piperidine complexes. At a
first moment, this can induce the thought that the R substituents
are innocent.
3.2. Synthesis of polymers from NBE
100
90
80
70
60
50
40
30
20
10
0
The yields showed favorable dependence on the volume of EDA
up to 2 ␮L ([EDA]/[Ru] ∼ 20), with a tendency of decreasing for
larger volumes (Fig. 2). The excess of EDA ([EDA]/[Ru] ∼ 20) can
inhibit the propagation reaction owing to a competitive coordi-
nation of EDA moiety to the catalytically active species or to the
occurrence of secondary reactions, as reported in other studies
[12,26–28]. This implies uncontrolled reactions, resulting in poly-
mers with high PDI values (Fig. 2; Table S1). It is worth noting the
same curve profiles in Fig. 2 when changing the initiator. How-
ever, the difference in results in each case is clear. The best yields
and PDIs (from c.a. 2.1 to 2.4) were obtained with 1; whereas
with 3, the yields dropped to nearly zero and the PDIs increased
significantly (from c.a. 1.8 to 6.6). Considering the good results
with 2 ␮L of EDA, this volume was selected for the next stud-
ies.
0
10
20
30
40
50
60
In experiments increasing the reaction time at 25 ◦C (Fig. 3),
the yields increased up to 85% for 60 min when the initiator was
changed, following saturation curve profiles in the order 1 > 2 > 3.
At 50 ◦C, the yields only diverged for 5 min of reaction; quantitative
yields were obtained for 5 min with 1 and for 15 min with 2 and 3.
Time (min)
Fig. 3. Dependence of yield on the reaction time for ROMP of NBE with 1 (᭹), 2 (᭹)
and 3 (᭹); [NBE]/[Ru] = 5000 with 1.1 ␮mol of the initiator in CHCl3 at 25 ◦C (dash
line) and 50 ◦C (solid line); 2 ␮L of EDA.

H.K. Chaves et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 46–53
49
Scheme 1. Proposed mechanism for the geometrical rearrangement (I → II) and the reaction with NBD (I → V).
For runs at [NBE]/[Ru] = 7000 in 2 mL of solvent, the yields
decreased using any initiator either at 25 or 50 ◦C (Fig. 4). This
can be associated with the fast gelation of solution, affecting the
propagation step. In experiments with 3 at [NBE]/[Ru] = 7000 in
10 mL, the gelation effect was minimized, resulting in 92% yield
(PDI = 2.8; Mw = 6.4 × 104 g mol−1). For runs with molar ratio of
10,000 at 50 ◦C in the presence of 3, the yield dropped to 76%, but
with large Mw (1.3 × 105 g mol−1; PDI = 2.8).
ROMP initiation. With phosphine, the induction period was prob-
ably delayed, but ROMP occurred. Although the initiation and
propagation reactions could also be affected, the major inhibitory
process must be the release of PPh3 in the induction periods,
as earlier discussed with similar [RuCl2(PPh3)(amine)] complexes
[19,20]. Thus, the polyNBE yield as a function of time at different
temperatures can be explained (Fig. 3), where favorable conditions
to release PPh3 at 50 ◦C were established, resulting in better yields.
Longer reaction time is then necessary for reaching high yields at
25 ◦C. In this context, a long induction period occurred at 25 ◦C,
whereas a rapid production of the catalytic species at 50 ◦C pro-
vided better yields for short periods. Consequently, the induction
period is critical to make use of these initiators for ROMP.
It is clear that the reaction time and monomer concentration
improved the polymer yields. However, the reaction appeared to be
more sensitive to temperature, where the yields were independent
of the initiator type at 50 ◦C.
Fig. 5 shows correlation among the PDI values for different reac-
tion times at [NBE]/[Ru] = 5000. In the case of 1, except for 5 min,
the PDI values did not show dependence neither on the reaction
time nor temperature, contrary to the case of 2 where large vari-
ance was noted. Excluding the value for 15 min at 25 ◦C with 2, the
remaining values are equal; the same behavior is observed with 3,
where the values are roughly the same at 25 ◦C and enhance up to
0.3 units at 50 ◦C. The results with 3 are the lowest in the set, either
at 25 or 50 ◦C.
