Synthesis and characterization of low-molecular-weight azo-acetoxystyrene and azo-naphthalene oligomers via stable free radical polymerization (SFRP)

Article abstract of DOI:10.1139/V10-072

As an approach toward controlled molecular architecture through stable free radical polymerization (SFRP), we have prepared a series of oligomers of controlled molecular weights (M_n)and low polydispersities (structures 2 and 3, with n values ranging from 2 to 52). Definitive evidence of structure was obtained through MALDI/MS (inter-peak interval of 162 m/z in azo-acetoxystyrene oligomers 2 and 260 m/z in azo-naphthalene oligomers 3, which correspond to the acetoxystyrene (AS) and naphthalenic (Np) repeating units, with corroborative evidence from NMR and GPC. Two synthetic pathways were explored. Pathway 1 yields azo-acetoxystyrene oligomer 2 via SFR addition of a TEMPO-capped unimer 1 to acetoxystyrene. Subsequent reactions would convert 2 into 3. In an alternative pathway 2, SFR addition of 1 to 4-(1-methoxynaphthyl) styrene gives azo-naphthalene oligomer 3 directly. Thus, the present reported methodology for controlled architecture has achieved synthesis of oligomers from low M_n (chlorobenzene, PhCl, solvent) to relatively high M _n (bulk), with incorporation of naphthyl (donor) and azobenzene (acceptor) moieties, as well as spacer moieties, in a controlled manner.

Full text of DOI:10.1139/V10-072

910
Synthesis and characterization of low-molecular-
weight azo-acetoxystyrene and azo-naphthalene
oligomers via stable free radical polymerization
(SFRP)
Dildar Ali, Zaheer Ahmed, Peter M. Kazmaier, and Erwin Buncel
Abstract: As an approach toward controlled molecular architecture through stable free radical polymerization (SFRP), we
have prepared a series of oligomers of controlled molecular weights (Mn) and low polydispersities (structures 2 and 3,
with n values ranging from 2 to 52). Definitive evidence of structure was obtained through MALDI/MS (inter-peak inter-
val of 162 m/z in azo-acetoxystyrene oligomers 2 and 260 m/z in azo-naphthalene oligomers 3, which correspond to the
acetoxystyrene (AS) and naphthalenic (Np) repeating units, with corroborative evidence from NMR and GPC. Two syn-
thetic pathways were explored. Pathway 1 yields azo-acetoxystyrene oligomer 2 via SFR addition of a TEMPO-capped un-
imer 1 to acetoxystyrene. Subsequent reactions would convert 2 into 3. In an alternative pathway 2, SFR addition of 1 to
4-(1-methoxynaphthyl)styrene gives azo-naphthalene oligomer 3 directly. Thus, the present reported methodology for con-
trolled architecture has achieved synthesis of oligomers from low Mn (chlorobenzene, PhCl, solvent) to relatively high Mn
(bulk), with incorporation of naphthyl (donor) and azobenzene (acceptor) moieties, as well as spacer moieties, in a con-
trolled manner.
Key words: azo-acetoxystyrene oligomers, azo-naphthalene oligomers, stable (controlled) free radical polymerization.
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Resume : Comme approche a une architecture moleculaire controlee par le biais d’une polymerisation a l’aide de radicaux
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libres stables (PRLS), on a prepare une serie d’oligomeres de masses moleculaires (Mn) controlees et de faibles polydisper-
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sites (structures 2 et 3, avec des valeurs de n allant de 2 a 52). Les donnees definitives permettant de determiner les struc-
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tures ont ete obtenues par spectrometrie de masse avec ionisation laser assistee par une matrice de desorption (SM-
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ILAMD) [intervalle entre pics de 162 m/z dans les oligomeres azo-acetoxystyrene 2 et de 260 m/z dans les oligomeres
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azo-naphtalene 3 qui correspondent aux motifs constitutifs de l’acetoxystyrene (AS) et naphtalenique (Np)] et corroborees
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par RMN et par chromatographie par permeation de gel (CPG). On a explore deux voies de synthese. La voie 1 conduit a
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l’oligomere azo-acetoxystyrene 2 par le biais d’une addition a l’aide de radicaux libres stables (RLS) de l’unimere 1 cou-
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vert d’un groupe tetramethylpiperidine oxyle (TEMPO) a l’acetoxystyrene. Des reactions subsequentes devraient permettre
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de transformer le produit 2 en 3. Dans une voie alternative 2, une addition a l’aide de radicaux libres stables aux 4-(1-me-
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thoxynaphtyl)styrene conduit directement a l’oligomere azo-naphtalene 3. La methodologie rapportee ici pour obtenir une
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architecture controlee a permis de realiser la synthese d’oligomeres a partir de produits de masses moleculaires basses
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(chlorobenzene, PhCl, solvant) a des masses moleculaires relativement elevees (en bloc), avec l’incorporation de portions
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naphtyles (donneur) et azobenzene (accepteur) ainsi que de portions agissant comme d’espacement d’une fac¸on controlee.
