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Rhodium Mediated
(Co)Polymerization
of Carbenes
O
ON2
ORO
n
OR
OR OR
O
O OR
Annelie Jongerius
25-07-2008
Rhodium Mediated (Co)Polymerization of Carbenes
by Annelie Jongerius
Molecular Design, Synthesis and Catalysis
Supervisor:
Dr. Bas de Bruin
Daily Supervisor:
Drs. Erica Jellema
Second Reviewer:
Prof. Dr. C. J. Elsevier
Universiteit van Amsterdam
25-07-2008
Abstract Ligand and substrate variation have been used to gain more insight in the mechanism of
rhodium catalyzed diazoalkane polymerization. A new rhodium complex is found to give
high yields as a catalyst precursor after being stored in air for several months. Thorough
characterization and kinetic analysis of both the new and the decomposed complex were
performed and indicated that repression of byproduct formation causes increased yields.
Variation of substrate leads to change in initiation efficiencies, suggesting that the
diazoalkane plays an important role in the initiation of the reaction. After using different
substrates, it was proved possible to prepare random and gradient block copolymers of
different diazoacetates. NMR effects indicate that π-stacking occurs in these new
materials, leading to the formation of microstructures in solution. Thermal analysis also
shows increased stability of polymers, again pointing to the presence of π-stacking.
Samenvatting Het ligand en het substraat zijn gevarieerd met als doel om meer informatie te krijgen
over het mechanisme van de rhodium gekatalyseerde polymerisatie van diazoalkanen.
Een nieuw rhodiumcomplex gaf hoge polymerisatieopbrengsten nadat het enkele
maanden aan de lucht was bewaard. Na gedetailleerde karakterisatie van zowel het
nieuwe als het complex dat met lucht heeft gereageerd, alsmede kinetische analyse van de
polymerisatiereactie gekatalyseerd door beide complexen, bleek dat de hogere
opbrengsten waarschijnlijk worden veroorzaakt door onderdrukking van de vorming van
bijproducten. Substraatvariatie veroorzaakt significante verandering van de initiatie-
efficiëntie. Dit suggereert dat het diazoalkaan erg belangrijk is voor de initiatie van de
reactie. Nadat verschillende substraten zijn gebruikt voor de polymerisatie is ook
bewezen dat gradiënt-blok-copolymeren en copolymeren met willekeurige volgorden
verschillende diazoacetaten kunnen worden gemaakt. NMR-analyse wijst erop dat π-
stapeling optreedt in sommige van de nieuwe polymeren. Dit lijdt waarschijnlijk tot
vorming van microstructuren in oplossing. Thermische analyse laat zien dat de
polymeren waar π-stapeling is gevonden ook stabieler zijn dan de andere polymeren.
Contents
1 Introduction 1
2 Polymer synthesis 5
2.1 Introduction 5
2.2 Results and Discussion 5 2.2.1 Catalyst precursors 5 2.2.2 Polymerization of ethyl diazoacetate (EDA) 6 2.2.3 Complex analysis 8
2.3 Subconclusion 12
3 (Co)polymerization of other diazo carbonyl compounds 13
3.1 Introduction 13
3.2 Results and discussion 13 3.2.1 Carbene polymerization reactions 13 3.2.2 Homopolymerization 14 3.2.3 Copolymerization 15
3.3 Subconclusion 18
4 Characterization of the new polymers 19
4.1 Introduction 19
4.2 Results and Discussion 19 4.2.1 NMR effects 19 4.2.2 Pyrolysis of copolymers containing tert-butyl acetate groups 21 4.2.3 Thermal analysis 23
4.3 Subconclusion 27
5 Conclusion 29
6 Future 31
7 Experimental 33
8 List of abbreviations 39
9 Acknowledgments 41
10 References 43
1
1 Introduction Carbon backbone polymers are widely used in industry all over the world. They form an
important basis for many commercial materials like plastics, fibers, rubbers and coatings.
Development of functionalized polymers is important because functional groups on the
polymer backbone give rise to improved material properties like toughness, miscibility
and desirable surface properties like paintability and printability. Block copolymers
containing functional polymer blocks with different properties are able to phase separate
to form micellar structures.
In ‘traditional’ olefin polymerization various methods can be used. Addition
polymerization is a form of living polymerization where radical or ionic initiators are
added to a solution of monomers. A chain reaction is initiated and polymers are formed
until all monomers are used. The molecular weight can be tuned by changing the amount
of initiator; however, there is no control over stereoselectivity. The discovery of the
Ziegler-Natta catalysts, a combination of a Lewis acid and a transition metal salt co-
catalyst, made it possible to control the stereoselectivity and produce isotactic or
syndiotactic polymers. Later on with the introduction of metallocene catalysis
polymerization, control over tacticity got even better and also higher molecular weight
polymers could be obtained.1 However, due to the sensitivity of most early transition
metal catalysts towards polar functional groups, these catalysts have limited use. Late
transition metals tend to be less sensitive than early transition metals, and significant
progress has been made in particular with Ni and Pd catalysts.2,3,4 Nevertheless for the
formation of stereoregular polymers with high amounts of polar functionalities a lot of
work has to be done.
An alternative for the formation of functionalized polymers is polymerization of
substituted diazoalkanes.
Ihara and co-workers found that PdCl2 catalyzes the polymerization of alkyl
diazoacetates.5 In the presence of an amine like pyridine or triethylamine a polymeric
2
product was obtained containing amine groups at both polymer ends. Spectroscopic data
indicated that a polymethylene having ethoxycarbonyl substituents on all backbone
carbon atoms was formed. During the propagation of the reaction, α-carbon atoms of
other diazoacetate monomers insert into the metal-carbon bond with elimination of N2. In
addition to polymerization of diazoacetates, this method can also be used for
polymerization and copolymerization of other diazocarbonyl compounds, thus creating
new polymers containing different functional groups. However, these polymers have
relatively low molecular weights, all under 3 kD.6,7,8
The polyhomologation reaction as introduced by Shea and co-workers uses the insertion
of ylides into a boron-carbon bond to make polymers build of C1 units.9 After performing
this reaction in preheated toluene solutions a low polydispersity of the product was
obtained. Because EDA was expected to react with BH3, Bai, Burke and Shea reported
this reaction and found not only the expected ethyl acetate but also oligomers consisting
of a carbon backbone with an ethylacetate group on every main chain carbon atom.10
The rhodium catalyzed polymerization of ethyldiazoacetate (EDA) as reported by de
Bruin and coworkers11,12 result in stereoregular highly functionalized high molecular
weight and low polydispersity polymers with polar ethylacetate groups on every main
chain carbon atom (Scheme 1). The radical polymerization of alkyl fumarates and
maleates reported by Otzu and co-workers also produces polymers with ethylacetate
groups on every main chain carbon atom. However, these methods give no control over
the polymer stereoproperties.13
O
ON2
cat 2 Mol%CHCl3
-N2
ORO
n
OR
OR OR
O
O OR
Scheme 1: Rhodium mediated polymerization of diazoacetates
PEA
3
This method allows the introduction of functional groups on every carbon backbone atom
whereas traditional olefin polymerization produces polymers with functional groups on
every other carbon backbone atom. It is also possible to make highly stereoregular
polymers with large functional groups. Olefin polymerization metallocene catalysts do
not function properly in the presence of large steric bulk.1 Polymerization of carbenes
leads to a whole new class of polymers with yet to be investigated chemical and material
properties.