Experiments performed with excess of PPh3 showed c.a. 18–32%
yield at 50 ◦C for 30 min at [NBE]/[Ru] = 5000. In the presence of the
respective amine, the yields were less than 2% of polyNBE under the
same conditions. In the latter case, the amine probably coordinated
into the SP vacant position of the starting complexes, preventing
100
90
80
70
60
50
40
30
20
10
The Mw values increased one order of magnitude (from 104 to
105 g.mol−1) in each case when the experiments were carried out
at 50 ◦C and [NBE]/[Ru] = 5000, with the tendency to decrease the
PDI values, which were not time dependent, as already commented
(Fig. 5). Considering the improvement in the yields at 50 ◦C for short
periods, the results from Mw and PDI can support the fact that tem-
perature provides short induction periods. The Mw values from the
experiments as a function of [NBE]/[Ru] molar ratio were within
the same order of magnitude (104 g mol−1), except for 5000 units
at 50 ◦C (105 g mol−1), with variation on the PDI values (Table S2).
0
1000 2000 3000 4000 5000 6000 7000
[NBE]/[Ru] molar ratio
Fig. 4. Dependence of yield on the [NBE]/[Ru] molar ratio for ROMP of NBE with 1
(᭹), 2 (᭹) and 3 (᭹); 1.1 ␮mol of initiator in CHCl3 at 25 ◦C (dash line) for 5 min and
50 ◦C (solid line) for 30 min; 2 ␮L of EDA.

50
H.K. Chaves et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 46–53
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
100
90
80
70
60
50
40
30
20
10
0
10
20
30
40
50
60
0
Time (min)
0
1000
2000
3000
4000
5000
[NBD]/[Ru] molar ratio
Fig. 5. Correlation among the PDI values for different reaction times for ROMP of
NBE with 1 (᭹), 2 (᭹) and 3 (᭹); [NBE]/[Ru] = 5000 with 1.1 ␮mol of initiator in CHCl3
at 25 ◦C (dash line) and 50 ◦C (solid line); 2 ␮L of EDA.
Fig. 7. Dependence of yield on the [NBD]/[Ru] molar ratio for ROMP of NBD with 1
(᭹), 2 (᭹), and 3 (᭹); 1.1 ␮mol of initiator in CHCl3 at 25 ◦C for 60 min; 2 ␮L of EDA.
Runs in air for 30 min at 50 ◦C and [NBE]/[Ru] = 5000, resulted
in yields in the range of 68–77% with 1, 2 or 3. Although this is a
decrease of c.a. 30% in yield compared with the runs carried out
in argon, these initiators presented reasonable reactivity in the
presence of O2 from the air.
Quantitative yields were also obtained for reactions at molar
ratios of 7000 and 10,000 when using complex 3 for 30 min at 50 ◦C.
The polyNBD were insoluble in CHCl3 and molar weights were
not obtained.
It is worth mentioning that the catalytic activities of 1, 2 and
3 for ROMP of NBE are comparable to other systems, where the
Ru carbene activity species were also generated in situ by diazo-
compounds [29–32].
3.4. Concerning the relative catalytic activities of initiators
The yields for both polyNBE and polyNBD were dependent on
the reaction time and starting [monomer]/[Ru] molar ratio with dif-
ferent trends in the curve profiles. At 25 ◦C, the curves for polyNBD
show quantitative yields (Figs. 6 and 7), whereas the curves for
polyNBE tend to present saturation profiles far from quantita-
tive yields (Figs. 3 and 4). At 50 ◦C, quantitative results for both
monomers are observed in spite of the different reaction times
(15 min for polyNBE and 30 min for polyNBD). In particular, the
yields for 5 min show a tendency to be lower for polyNBD than for
polyNBE (Figs. 3 and 6).
3.3. Synthesis of polymers from NBD
PolyNBD yields for different reaction times increased from c.a.
35% for 5 min to quantitative yields for 60 min at 25 ◦C with any
initiator. At 50 ◦C, quantitative results were obtained after 30 min
regardless of the initiator. Difference in the reactivity in the same
order 1 > 2 > 3 can be verified either at 25 or 50 ◦C, as observed for
ROMP of NBE (Fig. 6).
An explanation for the different profiles is the occurrence of
a double coordination of the NBD to the ruthenium metal cen-
ter, which can lead to a sequence of reactions following 31P NMR
analyses, as illustrated in the Scheme 1.
Quantitative consumption of NBD occurred at 25 ◦C for 60 min
with all initiators at [NBD]/[Ru] molar ratios from 2000 to 5000
(Fig. 7). Difference in the yields in the order 1 > 2 > 3 for [NBD]/[Ru]
molar ratios lower than 2000 can also be observed.