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Mots-cles : oligomeres de l’azo-acetoxystyrene, oligomeres de l’azo-naphtalene, stable (controlee), polymerisation par radi-
caux libres.
require specific architecture that, typically, incorporates en-
ergy donor (D) and energy acceptor (A) moieties, as shown
in Scheme 1. The donor group(s) absorb light energy, which
is then transferred from the excited D group (D*), with
varying degrees of efficiency, to the acceptors. In principle,
electron transfer could also occur, depending upon D and A
groups incorporated in the LH system.
Introduction
As part of a research program in materials science, we
have been engaged in studies of light harvesting (LH) sys-
tems and underlying energy transfer (ET) processes.1–4 En-
ergy transfer and (or) electron transfer are basic to the
process of light harvesting and processes such as photosyn-
thesis.5–10 In turn, the efficiency of ET has been shown to
Considerable effort has gone into understanding the pho-
Received 22 January 2010. Accepted 3 May 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 6 August 2010.
D. Ali. Department of Chemistry, Queen’s University, Kingston, ON K7L 3N6, Canada; H.E.J. Research Institute of Chemistry,
University of Karachi, Karachi 75270, Pakistan.
Z. Ahmed. H.E.J. Research Institute of Chemistry, University of Karachi, Karachi 75270, Pakistan.
P.M. Kazmaier. Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga ON L5K 2L1, Canada.
E. Buncel.1 Department of Chemistry, Queen’s University, Kingston, ON K7L 3N6, Canada.
1Corresponding author (e-mail: buncele@chem.queensu.ca).
Can. J. Chem. 88: 910–921 (2010)
doi:10.1139/V10-072
Published by NRC Research Press

Ali et al.
911
Scheme 1.
for subsequent conversion of acetoxy functions to naphthyl
methoxy donor groups, i.e., formation of 3. In an alternative
preparation (Scheme 2, pathway 2), the SFRP process in-
volved addition oligomerization of 4(1-methoxynaphthyl)-
styrene, 6, with the unimer 1 to give 3. The results of
preparation of the LH models containing 4-(1-methoxynaph-
thyl)styrene donor repeating units and their characterization
(MALDI-TOF/MS and GPC analyses), as well as corrobora-
tive NMR analyses, will be discussed in terms of optimiza-
tion of reaction time and the use of a suitable solvent to
control the molecular weight.
Results and discussion
Synthetic strategy
Two synthetic approaches (Scheme 2, pathways 1 and 2),
which differed in the structure of the arylvinyl monomer oli-
gomerized with unimer 1, were examined as routes to the
target oligomers, 3. Addition under SFRP conditions of the
benzylic radical derived from 1 (arising from facile scission
of the C–O bond of the TEMPO cap) to 4-acetoxystyrene, 5,
should yield oligomers, 2, with a range of acetoxystyrene re-
peating units, n. Hydrolysis of 2 followed by functionaliza-
tion with a naphthyl moiety yields the azonaphthyl oligomer
3.
The alternative route (Scheme 2, pathway 2) consists of
SFRP-style addition of the benzylic radical of 1 (formed as
before) to the 4-(1-methoxynaphtyl)styrene, 6, should again
give models, 3.