Diazocarbonyl compounds that serve as a substrate in this polymerization are easy to
make24 or commercially available and diazoesters are stable and safe to use, even in large
scale industrial synthesis. 14,15
With yields up to 50% and only minor knowledge on reaction proceedings like initiation,
propagation and termination, this subject leaves some room for improvement. The aim of
this research was to get higher yields and a deeper insight in the reaction by development
of new catalyst precursors. To explore the scope of the reaction, a number of different
diazocarbonyl compounds were investigated, and attempts to copolymerize them were
explored.
Here we report the formation of new highly functionalized polymers and gradient block
copolymers from diazoacetates. High yields are obtained by a new diene complex which
will be introduced and additional experiments will reveal new information on the
initiation and propagation of the reaction.
4
5
2 Polymer synthesis
2.1 Introduction
Not much is known about the initiation and termination mechanism for the rhodium
catalyzed polymerization of carbenes from diazoesters. New results on the influence of
reaction temperatures, the presence of several rhodium species and the formation of the
active species are presented in this chapter. This will shed some light on the formation of
the active species of this reaction.
2.2 Results and Discussion
2.2.1 Catalyst precursors
All catalyst precursors for the rhodium catalyzed polymerization of EDA consist of a
rhodium atom with a diene and a N,O-ligand. Previous research suggested that the N,O-
ligand most probably dissociates from the metal during the initiation of the reaction,
because it has no influence on the polymer properties. Varying the diene ligand leads to
polymers with different polymer properties, like Mw and polydispersity.12 In this
research, catalyst precursors containing different diene ligands will be tested for the
polymerization of EDA.
Complex 1 and 3 were previously reported to act as catalyst precursors for the
polymerization of carbenes from alkyl diazoacetates. These catalyst precursors as well as
the new complexes 2 and 4 were used for the polymerization of carbenes from EDA. The
new complex 2 shows complex 1H and 13C NMR spectra due to the presence of 2
diastereoisomers 2a and 2b. Reaction of enantiopure L-proline with racemic [{(1,5-
dimethyl-1,5-cyclooctadiene)Rh(µ-Cl)}2] in a 2:1 ratio is expected to result in formation
of two diastereomeric [(L-prolinate)Rh(dmcod)] species. According to 13C NMR, the
mixture contained mainly the 1,5-dimethyl-1,5-cyclooctadiene isomers 2a and 2b, and
signals at ~75 indicate only minor amounts of the1,6-dimethyl-1,5-cyclooctadiene
isomer.
6
The spectroscopic difference between the species 2a and 2b is small, only a few of the
signals in 1H and 13C NMR can be appointed, belonging to either one of these species.
Due to strong metal-to-olefin π-back-donation to the olefin trans to the π-donating
prolinate O-donor, these olefins shift upfield in both 1H and 13C NMR. Further
characterization of this complex is can be found in the experimental part of this report.
The new complex 4 was made with enantiopure ligands, and thus no complications from
mixtures of diastereoisomers should arise. However, no NMR characterization was
performed because of the small amount of complex that was obtained so the presence of
side products, although unlikely, is still possible.
RhN
O ORh
N
O OMe
Me
RhN
O O1 2 3
RhN
O O
Bn
Bn4
Figure 1: Catalyst precursors
2.2.2 Polymerization of ethyl diazoacetate (EDA)
Standard polymerization reactions were carried out by addition of 50 equivalents of EDA
to a solution of the catalyst precursor in chloroform at room temperature. After 14 h the
solvent was evaporated and the polymer (poly(ethyl 2-ylidene-acetate), PEA) was
isolated as a white powder and washed with methanol. Yields up to 50 % are normal, the
other 50% consist of dimer and oligomer byproducts. It is likely that the polymer is made
by a different active species than the dimers.12
Table 1 shows the results of EDA polymerization with complexes 1-4. Although yields
obtained with the new complexes 2 and 4 are not high, both complexes produce high
molecular weight polymers with relative low polydispersities (Mw/Mn). A remarkable fact
was that polymer made by catalyst precursor 4 showed 2 separate peaks in size exclusion
chromatography (SEC) of which one gave extremely high molecular weight PEA (Mw =
1197 kD) and a relatively narrow molecular weight distribution (Mw/Mn = 2.7). This
indicates that at least two different active species are formed from catalyst precursor 4 in
the presence of EDA.
7
Assuming that no chain transfer takes place (every polymer chain grows from a single
rhodium center, which dos not reinitiate chain growth after termination), the initiation
efficiency can be calculated from the number average molecular weight and the polymer
yield. Rhodium complexes tested previously for the polymerization of EDA all gave
initiation efficiencies of less than 5 percent.11,12 Presumably the rest of the rhodium is
active in dimerization and oligomerization. Also catalyst precursor 2 gives low initiation
efficiency and for complex 4 this calculation could not be made because of the bimodal
distribution.
Table 1: Polymerization of EDA
catalyst yield (%) Mw/Mn Mw (kD) Initiation (%)
1 50 5.3 147 5
3 30 2.0 540 0.3
2.7 1197 4 11
1.4 133
2 23 2.2 691 0.3
Figure 2 shows the SEC chromatograms of PEA obtained with different catalysts. On the
x axis, the retention time is shown; long polymers have a short retention time. The
difference in polymer length between polymers obtained with the cod complex and
polymers with the dmcod (2) and the dcp (3) complex is clear. The bimodal distribution
of the polymer obtained with dibenzylnorbonadiene complex (4) points to the presence of
two active species producing polymers of different length.