The 31P NMR spectra (Fig. S1) of the complexes in the presence
of NBD showed new species in solution (Scheme 1), with the dis-
appearance of signal at 62.7 ppm (species I) and the appearance
of new singlet signals at −4.86, 22.4, 23.2, 29.8, and 32.9 ppm. It
is reasonable to note the signal at 22.4 ppm to a six-coordinated
species with NBD (species III), which loses a PPh3 to produce a
five-coordinated complex with signal at 23.2 ppm (species IV); this
implies the presence of signals at −4.86 ppm for free PPh3 and
29.8 ppm for OPPh3. The species IV is in equilibrium with a NBD-
chelated six-coordinated isomer with signal at 32.9 ppm (species
V).
100
90
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70
60
50
40
30
20
10
0
The COSY-NMR spectra of the complexes obtained in the pres-
ence of NBD support the idea of a Ru NBD double coordination.
The spectra showed four groups of signals (Fig. S2), which were not
observed neither in the spectrum of the complex, nor in the spec-
trum of the NBD, when they were evaluated separately. Signals at
5.12 and 4.73 ppm were assigned to the H2,3 and H5,6 olefinic pro-
tons, respectively, with the NBD as a chelating agent (Scheme 1,
species V) [33–35]. In this case, the olefinic protons are not
magnetically equivalent because the two double bonds are in dif-
ferent chemical environments. Signals at 4.48 and 4.10 ppm were
attributed to the olefinic protons H2,3, corresponding to the species
IV and III, respectively, where the NBD is mono-coordinated. It was
0
10
20
30
40
50
60
Time (min)
Fig. 6. Dependence of yield on the reaction time for ROMP of NBD with 1 (᭹), 2 (᭹),
and 3 (᭹); [NBD]/[Ru] = 5000 with 1.1 ␮mol of initiator in CHCl3 at 25 ◦C (dash line)
and 50 ◦C (solid line); 2 ␮L of EDA.

H.K. Chaves et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 46–53
51
possible to observe the coupling of the H2,3 and H5,6 with the H1,4
(3.82 ppm), confirming that these signals are related to the NBD.
The occurrence of competitive reactions can afford long
induction period, resulting in low ROMP yields of polyNBD for
short periods. However, quantitative monomer consumption was
obtained when the reaction time was increased (Fig. 6). In addi-
tion, higher temperatures disfavor the double coordination and,
consequently, quantitative yields were obtained at 50 ◦C.
When comparing the catalytic activity of complexes, even in the
reactions with NBD, it is interesting to point out that 1 showed the
highest results under the studied conditions, followed by 2 and 3.
The unique difference in the complexes is the R substituent para-
positioned in the piperidine ring, that is, CH2(H) in 1, CH2(OH)
in 2, and CH2(Ph) in 3.
PPh3 and the ␲-electronic ring from the Ph in 4-CH2Ph-pip,
(PPh3)C H· · ·␲/Ph(pip), as observed for other complexes [36,37].
These cooperative effects stabilize the five-coordinated adduct
avoiding the release of the PPh3 ligand from the Ru center. In this
case, the rearrangements I → II (Scheme 1) were faster than with
3; the relative time is in the following order: 1 > 2 > 3.
The OH and the Ph interactions can explain the differences
noted in the yield results. Whereas the electronic interactions in
2 and 3 provide low Ru activities, the CH3 group in 1 is inno-
cent, resulting in the reactivity order 1 > 2 > 3. At 50 ◦C, however,
these inter/intramolecular interactions tend to disappear, afford-
ing quantitative yields for both monomers because of the absence
of Ru-deactivations from the piperidine R substituents.
Significant steric hindrance effect around the metal center from
the substituent was not expected, as they were axially positioned
(Fig. 1). This was supported by the fact that the R substituent size
order (H < OH < Ph) does not correlate to the general yield results of
ROMP (H > Ph > OH).
3.5. Synthesis of copolymers from NBE and NBD
Four ROMCP batches were carried out mixing a fixed amount
of NBE with four different amounts of NBD at RT for 60 min,
in the presence of an initiator. The samples were labeled
as [NBE]5000[NBD]n, where the subscript numbers indicate the
[monomer]/[Ru] molar ratio for each starting monomer. The n value
(500, 1000, 1500 and 2000) represents the loading of NBD in the
starting compositions.