tochemical and photophysical processes involved in light
harvesting and this effort is, partly, reflected in the range of
authoritative reviews that have appeared lately.11–16
We have taken a two-pronged approach to probing LH
systems. First and in common with a number of research
groups,5,6,17,18 we have prepared a range of dendrimer and
starburst polymeric LH model systems, including azoben-
zene moieties as A groups and naphthalenes as D moi-
eties.1–4 Second, we have prepared two LH model systems
(Scheme 1) using naphthalene D and azobenzene A groups
tethered by methoxy linkages: A–(OCH2)–D and D–
(CH2O)–A–(OCH2)–D. Our study of the photophysical/
chemical properties of two models (Scheme 1) revealed a
new criterion for efficiency of energy transfer, involving ra-
tios of rate constants for cis–trans isomerization and ratios
of light energy absorption; the LH models were each com-
pared to relevant benchmark molecules. According to this
new criterion, energy transfer was virtually complete for
both model systems. Electron transfer, under the conditions
studied, was not found to be feasible. However, a need for
a greater loading of light harvesting donor groups was also
identified, as a pre-requisite to the preparation of more prac-
tical LH systems.4 To this end, we have now prepared a ser-
ies of oligomers 3 containing an azobenzene acceptor group
at one end of the chain and a progressively larger number of
donor naphthalenic repeating units, as shown in Scheme 2.
The long-term goal is to study the LH 3 to 4 interconversion
and the photophysics involved (Scheme 2).
Herein, we describe our synthetic strategy, which is based
on stable free radical polymerization (SFRP, also termed ni-
troxide-mediated polymerization, NMP)19–27 to yield LH
oligomers in a controlled manner. SFRP has proven to be
particularly valuable in the preparation of polymers with
controlled architecture. The Georges,21,23 Hawker,20,22,25
Fischer,19 and Matyjaszewski26 groups, among others, have
pioneered and elaborated on this approach to the preparation
of high-molecular-weight polymers. Our aim here, however,
is the preparation of low-molecular-weight oligomers for the
purpose of preparation of controlled oligomeric structures.
The starting point in two different synthetic approaches is
the same: the azobenzene-containing unimer, 1, which is
capped reversibly with the persistent nitroxyl radical
TEMPO. Reaction of the 1 with 4-acetoxystyrene, 5, was ex-
pected to give low-molecular-weight oligomers 2 (Scheme 2,
pathway 1) incorporating the acetoxystyrene repeating unit, n,
The difference in the two routes lies, then, in whether the
naphthyl donor group is incorporated into the product 3 after
oligomerization (pathway 1) or, alternatively, preparation of
a suitable arylvinyl monomer, 6, leads to direct oligomeriza-
tion to give 3 (pathway 2).
In the preparation of model 3, controlled low-molecular-
weight oligomers were desired. To this end, both the reaction
medium and the reaction times were empirically optimized.
For the azo-acetoxystyrene oligomerization to yield 2, bulk
(at 130 8C) and solution media (at 120 8C) were studied,
where varying amounts, i.e., 1:1 to 1:5 w/w ratios of the ar-
ylvinyl monomer to chlorobenzene (PhCl) solvent,20,22,25,28
were used. Reaction times were varied from 1 min to 5 h in
a given set of solvent conditions by quenching a reaction
run by pouring the reaction mixture into excess cold metha-
nol (~10 mL). For the azo-naphthalene oligomerizations to
yield 3 directly, only the solution conditions (i.e., dilution
with PhCl at 120 8C) were studied, as a consequence of the
information gleaned from the results in the azo-acetoxystyrene
systems, 2. Reaction times were varied from 10 min to 1 h,
again as prompted by the results in the related oligomeriza-
tion system.
The results of these two preparatory routes are listed in
Tables 1 and 2 and will be discussed below.
Azo-acetoxystyrene oligomers, 2
The results of the SFRP between the unimer 1 and 4-ace-
toxystyrene, 5, as the arylvinyl monomer, are summarized in
Table 1. Specifically, the reaction conditions including reac-
tion times, the analysis of the maximum number of acetox-
ystyrene repeating units (n) as determined by MALDI-TOF/
MS,29–31 the number average molecular weight of the
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912
Can. J. Chem. Vol. 88, 2010
Scheme 2.
Table 1. Characterization of azo-acetoxystyrene (AS) oligomers, 2, as a function of reaction conditions: MALDI-TOF/MS and GPC results.