8
0
500
1000
1500
2000
8 10 12 14 16 18
Retention time
Inte
nsity
1
2
3
4
Figure 2: SEC chromatogram of polymers from EDA using different catalysts
2.2.3 Complex analysis
[(dmcod)Rh(pro)] (2) stored in air partly decomposes within a few months and shows an
even more complicated and crowded 1H NMR spectrum, see Figure 3. However, when
using this partly decomposed ‘old’ rhodium complex in the polymerization of EDA,
higher yields are obtained than when a freshly made ‘new’ complex is used. There may
be several explanations for this behavior:
The decomposed complex reacts faster with EDA to form the active species for
polymerization.
The species responsible for dimerization decomposes, resulting in a higher
polymer yield.
The catalyst performs better at lower concentrations.
9
In the first two cases it is beneficial to know what kind of decomposition takes place,
how many complexes are present in the reaction mixture and which complex forms the
active species.
Figure 3: 1H NMR spectra of new and old [(dmcod)Rh(pro)] in CDCl3 at RT
Because 1H and 13C NMR studies result in complicated spectra to such an extent that no
useful information can be gained from this technique (Figure 3), it was necessary to look
for different methods. With 103Rh NMR more than 14 different rhodium species were
found in the old complex, only 5 of these were also present in the new complex that
shows 6 different rhodium species. [(cod)Rh(pro)] (1) shows only one species in 103Rh
NMR.
These results together with MS-MS experiments of the old and new complex that show
fragmentation peaks due to loss of oxygen lead to the conclusion that reaction with
oxygen from the air causes decomposition of the complex. Results of elemental analysis
performed on the old complex also indicates the presence of oxygen.
New
Old
10
In order to understand the decomposition process, several experiments to speed up the
decomposition of a freshly made new batch of complex 2 were performed. Imitation of
the decomposition process via heating and addition of water was tried and polymerization
results are shown in Table 2. Storing the compound in solid state under argon seems to
elongate the durability of the complex. Leaving the fresh complex in dissolved in CDCl3
under inert atmosphere at room temperature for several weeks also did not result in any
decomposition. Heating this solution to 100˚C resulted in proline loss and low
polymerization yields. Adding water to the reaction resulted in low yields and low
molecular weight polymers. Heating solid 2 to 100˚C in air resulted in a huge decrease in
solubility but increased the initiation efficiency. Polymer yield (41 %) did not increase
much although it was a bit higher. Also filtration of the heated complex before
polymerization resulted in yields up to 40 % just like heating the complex in absence of
oxygen.
Table 2: Polymers from EDA by [(dmcod)Rh(pro)] (2)
yield % PDI Mw (kD) Initiation % special
23 2.2 691 0.3 new
10 2.8 83 1.5 new + H2O
53 4.4 397 2.6 new 25 ml
65 5 500 2.7 old
62 4.4 326 3.7 old 25 ml
41 4.9 488 1.8 heated air
40 4.5 628 1.3 heated argon
42 2.3 900 0.5 heated air + filtered
Because addition of the dimers diethylmaleate and diethylfumarate to the reaction with
complex 1 mixture lowers polymer yield (~20%), it is likely that the dimer poisons the
active species for polymerization. In that case, catalysts that make a lot of dimer
automatically further reduce polymer yield. SEC data of polymers made at lower
concentrations of substrate and catalyst 1 show evidence for the existence of different
active species. These polymers give a bimodal distribution. Possibly one of more active
species is poisoned by dimer at higher concentrations. Also performing polymerization
11
with the new complex 2 in 25 ml of chloroform instead of the usual 5, resulted in higher
polymer yields. Using a larger volume in combination with the old complex (2) had no
significant effect on the yield (Table 2).
Polymerization reactions can be followed in time with 1H NMR. The amounts of
polymer, dimer and monomer can be established from the integrals and concentrations of
oligomer can be calculated from the other concentrations. When comparing the integrals
with the isolated yields at the end of the reaction it appears that the amount of polymer is
underestimated by 10 to 15 percent in 1H NMR, subsequently the amount of oligomer is
overestimated.
Following the polymerization of EDA with both the ‘old’ and the ‘new’ complex (2) in
time reveals a significant difference in the amount of ethyl acetate-dimer formed by both
complexes (Figure 4 and Figure 5). Possibly, more dimer is formed because less EDA
was used for polymer formation. However, it is also possible that the species responsible
for dimer formation is more decomposed than species active in polymerization.
On the other hand, initiation efficiency is higher for the old than for the new complex (2)
which could also mean that this catalyst performs better at lower catalyst loadings. After
all, when a part of the complex is decomposed and ‘inactive’ the overall active catalyst
loading is lower. It is possible that when a low concentration of active catalyst precursor
is present, les dimer forming species will be formed. This results in a lower dimer
concentration and less catalyst poisoning.
12
0 50 100 150 200 250 3000
20
40
60
80
100
Am
ount
(%)
Time (minutes)
EDA polymer dimer oligomer
0 50 100 150 200 250 3000
20
40
60
80
100
Am
ount
(%)
Time (minutes)
EDA polymer dimer oligomer
0 50 100 150 200 250 3000
20
40
60
80
100
Amou
nt (%
)
Time (minutes)
EDA polymer dimer oligomer
0 50 100 150 200 250 3000
20
40
60
80
100
Amou
nt (%
)
Time (minutes)
EDA polymer dimer oligomer
Figure 4: Polymerization of EDA with new
[(dmcod)Rh(pro)] (2) followed in time with 1H NMR
Figure 5: Polymerization of EDA with old
[(dmcod)Rh(pro)] (2) followed in time with 1H NMR
2.3 Subconclusion Two new precatalyst complexes are tested in the polymerization of EDA. Both
[(dmcod)Rh(pro)] (2) and [(dbn-nbd)Rh(pro)] (4) gave relatively low yields but high
molecular weight polymers. When [(dmcod)Rh(pro)] decomposes in the presence of
oxygen, initiation efficiency of the complex increases 9 times and polymer yields up to
65% are observed. Increased yields are possibly caused by repressing dimer formation.
Performing the reaction under dilute conditions for some catalyst precursors also leads to
higher yields suggesting that the lower dimer concentration (the dimers are a poison to
the catalyst) is beneficial.