The other parameter to evaluate yield results should be asso-
ciated with the electronic effects. The methyl group in 1 (R = H) is
electron donor, whereas the benzyl in 2 (R = Ph) and the methanol
groups in 3 (R = OH) behave as electron-withdrawing. Thus, the
hydroxyl substituent in the case of 3 should afford the lowest
piperidine ␴-donation to activate the monomer via electronic
synergism, with small tendency to produce polymer. Similar eval-
uation could be elaborated for 2. On the other hand, enhanced
yields should be expected in the case of 1 because of the con-
trary electronic effect. However, an electronic effect from the
substituent to modulate the behavior of amine ligands through
amine → Ru → monomer electronic synergism is contradictory to
the very small difference in the expected pKa values.
Apart from steric and electronic effects from 4-CH2R-pip, a
suggestion to understand the difference in the reactivity of the ini-
tiators is the occurrence of intra/intermolecular interactions. In the
case of 3 (R = OH), an intermolecular Ru O interaction between
neighbor Ru complexes can block the vacancy positions in the
Ru metal centers, increasing the induction period. The occurrence
of Ru O interactions in pure solutions hinders the geometrical
rearrangement from SP to TBP (species I → II; Scheme 1), as well
as the progress of ROMP when in the presence of monomer and
EDA. This assumption is supported by the fact that the signals at
45.0 ppm in the 31P NMR spectra from complex 3 in pure solutions
took relatively less time to appear than in the cases of complexes 1
and 2.
The isolated materials were analyzed by 13C NMR spectroscopy
and the signals in the spectra suggested the presence of M1M2
and M2M1 heterodyads, confirming the production of poly[NBE-co-
NBD] copolymers (Fig. S3); the M1 and M2 representations denote
the NBE and NBD mer-units, respectively. The spectra showed
signals associated with the olefin (132–135 ppm) and methylene
(32–49 ppm) carbons, following assignments early described (Table
S3) [38,39].
The cis-double bond fractions (ꢁc) were determined from the
ꢀ
ꢀ
C1,4 and C1 ,4 methylene carbon signals (Table 1). The ꢁc values
show similar stereo-selectivity in the copolymer samples for both
NBE and NBD, with a slight tendency to trans stereo-selectivity for
the NBE mer-units. The obtained results correlated well with the
literature [38,39], where similar Ru-based catalysts usually do not
produce selective synthesis of highly cis-polyNBEs. These initiators
afforded a slight trans configuration of the carbon-carbon double
bond and a blocky distribution of the double bond dyads [34,35].
Further, the microstructures of the resulting polymers were similar
to the data previously reported for the copolymers prepared with
the parent complexes [21,33].
The calculated reactivity ratios rc (cc/ct) and rt (tt/tc) of c.a.
0.8 and 1.4, respectively, with an rc·rt value of c.a. 1.2, suggest a
blocky distribution, following the literature (ꢁc values in the range
of 0.35–0.85 with rc·rt > 1 [1]). The rc·rt values for NBD units were
In the case of complex 2 (R = Ph), weak intramolecular CH
␲
interactions can be formed between the CH of the Ph ring from
Table 1
Fraction of cis double bonds (ꢁc), rc·rt and M2/M1 ratios determined from the 13C and 1H NMR spectra of the copolymers obtained from different starting [NBE]5000[NBD]n
compositions.
1,4
1ꢀ ,4ꢀ
Initiator
n value
ꢁc
ꢁc
rc·rt
M2/M1 ratio
13C
1H
13C
1H
(C1,4
)
Found
Calcd.
1
500
1000
1500
2000
0.44
0.44
0.45
0.45
0.45
0.44
0.45
0.46
0.44
0.58
0.49
0.50
0.45
0.56
0.50
0.50
1.18
1.22
1.29
1.24
0.08
0.11
0.18
0.23
0.10
0.20
0.30
0.40
2
500
1000
1500
2000
0.44
0.44
0.45
0.46
0.46
0.46
0.44
0.47
0.51
0.57
0.58
0.59
0.52
0.56
0.58
0.60
1.13
1.33
1.30
1.20
0.05
0.12
0.14
0.16
0.10
0.20
0.30
0.40
3
500
1000
1500
2000
0.44
0.44
0.46
0.45
0.45
0.44
0.45
0.46
0.51
0.52
0.60
0.56
0.50
0.53
0.58
0.55
1.27
1.25
1.21
1.24
0.05
0.10
0.15
0.19
0.10
0.20
0.30
0.40
M2 = NBD; M1 = NBE.