Conditions (w/w ratio AS:PhCl)
Reaction time (min)a
MALDI/MS repeating units nb
0,e 37, 50, 52
25, 30, 39
GPC (Mn,c kDa (PD))d
0,e 6.7(1.6), 7.8(1.4),8.2(1.3)
4.6(1.4), 5.3(1.3), 9.7(1.3)
NAf
A = Bulk
A-1, A-2, A-3, A-4
1, 3, 5, 10
B = 1:1
B-1, B-2, B-3
5, 30, 240
C = 1:2
C-1, C-2, C-3
5, 20, 90
0,e 5, 10
D = 1:3
D-1, D-2, D-3
5, 20, 114
0,e 20, 24
0,e 5.1(1.2), 8.49(1.1)
NA
E = 1:4
E-1, E-2, E-3
5, 20, 90
0,e 7, 17
F = 1:9
F-1, F-2, F-3
5, 20, 300
0,e 0,e 0e
0,e 0,e 0e
aBulk reactions were carried out at 130 8C. Solution reactions with PhCl diluent were carried out at 120 8C. Both were quenched by addition to cold MeOH
accompanied by precipitation of oligomeric product(s) at times cited.
bn is the maximum number of repeating acetoxystyrene (AS) units in 2 observed ( 5%–10% error).
cNumber average molecular weight (kDa) from GPC, calibrated with polystyrene standards.
dPolydispersity, Mw/Mn.
ePeaks (m/z) corresponding to oligomer formation were not observed; the only major peak recorded is ascribed to the parent molecular ion for protonated
azobenzene unimer at [M + H]ꢀ+ = 472. Oligomer formation was not observed in GPC traces.
fNA: not applicable.
oligomers (Mn) as determined by GPC, as well as the poly-
dispersity (PD = Mw/Mn) are listed in the Table.
long chain oligomers, 2. For example, a 3 min reaction time
produces 2 with Mn of 6.7 kDa, corresponding to about a
maximum of 37 acetoxystyrene repeating units (n = 37),
and a relatively broad molecular weight distribution (PD =
Mw/Mn = 1.4). In fact, repetition of the oligomerization at a
somewhat lower temperature (120 8C here versus 130 8C in
Mechanistic effects on oligomerization
As can readily be seen in Table 1, even at short reaction
times, bulk conditions (experiment series A) yield relatively
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Ali et al.
913
Table 2. Characterization of azo-naphthalene oligomers, 3, as a function of reaction conditions: MALDI-TOF/MS and GPC results.
Conditions (w/w ratio AS:PhCl)
Reaction time (min)a
MALDI/MS repeating units nb
GPC (Mn,c kDa (PD))d
0.75 (1.2), 3.8 (1.3)
0.69 (1.1)
Q = 1:3
Q-1, Q-2
15, 40
10
2, 12
2
R = 1:4
S = 1:5
T = 1:49
30
10
2.5 (1.2)
60
5
1.8 (1.1)
aSolution reactions (PhCl diluent) were carried out at 120 8C and were quenched by addition to cold MeOH accompanied by precipitation of oligomeric
product(s) at times cited.
bn is the maximum number of repeating 4-(1-methoxynaphthyl)styrene (Np) units in 3 observed ( 5%–10% error).
cNumber average molecular weight (kDa) from GPC, calibrated with polystyrene standards.
dPolydispersity, Mw/Mn.
the bulk system) and with an equal weight of chlorobenzene
(experiment series B) requires 240 min (B-3) to yield 2 with
approximately the same number of repeating units (according
to MALDI/MS), i.e., n = 39. Representative MALDI/MS
spectra for experimental series C-1 to C-3 are shown in
Fig. 1 and illustrate the number of repeating units, n, ana-
lyzed via this method.
These results are consistent with the mechanism of SFRP,
where at the temperature of the reaction the TEMPO-capped
unimer 1 dissociates to give persistent TEMPO nitroxyl radi-
cal and a resonance-stabilized benzylic radical.32 The benzylic
radical concentration is partitioned between recombination
with the TEMPO radical and addition to 4-acetoxystyrene, 5.
Under bulk conditions, the TEMPO-benzylic radical pair is
effectively caged by 5, the arylvinyl monomer. Addition of
the benzylic radical to the vinyl functional group of the sur-
rounding acetoxystyrenes should be facile, and relatively
high Mn oligomer, 2, arises in accord with the results listed
in Table 1. Recombination of TEMPO with the growing
benzylic radical end of the chain provides a termination
step in the chain mechanism, and through potential further
dissociation, provides an avenue for conversion to a
‘‘living’’ chain mechanism.
with all other series, a 5 min reaction time yields no
oligomer detectable by MALDI/MS or GPC. The results at
20 min appear anomalous; a 1:2 dilution (AS:PhCl; Table 1,
C-2) yields an oligomer with five repeating units, while 1:3
dilution (D-2) gives 20 repeating unit oligomer and 1:4 dilu-
tion gives seven repeating-unit oligomer, but 1:9 dilution
(F-2) yields no detectable oligomer, 2.