13
3 (Co)polymerization of other diazo carbonyl compounds
3.1 Introduction
Rhodium catalyzed polymerization of ethyldiazoacetate (EDA) is an interesting type of
polymerization because it leads to a highly functionalized polymers with a high amount
of polar functional groups.11,12 Only a limited amount of reactions are known to produce
polymers with polar functional groups or functional groups at every main chain carbon
atom like the radical initiated polymerization of alkyl fumarates reported by Otzu and co-
workers.13 To broaden the scope of the rhodium catalyzed polymerization of ethyl
diazoacetate it is interesting to look for other diazoalkanes that can be polymerized and
the possibility to copolymerize different diazoalkanes. In this way material properties of
the polymer, like solubility and toughness, may be tuned leading to polymers that may be
used in new materials.16 When different diazoalkanes are available in this reaction, it is
interesting to see if these can be copolymerized. Block copolymers containing immiscible
blocks may lead to phase separation and the formation of microstructures in solution or in
the bulk polymer. By adjusting the amount of each monomer, material properties can be
tuned.
3.2 Results and discussion
3.2.1 Carbene polymerization reactions
Until now all material reported on the rhodium mediated polymerization of carbenes from
diazoalkanes (Figure 6) focuses primarily on the use of EDA (a) as carbene precursor.
However, also other diazoacetates and diazoketones are known to form polymers in the
palladium mediated polymerization of diazoketones.5,6,7,8 Therefore, it is likely that also
other substrates (b-e) may be suitable to form polymerizable carbenes in the presence of
rhodium. These five carbene precursors were tested in the rhodium catalyzed
homopolymerization and copolymerization.
14
Diazoacetates a and c are commercially available and b is easily produced in two steps.
Both diazoketones are produced by diazotransfer reactions.
N2
O O
F3CN2
O
O
O
O
O
O
N2 N2N2
a b c
d e
Figure 6: Diazoalkanes
3.2.2 Homopolymerization
Reactivity of EDA (a) was already described in the previous chapter. Diazoketones d and
e showed no reactivity in the presence of rhodium complex 1 and during the reaction with
the sterically demanding diazoacetate c, only dimeric species were formed. Treating
benzyldiazoacetate (BnDA, b) with catalyst 1 and 2 resulted in the formation of
stereoregular polybenzylacetate (PBnA). The polymers were analyzed by size exclusion
chromatography (SEC) calibrated with polystyrene samples (Table 3). In the
polymerization of EDA as well as in the polymerization of BnDA, the use of dmcod
catalyst precursor 2 gives much higher yields, lower polydispersity and much higher
molecular weight polymers (Mw = 438 kD) compared to the cod analogue 1 (Mw = 83
kD). Also in the polymerization of BnDA, much higher initiation efficiencies, up to 11.8
%, are observed compared to the polymerization of EDA. This indicates that both
substrate and diene ligand are needed to form the active species. Otherwise you would
expect no differences in polymerization of different diazo substrates and there would be
no explanation for the difference in yield, Mw, polydispersity and initiation efficiency for
polymerization of EDA and BnDA. The high initiation efficiency also indicates that at
lower catalyst concentration, higher yields and longer polymers can be obtained.
Table 3 Polymerization of BnDA (b)
catalyst yield (%) Mw/Mn Mw (kD) Initiation (%)
1 20 6.3 83 11.8
2 65 4 438 3.5
15
3.2.3 Copolymerization
With the use of different substrates, it is possible to make copolymers. There are different
possibilities to copolymerize diazocarbonyl compounds.
Gradient block copolymerization
Block copolymers can be made by starting the reaction with one of the monomers and
wait until all of the first monomer has reacted. After that, the second monomer is added
and the reaction continues, elongating the polymer with a block of different monomers.
Because it is not clear if the polymerization is completely living and the possibility exists
that the catalyst dies in absence of substrate, making a defined block copolymer is
difficult. However, it is possible to initiate polymerization using only one substrate and
adding a second substrate after a period of time when most, but not all of the first
monomer has reacted (Scheme 2). This way a polymer is formed containing a block of
pure homopolymer followed by a gradient block containing both monomers and where
the concentration of the second monomer increases to the end of the polymer. (Figure 7)
These types of polymers are also called gradient block copolymers.17 Conditions for the
gradient block copolymerization have not been optimized. Therefore, it is probable that
higher yields can be obtained when using different catalyst loadings and lower
concentrations.
O
ON2
cat 2 Mol%CHCl3
-N2
OR1O
n
OR1
OR1 OR1
O
O O
R2
OR1O
n
OR1
OR2 OR2
O
O O
m
O
ON2
R1
Scheme 2: Formation of copolymers by rhodium mediated copolymerization of carbenes
16
Figure 7: Gradient copolymer
Random copolymers
A second method for the formation of copolymers is random copolymerization. When
two or more different substrates are used to initiate the polymerization a random
copolymer is formed (Figure 8). The two different monomers are ordered in a random
fashion. The ratio of both monomers in the polymer can be tuned by changing the amount
of both substrates added to the precatalyst solution.
Figure 8: Random copolymer
Homo-polymerization of c is not possible because no initiation of the polymerization
takes place. However, the monomer can be built into a polymer once the reaction has
been initiated by another monomer. In this way it is possible to make gradient and
random copolymers containing monomer c in the second gradient block.
In these experiments, the ratio between the monomers added to the reaction mixture was
1:1.7 for gradient block copolymers and 1:1 for random copolymers. As can be seen in
Table 4, according to 1H NMR integrals the composition of some polymers differs from
what would be expected by the feed ratio. Where polymers containing ethylacetate and
benzylacetate groups show more or less the expected composition, polymers with tert-
butylacetate groups contain only low amounts of tert-butylacetate. A possible explanation
for this is the steric bulk of the tert-butyl group. Because addition of tert-
butyldiazoacetate (tBuDA, c) leads to faster termination of the reaction, yields of gradient
copolymers with tBuDA (c) are strongly dependent on the amount of tBuDA (c) added
and the moment on which it was added. Also during random copolymerization the
amount of tert-butyl groups incorporated into the polymer is lower than could be
Homo block Gradient block
17
expected according to the ratio of monomers added to the reaction mixture. In
combination with BnDA (b), more tBuDA (c) is incorporated than when a copolymer of
EDA (a) with tBuDA is formed.
Not only in homo-polymerization but also in copolymerization, the initiation efficiency is
higher when precatalyst 1 or diazoacetate b is used. However, yields still seem dependent
on both catalyst and substrates used.