52
H.K. Chaves et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 46–53
94
92
90
88
86
84
82
80
78
76
74
2500
2000
1500
1000
500
310
n = 500
1
300
n = 1000
n = 1500
n = 2000
290
280
270
260
250
240
230
220
500
1000
1500
2000
n value
0
-50
0
50
100
150
200
Fig. 9. Dependence of Mc (dash lines) and Tg (solid lines) on the n values for the
copolymers obtained from different starting [NBE]5000[NBD]n compositions with 1
(᭹), 2 (᭹), and 3 (᭹).
Temperature (ºC)
2500
2000
1500
1000
500
n = 500
2
n = 1000
n = 1500
n = 2000
[NBE]5000[NBD]n compositions (Table 1). The data also suggested
that there is a slight tendency for more insertion of NBD when using
1, whereas the results from 2 and 3 were roughly the same, consid-
ering the expected calculated values. The same profile was noticed
for the production of homopolymers, where 1 showed a tendency to
produce more polyNBE and polyNBD than the other two initiators.
The storage modulus (Eꢀ) curves as a function of temperature
indicated that all the copolymers were in the glassy state at the
beginning of the measurements at −50 ◦C (Fig. 8). In this region,
the molecular chains were still restricted to molecular movements,
where the thermal energy supplied was not enough to provide
polymer chain movements [40–42]. When increasing the NBD
content in the copolymers, the E’ values in the glassy state also
increased. Moreover, this characteristic was superior when 1 was
used as initiator.
0
-50
0
50
100
150
200
Temperature (ºC)
2500
2000
1500
1000
500
When the temperature was increased, the samples with more
NBD tended to be less sensitive to the viscous component in each
case (˛ relaxation), where the region of glass transition initiated at
high temperatures compared with copolymers with low n values.
In the temperature range of c.a. 0–90 ◦C, the storage modulus
curves showed a drop of at least one order of magnitude related
to the Tg for all samples. The Tg values were determined from the
maxima of tan ı, which raised as the NBD amount in the isolated
materials increased (Fig. 9).
After the ˛ relaxation, the polymers reached the rubbery plateau
with Eꢀ proportional to the amount of NBD monomers. This fact
is an evidence for the existence of cross-linked networks in the
copolymers, provided by second ring-opening from the NBD [21].
The molecular weights between the cross-linked chains (Mc)
were determined from the rubbery modulus [40–42], decreasing
as the NBD amount increased from 500 to 2000 (Fig. 9). As the Mc
values decreased, the molecular motions become more restricted,
decreasing the amount of energy that could be dissipated through-
out the polymers [40–42]. This confirms that the increase of NBD in
the copolymers resulted in more cross-linked networks, increasing
the Tg and, consequently, enhancing the stiffness of materials.
The values from DMA analyses were also sensitive to the
amount of NBD in the copolymers using any initiator. At low n
values (n = 500 and 1000), a drastic increase in the Tg values was
observed, while for higher n values (n = 1500 and 2000), the Tg val-
ues almost did not change. According to the catalytic activities for
the homopolymers and copolymers, it is reasonable to affirm that
initiator 1 provided the highest NBD insertion at low NBD contents.
Nevertheless, when increasing the n values, the M2/M1 values pre-
sented saturation profiles for the three initiators, justifying similar
Tg values with n = 1500 and 2000.
n = 500
3
n = 1000
n = 1500
n = 2000
0
-50
0
50
100
150
200
Temperature (ºC)
Fig. 8. Storage modulus curves for the copolymers obtained from different starting
[NBE]5000[NBD]n compositions with 1, 2 and 3.
not calculated because the signals for the M2M2 homodyads were
not well split.
The ꢁc values determined from the 1H NMR spectra at 2.41
and 2.77 ppm (HC1,4; trans- and cis-NBE units, respectively) and at
ꢀ
ꢀ
3.18 and 3.54 ppm (HC1 ,4 ; trans- and cis-NBD units, respectively)
were in agreement with those obtained from the 13C NMR spectra
(Table 1).
ꢀ The M2/M1 ratios were also determined from the C1,4 and
ꢀ
C1 ,4 methylene carbon signals, which correspond to the relative
amount of NBD in the chains of the isolated copolymers. In gen-
eral, the M2/M1 ratio increased when increasing the n value in the

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