One possibility that accounts for the observed behaviour,
namely, an increase in number of repeating units for D-2,
as compared to the C-2 and E-2 experiments (Table 1), is
that the solvent effect of dilution (i.e., preferential stabiliza-
tion of the dissociated radicals that would favour dissocia-
tion) opposes the concentration effect, that would reduce
the probability of radical addition to the arylvinyl monomer.
In short, dilution favours dissociation freeing the benzylic
radical derived from 1 to add to the vinyl function of 5, but
the decreased concentration of the styrene counteracts this
effect partly. The interplay of these factors could lead to the
behaviour observed.
At extreme dilution (series F), none of the reaction times
studied (5 to 300 min) were sufficient to lead to any
oligomer formation. The radicals derived from 1 are effec-
tively free in solution but the concentration of AS is too
low to lead to oligomerization.
Chlorobenzene solvent effects on oligomerization
Also in accord with the foregoing interpretation are the
polydispersities determined by GPC (Table 1). In Fig. 2, the
GPC traces for experimental series B, D, and F (Table 1) are
shown. The bulk run even at 3 min reaction time (Table 1,
A-1) gives a PD of 1.6, and modest dilution (B-1, 5 min)
yields only a somewhat narrower molecular-weight distribu-
tion (PD = 1.4). In living polymerizations, an effective equi-
librium is established between dormant or dead polymer
(oligomer). This arises from recombination of the benzylic
radical end of the growing chain and the TEMPO radical,
on the one hand, and propagation of the chain by addition
of the benzylic radical to the arylvinyl monomer, on the
other. The closer the PD value is to 1 (i.e., monodispersity),
the more nearly the polymerization is considered a living
process. With a 1:3 dilution with PhCl (D-2, 20 min), the
PD drops to 1.3.
In principle, addition of chlorobenzene solvent could have
two effects.20,22,25,28 The first and presumably smaller effect
is a solvent effect on the dissociation of the unimer. Prefer-
ential solvation of the dissociated radicals (e.g., via p–p
stacking) relative to the bulky TEMPO-capped unimer could
favour dissociation of the unimer. On the other hand, the
concentration of acetoxystyrene in the cage surrounding the
radical pair would be reduced as a function of increasing
added PhCl and the probability of addition to the vinyl
group reduced. At short reaction times, the number of re-
peating groups incorporated in the oligomer would be re-
duced, as is observed (Table 1).
Generally, then, increasing dilution (Table 1, experiment
series
C through F) leads to lower-molecular-weight
oligomer at comparable reaction times. At 5 min, a 1:1 dilu-
tion (AS:PhCl, B-1) gives an oligomer, 2, incorporating 25
repeating units according to MALDI/MS analysis, while
The relatively low overall mass recovery found for the
acetoxystyrene oligomer 2 indicated that additional steps
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Can. J. Chem. Vol. 88, 2010
Fig. 1. MALDI/MS spectra of azo-acetoxystyrene oligomers with varying acetoxystyene repeating unit (n) values: (a), n = 0; (b), n = 5; (c),
n = 10, corresponding to experiments C-1, C-2, and C-3, respectively (see Table 1).
leading to the desired LH-model oligomer, 3, could result in
little final product. The time-consuming nature of the neces-
sary oligomer characterization at each subsequent step was
another consideration that prompted us to consider direct in-
corporation of the naphthalenic donors (Np) in the SFRP
step (Scheme 2). However, the results of the preparation of
oligomer 2 indicated that dilutions of 1:3 to 1:5 could yield
the appropriate LH oligomer 3 comprising a controlled
range of repeating donor units.
6, under the range of conditions identified as favouring for-
mation of low-molecular-weight oligomer 2, was undertaken
to prepare 3 directly (Scheme 2). Table 2 summarizes the
MALDI-TOF/MS and GPC characterization of 3.
With a ratio of 1:3 of the arylvinyl monomer, 6, to PhCl
solvent (120 8C; experiment Q, comparable to the D series
of experiments in Table 1), SFRP yields low average molec-
ular weight oligomer (Mn = 0.75 kDa) with low polydisper-
sity (PD = 1.2). MALDI/MS analysis shows the expected
inter-peak m/z interval of 260 that corresponds to the naph-
thalenic repeating unit (Np) as shown in Fig. 3 for experi-
ment R. At 15 min reaction time (experiment Q-1), the
Azo-naphthalene oligomers, 3
SFRP using unimer 1 and 4-(1-methoxynaphthyl)styrene,
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Ali et al.