Monomer d and e could not be built into the polymer, not even after initiation with EDA.
Possibly, the electronic properties of the diazoketones stabilize the diazogroup so that the
reaction is unfavorable and less electron withdrawing diazoketones are needed for this
reaction.
Table 4: Copolymers of diazoacetates
Catalyst
yield
(%)
1H NMR
Mw/Mn
Mw
(kD)
Initiation
(%)
a - ab b (gradient) 2 13 1:1.7 2.1 224 1.2
a - ab c (gradient) 2 7 1:0.33 5.1 417 0.8
a - ab c (gradient) 1 20 1:0.22 3.4 137 5.2
b - bc c (gradient) 2 6 1:0.43 6.5 322 1.2
ab (random) 2 59 1:1 4.4 432 3.6
ab (random) 1 29 1:0.8 2.8 81 5.9
ac (random) 2 27 1:0.31 3.8 334 1.5
bc (random) 2 27 1:0.68 3.4 107 5.5
Figure 9 shows SEC chromatograms of the EDA-tBuDA gradient copolymer made with
catalyst 2 just before addition of the second monomer (c) and after finishing the reaction.
Just before addition of the second monomer, the growing polymer chain shows a bimodal
distribution and a high polydispersity. After addition of the second monomer (c), the
shortest polymer chains keep growing, incorporating the newly added monomer whereas
the longest polymer chains do not grow much further. That way polydispersity decreases
after adding the second monomer as the total length of the polymer increases (from 250
18
to 400 kD). Other gradient copolymerizations from catalyst 2 show similar SEC
chromatograms.
0
200
400
600
800
1000
1200
1400
10 11 12 13 14 15 16 17
Retention time
Inte
nsity
beforeafter
Figure 9: SEC chromatogram of growing polymer (precatalyst 2)
3.3 Subconclusion
After polymerization of different diazoacetates it appeared that initiation efficiency is
much dependent on the monomer that was used to initiate the reaction. Because no
diazoketones could be incorporated in the polymers, it is likely that the ester group is
important for the propagation of the reaction as well, but we cannot exclude an electronic
influence.
It proved possible to prepare random and gradient block copolymers of different
diazoacetates, leading to new polymers.
19
4 Characterization of the new polymers
4.1 Introduction
There are various methods to characterize polymers. Apart from 1H and 13C NMR also
other NMR spectroscopy techniques like 2D NMR, thermal analysis (DSC and TGA) and
size exclusion chromatography (SEC) can be applied. For polymers with molecular
weights above 1000 dalton, it is not possible to use mass spectrometry. In this chapter,
the results of thermal analysis and NMR experiments on the new polymers will be
discussed.
4.2 Results and Discussion
4.2.1 NMR effects
Because of the different steric and electronic properties of benzyl acetate compared to
ethylacetate, this polymer is expected to show some change in material properties.
After comparing 13C NMR spectra of homo PBnA and homo PEA with data on
assignment of stereochemistry, the shifts at ~171 ppm (carbonyl) and 45.1 ppm (methine)
indicate that these polymers have the same tacticity (most likely syndiotactic).12
In 1H NMR the backbone protons of PBnA shift significantly compared to the same
protons in PEA, even though the 13C NMR signals of these polymers show the same shift
for the adjacent carbons. As can be seen in Figure 10 (middle spectrum, 1:1 mixture of
homo polymers PEA and PBnA), this difference is almost 0.5 ppm. When combining
EDA and BnDA in a random copolymer, backbone peaks of both monomers shift
towards each other (Figure 10 lower spectrum).
Backbone protons shift downfield on going from homopolymers of EDA via copolymers
of EDA/BnDA to homopolymers of BnDA, whereas the O-CH2-R signals shift upfield
with increasing content of BnDA. It is proposed that clustering of the phenyl groups via
π-π interactions, as shown in Figure 10, causes the deshielding of the backbone CH and
shielding of the O-CH2-R protons. The effects are maximized in homopolymers of
BnDA, and smaller in the copolymers of BnDA and EDA. In a gradient copolymer
20
(Figure 10 upper spectrum) the first part of the polymer consists only of PEA and the last
part of the polymer consists mainly of PBnA. Therefore, the effect of the π stacking on
NMR chemical shifts is slightly less compared to the random copolymer. Because part of
the polymer chains stop growing after addition of the second monomer, also peaks of the
EDA homopolymer at δ 4.08 and δ 3.18 ppm are observed.
Figure 10: 1H NMR effects in PBnA containing polymers
A different method to see if folding of these polymers occurs is measuring the Nuclear
Overhauser Effect (NOE). When ethyl ester groups of one part of the polymer are placed
between two aromatic rings as shown in Figure 12, NOE couplings between aromatic and
ethyl protons should be present. However, when the ethyl ester and the benzyl ester are
neighboring monomers in the same polymer chain (Figure 11), also NOE interactions
between the ethyl group and the CH2 of the benzyl are expected. This is the case in the
random copolymer: NOE signals appear between the CH2 benzyl and the ethyl group. In
the gradient copolymer, there is coupling between the aromatic protons and the protons of
the ethyl but not between the benzyl CH2 and the ethyl. This suggests the polymer folds
back or two different polymer chains are folding into each other so that the ethyl groups
get close to the phenyl groups (Figure 12). Because the ethyl group is not close to the
benzyl-CH2 it is not possible that the monomers are just next to each other in the chain
(Figure 11).
Gradient block copolymer Mixed homopolymers Random copolymer
21
Because of the possibility of two chains folding into each other it is also possible that the
molecular weight found by SEC is overestimated.
OOO
O
R
OOO
O
R
OO
OO
OO
R
Figure 11: Neighboring ethylacetate and
benzylacetate monomers
Figure 12: Folding of polymers
4.2.2 Pyrolysis of copolymers containing tert-butyl acetate groups
When heating poly tert-butylacetate containing polymers to high temperatures, isobutene
is eliminated from the polymer leaving poly acetic acid groups as was reported by Otzu et
al.18 (Scheme 3) In this way also other functional groups than esters can be incorporated
in the polymer and poly acetic acid containing copolymers are known to be able to form
microstructures in solution.19
R1R2 H
R2 H
R2H
R2 HR2 H
R2H
R1 H
R1 H
R1 H
R1H
R1 HR1 H
R1H
R1 H
H
R1R3
H
R3 H
R3H
R3 HR3 H
R3H
R1 H
R1H
R1 H
R1H
R1 HR1 H
R1H
R1 H
H
O
O
O
O
O
OH
- isobutene
R2 =R1 = R3 =
∆
Scheme 3: Isobutene loss from tBuA containing polymer
22
After heating a tBuA containing polymer to 180 ˚C for several hours under vacuum, the
polymer became partly insoluble even in mixtures of polar and apolar solvents and tBu
peaks disappeared from the 1H NMR spectra leaving only broad PEA signals. This
indicates that isobutene loss took place.
Solid state 13C NMR of this tBuA containing polymer before and after heating are shown
in Figure 13 and Figure 14. The relative intensity of all peaks in both spectra is the same,
only the tert-butyl peak is significantly decreased. Remarkable is that no extra carbonyl
peak formed. This could mean that croslinking between different polymer chains takes
place via formation of anhydrides. This would also explain the insolubility of the polymer
after heating (Scheme 4).
O
O OR
R
R
R
O OR
R
R
ROt-Bu t-BuO
∆
Scheme 4: Possible formation of anhydride, crosslinking polymers after thermally induced isobutene loss
23
Figure 13: Solid state NMR of the a – ac c gradient copolymer
Figure 14: Solid state NMR of the a – ac c gradient copolymer after isobutene loss
4.2.3 Thermal analysis
Differential Scanning Calorimetry (DSC) is a technique in which the difference in the
amount of heat required to increase the temperature of a sample is measured as a function
of temperature. There are several thermal transitions that can take place during the
24
heating and the temperature at which they occur gives information about the crystallinity
and brittleness of the polymer. Glass transition temperature Tg is the temperature where
the amorphous part of the polymer goes from the glassy hard/brittle state to the rubbery
bendable state. Crystallization temperature Tc is the temperature where the polymer
crystallizes again when cooling from the melting point and the polymer chains move into
ordered arrangements. Only polymers that can crystallize have a crystallization
temperature. Melting temperature Tm is the point where the crystalline part of the
polymer melts, polymers with high melting points have stable crystal structures. Not all
polymers have a crystalline part, but all polymers have an amorphous part, even the
polymers with high crystallinity. Therefore, all polymers have Tg but not all polymers
have an Tc and Tm.17
Tg and Tc of PBnA are more clearly visible and appear at higher temperatures than those
of PEA, indicating the former polymer is more crystalline than PEA (Table 5). These data
support the supposed clustering of phenyl groups by π-π-stacking. Random polymers
show only a Tg and no Tc or Tm . This means that these polymers are purely amorphous
and contain no crystalline parts because only crystalline parts undergo melting. Gradient
copolymers containing PBnA have Tm higher than PEA but not as high as pure homo
PBnA. Examples of the DSC chromatograms of a random and a gradient copolymer are
shown in Figure 15 t/m Figure 17 on the next page.
Table 5: DSC data
Polymer Tg (˚C) Tc (˚C) Tm (˚C)
PEA 22 80 105
PBnA 50 165 190
random PEABnA 42 - -
gradient PEABnA 46 91 120*
random PEAtBuA 51 - -
gradient PEAtBuA n.d. 89 113*
random PBnAtBuA 62 - -
*weak signal
25
Figure 15: Heating curve of the a – ab b gradient block copolymer
Figure 16: Cooling curve of the a – ab b gradient block copolymer
Figure 17: Heating curve of the ab random copolymer
Tg
Tm
Tc
Tg
26
With thermogravimetric analysis (TGA) the change in weight of the polymer can be
studied in relation to the change in temperature. Thermal decomposition of pure PEA and
PBnA measured by TGA takes place in one step when heated to temperatures above 300
˚C. Gradient copolymers containing tert-butyl acetate show decomposition in several
steps (Figure 18), starting with a small weight loss at 250 ˚C which is consistent with
isobutene loss. Also thermal degradation of the rest of the polymer proceeds in two steps,
presumably because there are different decomposition temperatures for the poly ethyl
acetate and poly acetic acid part. Random copolymers also show weight loss at 250 ˚C
due to isobutene loss. This process occurs slower and less clear than with the gradient
block copolymers and is not visible when only small amounts of tBuDA are incorporated
in the polymer.
0
20
40
60
80
100
120
200 300 400 500 600 700 800
temperature (˚C)
Wei
ght (
%)
PEAgradient PEAtBuArandom PBnAtBuA
Figure 18: TGA analysis of copolymers
27
4.3 Subconclusion
PBnA containing polymers are able to form microstructures in solution and homo PBnA
is able to form stable crystalline structures in solid state which is proven by 1H NMR and
NOE NMR spectra together with DSC analysis. tBuA containing polymers show
isobutene loss when heated above 250 ˚C. The tert-butyl group is then cleaved of and
possibly an cross link between two polymer chains in the form of an anhydride is formed.
This could also explain the insolubility of the heated polymer.
28
29
5 Conclusion Apart from EDA, also BnDA can be polymerized by a [(diene)Rh(pro)] complex forming
high molecular weight stereoregular polymers. Homopolymerization of tBuDA is not
possible; however, it can be build into a polymer chain when the polymerization reaction
is initiated by EDA or BnDA. This suggests that the diazoacetate is involved in initiation
of the reaction. TGA data shows a loss of isobutene from tert-butyl acetate containing
polymers at 250˚C, thermal decomposition of the rest of the polymer takes place above
300˚C. After loss of the isobutene group the polymer becomes insoluble. Possibly this is
caused by crosslinking between different polymers via formation of anhydrides.
Random and gradient copolymers can be made with different combinations of EDA,
BnDA and tBuDA. GPC data show elongation of the polymer chain after addition of a
second monomer to the reaction mixture.
Comparison of 1H NMR chemical shifts of PBnA and PEA to the 1H NMR chemical
shifts of gradient and random copolymers from PEA and BnDA suggest that BnA
containing polymers form microstructures in solution by π-stacking of the benzylic
groups. This theory is supported by NOE couplings between benzylacetate and
ethylacetate in random and gradient copolymers. The higher Tg and Tc led us to believe
that also in solid state π-stacking occurs.
Decomposition of [(dmcod)Rh(pro)] (2) in air leads to higher yields in polymerization of
EDA. It is not clear what causes this increase in activity; but possibly the old complex
represses formation of the ethylacetate dimer. It is clear that the decomposed complex
contains more than 14 different rhodium species and produces less dimer during the
polymerization of EDA
.