915
Fig. 2. GPC chromatograms for experiments (a) B (B-1, B-2, B-3);
(b) D (D-1, D-2, D-3); and (c) F (F-2, F-3) (see Table 1).
oligomer with a lower maximum number of repeating units
(n = 5) and a lower number average molecular weight
(1.8 kDa). This parallelism, namely, first increase and then
decrease in molecular weight and maximum number of
repeating units, which was found in the acetoxystyrene oli-
gomerization runs to give 2, is consistent with the behaviour
found in the oligomerization to give 3. The same reasons ap-
pear to apply: compensatory interplay of solvent and dilu-
tion effects on the SFRP chain mechanism.
Highlights of characterization and fractionation
MALDI-TOF/MS
Figure 1 shows the MALDI-TOF/MS analysis29–31 for
three experimental runs (Figs. 1a to 1c, C-1 to C-3,
Scheme 2, pathway 1). As can be seen in run C-1, no ace-
toxystyrene oligomer 2 formed (i.e., n = 0); the major MS
peak corresponds to unmodified unimer 1 (M – H+
=
472 m/z) while in the subsequent runs, C-2 and C-3, the ma-
jor peaks correspond to the oligomers with n = 5 and n = 10,
repeating units, respectively. Significantly, for C-2 and C-3,
the main fragmentation route involves sequential scission of
the acetoxystyrene repeating units. Accordingly, in Fig. 1b,
main peaks are seen at n = 5 (m/z = 1282) and at consecu-
tively smaller m/z values with intervals of 162 m/z for n = 4,
3, 2, and 1. This pattern applies equally to C-3 and all ex-
perimental runs summarized in Table 1.
An additional feature of these MALDI/MS spectra is the
presence of minor peaks at m/z values significantly greater
than that of the main peak. Representative is C-2 (Fig. 1b).
Arising from the presence of TEMPO radical in the system
and multiple benzylic sites for H-abstraction and coupling,
the possibility exists for incorporation of multiple TEMPO
moieties at the oligomeric end groups. This accounts for
these higher m/z peaks.
Beyond the major sequential splitting off of repeating
units, small fragmentation peaks are evident between the
162 m/z intervals, e.g., between n = 5 and n = 4 in C-3
(Fig. 1c). These small peaks may result from minor defects
in the oligomer structure.
Turning to pathway 2 (Scheme 2), the direct preparation
of oligomer 3, representative MALDI/MS spectra are shown
in Fig. 3 for experiment R; reaction conditions are given in
Table 2. Once again major peaks result from the oligomer
molecular ions with the main fragmentation route involving
sequential repeating-unit scission with intervals now of
260 m/z. Minor fragmentation routes for 3 are accounted for
as previously outlined for 2 (Scheme 2).
Importantly, the dominant features of the MALDI/MS
analysis of 2, which apply equally to 3, are the observation
of the M – H+ molecular ions corresponding to the maxi-
mum number of repeating units in the respective oligomers.
Further, the sequential loss of the repeating moiety empha-
sizes the source of these oligomers: a living SFRP process.
Thus, the nature of any minor peaks, arising from impurities
or defects, was not investigated in detail, but their presence
cannot affect the conclusions or impact the main theme of
this study.
maximum number of repeating units found was two; the
‘‘oligomer’’ here consists solely of unimer and dimer. An in-
crease in reaction time to 40 min (Q-2) gives higher-molecular-
weight oligomer (Mn = 3.9 kDa; 12 repeating units by
MALDI/MS analysis) with only a slight broadening of the
molecular weight distribution (PD = 1.3 by GPC). Note, for
comparison, that at 20 min reaction time in the acetoxystyr-
ene series (Table 1, D-2), MALDI/MS analysis gave
oligomer 2 containing a maximum of 20 repeating units and
GPC traces indicated a number average molecular weight of
5.1 kDa for the oligomer with a PD of 1.2, all similar to
the results obtained here for experiment Q-2 with a
reaction time of 40 min. Clearly, the experimental condi-
tions optimized for the acetoxystyrene SFR oligomerizations
(Scheme 2, pathway 1) extend reasonably to the 4-(1-methoxy-
naphthyl)styrene additions, i.e., formation of 3 directly via
pathway 2 in Scheme 2.