30
31
6 Future For future research it is important that the reaction conditions for the copolymerization,
especially the gradient block copolymerization are optimized. Also the optimal
concentration for homo polymerization reactions should be reconsidered. From
economical and environmental perspective it is desirable to develop a catalyst that
performs at higher concentrations and low catalyst loadings in order to use less expensive
rhodium and less chlorinated solvents. Therefore it may be beneficial to screen more
catalysts and look for higher initiation efficiencies.
32
33
7 Experimental General procedures
All manipulations were performed under an argon atmosphere using standard Schlenk
techniques. Toluene distilled from sodium was used for synthesis of [((R,R)-Bn-
nbd)Rh(Cl)]220
. Chemicals have been purchased from commercial suppliers and, if not
stated otherwise, used without further purification. NMR experiments were carried out on
a Varian Inova 500 spectrometer (500 and 125 MHz for 1H and 13C respectively), a
Varian Mercury 300 spectrometer (300, 75 and 280 MHz for 1H, 13C and 19F
respectively), a Bruker ARX 400 spectrometer (400 and 125 MHz for 1H and 13C
respectively) and a Bruker DRX 300 spectrometer (9.483 MHz for 103Rh). Assignment of
the signals was aided by COSY, NOESY, 13C HSQC and APT experiments. All 103Rh
NMR experiments were performed two times. Solvent shift reference for 1H NMR
spectroscopy: CDCl3: δH = 7.26, DMSO-d6: δH = 2.50. For 13C NMR spectroscopy:
CDCl3: δC = 77.0. High Resolution Mass Spectra were recorded at the Department of
Mass Spectrometry at the University of Amsterdam using FAB+ ionization on a JOEL
JMS SX/SX102A four sector mass spectrometer with 3-nitrobenzylalcohol as the matrix.
Elemental analysis (CHN) was performed by the Kolbe analytical laboratory in Mülheim
and der Ruhr (Germany). Molecular mass distributions were measured using size
exclusion chromatography (SEC) on a Shimadzu LC-20AD system with two PLgel 5µm
MIXED-C columns (polymer laboratories) in series columns and a Shimadzu RID-10A
refractive index detector, using dichloromethane as mobile phase at 1mL/min and T = 35
˚C. Polystyrene standards in the range of 760-1880000 g/mol (Aldrich) were used for
calibration. Thermal analysis (DSC and TGA) was measured at the University of
Groningen on a DSC Q1000 V9.8 from Universal V4.3A TA instruments.
34
Synthesis of [(1,5-dimethyl-1,5-cyclooctadiene)Rh(pro)] (2)
A solution of proline (1 mmol) and NaOH (1mmol) in 10 ml MeOH
was added to a solution of rhodium 1,5-dimethyl-1,5-cyclooctadiene
chloride dimer (0.5 mmol) in 5 ml MeOH. 5 ml MeOH was added and
the reaction was stirred at room temperature for 90 minutes. The
solvent was evaporated and the remaining solid extracted with dichloromethane and
filtered over celite. After evaporation of DCM the complex was obtained in 90 % yield.
MS (FAB) calcd for RhC15H24O2N (M+H) 354.0940 Found 354.0932
Table 6: 1H (500 MHz) and 13C NMR (125 MHz) shift values (CDCl3, 292 K, δ in ppm) of 2
abc d
efgh
[(dmcod)Rh(pro)]
abc d
efgh
ij
[(dmcod)Rh(pro)]
proton 2a 2b carbon 2a 2b
a 4.05 a 100.23 and 99.83
b 3.48 3.20 b 85.84 and 84.79
c c 73.20 71.82
d 1.57, 1.46 and 1.40
d 79.58
e e
f f
g g
h
1.5 - 3.0 *
h
23 - 53 **
NH 3.97 and 3.38 i
COO-CH 4.03 3.71 j
30.44, 29.04, 28.21
and 27.58
NH-CH2 1.5 - 3.0 * COO 182.74 and 180.36
CH2-CH 1.5 – 3 * 2.14 COO-CH 65.16 63.42
COO-CH NH-CH2-CH2
NH-CH2-CH2 1.5 - 3.0 *
CH2-CH-COO
NH-CH2-CH2
23 - 53 **
* Indistinguishable signals, partially overlapping in this region ** indistinguishable signals
RhN
O OMe
Me
35
Synthesis of [(C2H4)2Rh(Cl)]221
2 grams of RhCl3·3H2O (8 mmol) was dissolved in 3 ml H2O and transferred to a flask
with 50 ml MeOH. The content of the flask was freed from oxygen and repressurized
with ethylene to 1 atm. The reaction was stirred at room temperature for 7 h after which a
brown solid was collected by filtration under argon. The product was washed with MeOH
and dried in vacuo yielding 400 mg (13%) of product.
Synthesis of [((R,R)-Bn-nbd)Rh(Cl)]220
86 mg of (R,R)-benzyl-norbornadiene (0.25 mmol) was
dissolved in 2,5 ml toluene and 50 mg of [(C2H4)2Rh(Cl)]2
(0.125 mmol) was added. The solution was stirred for 2h
under inert atmosphere, dried and purified over silica gel.
Synthesis of [(R,R)-Bn-nbd)Rh(pro)] (4)
A solution of proline (0.2 mmol) and NaOH (0.2 mmol) in 2 ml
MeOH was added to a solution of ((R,R)-Bn-nbd)Rh(Cl)]2 (0.1
mmol) in 1 ml MeOH. 1 ml MeOH was added and the reaction was
stirred at room temperature for 90 minutes. The solvent was
evaporated and the remaining solid extracted with DCM. After evaporation of DCM the
complex was obtained in 83 % yield.