Reasonably, the considerations of solvent enhancement of
the dissociation of the unimer 1 and the opposing influence
of dilution on the addition step apply consistently to path-
way 2 to give 3. It follows that experiments R (1:4, 6:PhCl
solvent) through T (1: 49, 6:PhCl; Table 2) mimic the trend
in degree of oligomerization and polydispersity found for
the comparable experimental series D through F (Table 1)
found for pathway 1. The number average molecular weight
of the oligomer increases, according to GPC, in going from
R (1:3) to T (1:4), i.e., from 0.69 kDa to 2.5 kDa with sim-
ilar PD values and in a maximum number of repeating units
of two rising to 10 in the latter run. Experiment T gives an
GPC
Oligomers 2 and 3 were analyzed by GPC30 (UV–vis de-
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Can. J. Chem. Vol. 88, 2010
Fig. 3. MALDI/MS spectra for azo-naphthalene oligomer, 3, experiment R (n = 2).
tector; see Experimental). The results of molecular weight
determination (polystyrene calibration) and polydispersity
(PD) are summarized in Tables 1 and 2 and discussed earlier
in the text (see Fig. 2). Typical GPC traces for 4-(1-methox-
ynaphthyl)styrene monomer 6 (Fig. 4, experiments R, T, and
S) show the separation of oligomeric fractions 3 with elution
time, as well as the presence/absence of residual monomer.
yielded a mixture of oligomers containing an oligomer 3
with a maximum of 14 Np repeating units. Flash column
chromatography (dichloromethane eluant) gave, after recom-
bination based on TLC, five fractions. The first (H-1) con-
sisted only of unmodified arylvinyl monomer, 6.
The remaining fractions contained mixtures of oligomers
3; the longest chain oligomers eluted first. Hence, fraction
H-2 contained oligomers with a maximum of 14 Np repeat-
ing units according to MALDI/MS. The next in order had
n = 8 (H-3), n = 2 (H-4), and n = 1 (H-5).
1H NMR
1H NMR spectra (Supplementary data, Figs. S2 and S3)
for the acetoxystyrene oligomerizations (Scheme 2, pathway
1) are in accord with the MALDI/MS and GPC analyses
summarized in Table 1. Specifically, the spectra for
oligomer 2 contain a diagnostic signal at 2.27 ppm for the
acetoxy methyl of the repeating unit. This signal increases
in relative intensity with increasing n values for 2. Similarly,
for samples A-2 to A-4, B-1 to B-3, C-2, and C-3 (Fig. S3),
increasingly broad signals are found centred at 6.2 and
6.9 ppm and are ascribed to the aromatic protons of the ace-
toxystyrene residues. These NMR peaks for the acetoxystyr-
ene repeating unit of 2 are absent in those runs (e.g., A-1)
where oligomerization was deemed to be absent, on the ba-
sis of MALDI/MS and GPC determinations.
GPC analysis of the crude reaction product (H) prior to
fractionation gave a number average molecular weight of
0.73 kDa with a relatively broad molecular weight distribu-
tion (PD = 1.5). Fractions H-2 and H-3 gave GPC peaks cor-
responding to lower-molecular-weight oligomer (Mn
=
0.57 kDa) and narrower distribution (PD = 1.4), while frac-
tion H-4 was found to have Mn = 1.1 kDa with a PD of 1.2.
Figure 6 shows an ESI-MS spectrum for oligomer 3 where n =
1 (fraction H-5).
The results show that the number of Np repeating units, n,
in the oligomer 3 (via pathway 2 in Scheme 2) and the cor-
responding molecular weight with low polydispersity
(Table 2) may be achieved through careful control of the
SFRP reaction conditions (temperature and dilution with
PhCl). The oligomeric mixture may be further fractionated
by standard flash chromatography.
1
Considering formation of 3 (Scheme 2, pathway 2), H
NMR spectroscopic analysis agrees with the results given in
Table 2 in that higher degrees of oligomerization are accom-
panied with more intense signals attributable to the Np re-
peating units and overall broadening of all signals in the
relevant spectrum. This is shown in Fig. 5 for experiments
T and S.
In conclusion, it can be envisaged that the described
methodology will lead to a controlled range of oligomers,
varying in the numbers of naphthalenic donor units.