Synthesis of Benzyl diazoacetate (b) 22,23
A roundbottom flask containing a solution of 3.75 gram glycine
(50 mmol) in 350 ml benzyl alcohol was cooled in ice. 11 ml of
thionylchloride was added dropwise. The mixture was heated to
110˚C for 5 h after which it was cooled to room temperature and the excess of
thionylchloride was evaporated in vacuo. Diethylether was added until some permanent
turbidity appeared. The mixture was stored at -22˚C for 36 h. A white solid was filtered
from solution, washed with cold Et2O and dried. Yielding 7.54 grams (75%) of white
solid. 1H NMR (300 MHz, DMSO d6) δ 8.56 (s, 3H), 7.37 (m, 5H), 5.20(s, 2H), 3.83 (s,
2H)
RhN
O O
Bn
Bn
Rh
Bn
Bn
Rh
Bn
BnCl
Cl
O
ONH3
Cl-
36
This was dissolved in 9 ml of water and 20 ml of dichloromethane
and 8.12 mL 5M NaNO2 were added. The solution was cooled to
-10˚C and 3.11 mL 5% H2SO4 was added. The mixture was stirred for
1 h at -10˚C after which the layers were separated, the water layer was extracted with
DCM and the organic layer washed with NaHCO3 (aq). The organic layer was dried over
Na2SO3 and the solvent was evaporated. Yield, 6.09 gram of yellow oil (71%) 1H NMR
(300 MHz, CDCl3) δ 7.35 (m, 5H), 5.22 (s, 2H), 4.80 (s, 1H)
Synthesis of α-diazophenone (d) 24
α-bromoacetophenone (5 mmol) and N,N’-ditosylhydrazine (10 mmol)
were dissolved in 25 ml of THF and cooled to 0˚C. DBU (25 mmol) was
added dropwise and the reaction mixture was left stirring at 0˚C for 10
min. The reaction was quenched with NaHCO3, extracted with Et2O and washed with
brine. The organic layer was dried over MgSO4 after which the solvent was evaporated.
The remaining solid was purified over silica with hexane:EtOAc 1:1. 1H NMR (300 MHz,
CDCl3) δ 7.75 (d, 2H) 3J = Hz, 7.53 (m, 1H), 7.42(m, 2H), 5.89 (s, 1H)
Synthesis of 1,1,1-trifluorodiazoacetone (e) 25
10 ml of trimethylsilyldiazomethane (2 M in ether) was dropped slowly into
1.4 ml trifloroacetocanhydride (10 mmol) solution in ether at 0˚C under inert
atmosphere. After removal of the solvent in vacuo the product was purified
by over silica with hexane:EtOAc 1:1 1H NMR (300 MHz, CDCl3) δ 5.80 (s, 1H) ; 19F
NMR (280 MHz) δ -77.25 (s, 3F)
General procedure for polymerization of diazocompounds
Diazocompound (2 mmol) was added to a solution of catalyst precursor (0.04 mmol) in 5
ml chloroform. The reaction mixture was left stirring under argon during the night, after
which the solvent was removed under vacuo and the remaining polymer was washed with
MeOH and dried under vacuum.
Experiments in which the polymerization was followed in time were performed using
identical conditions (1H NMR: 300 MHz, CDCl3, 292 K).
N2
O
O
F3CN2
O
O
N2
37
General procedure for gradient block copolymerization of diazocompounds
The first diazocompound (1.2 mmol) was added to a solution of catalyst precursor (0.04
mmol) in 5 ml chloroform. After 30 minutes the second diazocompound (2 mmol) was
added and the reaction mixture was left stirring under argon over night. The solvent was
removed and the polymer was washed with MeOH and dried under vacuum.
General procedure for random copolymerization of diazocompounds
Both diazocompounds (1 mmol) were added to a solution of catalyst precursor (0.04
mmol) in 5 ml chloroform. The reaction mixture was left stirring under argon during the
night, after which the solvent was removed under vacuo and the remaining polymer was
washed with MeOH and dried under vacuum.
OO
OO
OO
1
2
3
4
5 6
1 1
Table 7: 1H NMR shifts (CDCl3, 292K, δ in ppm) of polymers and copolymers
Polymer MHz 1 2 3 4 5 6
PEA 500 3.16 x x 4.06 1.21 x
BnEA 500 3.59 4.72 7.03 - 7.10 x x x
Random PEABnA 500 3.38 4.9 7 - 7.5 3.9 1.05 x
Random PEAtBuA 400 3.18 x x 4.07 1.23 1.43
Random PBnAtBuA 400 3.62 sh-r 4.79 6.9 - 7.4 x x 1.17
Gradient PBnAtBuA 400 3.61 4.74 6.9 - 7.5 x x 1.22, 1.35
Gradient PEAtBuA 500 3.16 x x 4.06 1.22 1.41
Gradient PEABnA 500 3.40, 3.17 4.87 7.0 - 7.5 4.07, 3.88 1.22, 1.01 x
38
OO
OO
OO
a
b
c
d
ef
g
h
a a
b b
Table 8: 13C NMR shifts (CDCl3, 292K, δ in ppm) of polymers and copolymers
Polymer MHz a b c d e f g h
PEA 125 45 171 x x 61 14 x x
BnEA 125 45 171 67 135, 128, 127 x x x x
Random PEABnA 125 45 171 67 136, 128, 127 61 14 x x
Random PEAtBuA 100 45 171 x x 60 14 81 28
Random PBnAtBuA 100 x 171 67 136, 128, 127 x x x 28
Gradient PBnAtBuA 100 x 171 x 136, 128, 127 x x x 28
Gradient PEAtBuA 125 45 171 x x 61 14 81 28
Gradient PEABnA 125 45 171 67 136, 128, 127 61 14 x x
39
8 List of abbreviations
EDA ethyldiazoacetate
BnDA benzyldiazoacetate
tBuDA tert-butyldiazoacetate
PEA poly(ethyl 2-ylidene-acetate)
PBnA poly(benzyl 2-ylidene-acetate)
cod 1,5-cyclooctadiene
dmcod 1,5-dimethyl-1,5-cyclooctadiene
pro L-prolinate
nbd 2,5-norbornadiene
dcp dicyclopentadiene
dbn-nbd (R’,R)-benzyl-norbornadiene
SEC Size Exclusion Chromatography
DSC Differential Scanning Colorimetry
TGA Thermo Gravimetric Analysis
NOE Nuclear Overhauser Effect
FAB Fast Atom Bombardment
Mw Weight distributed molecular weight
Mn/Mw polydispersity
Tg Glass transition point
Tm Melting temperature
Tc Crystallization temperature
s singlet
m multiplet
40
41
9 Acknowledgments This work was supported by the Netherlands Organization for Scientific Research
(NWO-CW) and the University of Amsterdam. I want to thank B. de Bruin and E.
Jellema for their support and guidance during this project, prof. J. N.H. Reek for helpful
discussions, J.A.J. Geenevasen and J.M. Ernsting for their assistance with the NMR
measurements; J.W.H. Peeters for recording the high resolution mass spectra; G.O.R.
Alberda van Ekenstein for thermal analysis of the polymers and Guillaume Berthon for
providing the dibenzylnorbornadiene ligand.
42
43
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