Fractionation of azo-naphthalene oligomers 3 via flash
chromatography
Experimental
Instruments and materials
1H NMR spectra were recorded using a Bruker Avance-
Reaction of a scaled up quantity of 1 and 1-[(4-vinylphen-
oxy)methyl]naphthalene (20 min; 1:4, monomer:PhCl)
Published by NRC Research Press

Ali et al.
917
Fig. 4. GPC chromatograms showing the increasing molecular
weights of 3 and peaks for 1, 3, and 6 (Scheme 2), experiments R,
T, and S.
40 8C) with a separation module consisting of four Waters
Ultrastyragel1 (HR5.0, HR3.0, HR1.0, and HR0.5) in series.
Distilled THF was used as eluant with a flow rate of
1.0 mL min–1. The GPC system was calibrated with standard
polystyrenes up to an elution time of 35 min. MALDI-TOF/
MS traces were recorded using an Applied Biosystems Voy-
ager DE-STR MALDI-time of flight/mass spectrometer with
a nitrogen laser (337 nm), delayed extraction and reflectors.
An accelerating potential of 20 kV was used in the MS de-
tector in both linear and reflector modes. The optimum matrix
used for oligomer characterization was 2,5-dihydroxybenzoic
acid (DHB) containing no added salt. Typically, oligomeric
samples were prepared by mixing a THF solution of the
oligomer(s) (1 mg mL–1) with
a solution of DHB
(20 mg mL–1 in 1:1 MeCN:MeOH) in 1:1 (v/v) proportions
to give the analytical matrix. A 1 mL aliquot of the analyti-
cal matrix was placed onto a sample plate and the solvent
was evaporated at room temperature to give the MALDI-
TOF/MS sample for characterization.
All common solvents (acetone, THF, and so forth) used in
the preparations outlined below were purchased commer-
cially in HPLC-grade or better and, where necessary, were
further purified by standard methods.33 4-Hydroxyazoben-
zene (Aldrich) was recrystallized from ethanol and THF
(VWR) was dried over and distilled from sodium metal/ben-
zophenone. All inorganics (e.g., NaBH4) and the remaining
requisite chemicals and organic reagents were purchased
commercially and used without further purification. 4-Ace-
toxystyrene (96%, Aldrich), di-t-butyl peroxide (3.5 mol/L
in decane, Aldrich), 18-crown-6 (98%, Aldrich), 4-ethylbenzal-
dehyde (98%, Aldrich), oxalyl chloride (95%, Aldrich), and
2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO, gift of Xerox
Research Canada) were used as received.
The azo unimer 1 was prepared in a five-step procedure
as previously described1,27 starting from 4-ethylbenzalde-
hyde that was reduced with NaBH4 to 4-ethylbenzyl alcohol,
converted to the bromide, and capped with 2,2,6,6-tetra-
methyl-1-piperidinoxyl) using TEMPO and di-t-butylperoxy-
oxalate. Further reaction with NaI in acetone converted the
bromide to the iodide, which, in a final step, was reacted
with 4-hydroxyazobenzene in the presence of K2CO3 and
18-crown-6 in acetone to give 1. The overall yield of this
five-step procedure was 24%. All physical and spectroscopic
properties were in good agreement with those previously
published.1 The MALDI/MS of 1 contained small extrane-
ous peaks at greater m/z values than the protonated molecu-
lar ion for 1 (i.e., M – Hꢀ+ at 472.28 m/z). The minor
impurity was not removed by column chromatography (60
mesh silica gel; dichloromethane eluent), and 1 was used as
is in the following syntheses.
400 spectrometer operating at 400.1 MHz, broad band
probe, auto-tuned. All chemical shifts are reported as d in
parts per million (ppm) relative to residual CHCl3 (d =
7.28 ppm) and CHDCl2 (d = 5.30 ppm) in CDCl3 (source:
CDN) and CD2Cl2 (CDN) solvents, respectively. Coupling
constants (J) are reported in Hz. IR samples were recorded
using a Bomem MB-120 FTIR spectrophotometer. Dilute
samples dissolved in a suitable volatile solvent were allowed
to evaporate to provide a film on a KBr plate. Melting
points were measured using a Fisher–Johns melting point
apparatus and are reported as uncorrected. Gel permeation
chromatography (GPC) was performed using Waters 2695
HPLC (Waters 410 differential refractometer detector,
Preparation of azo-acetoxystyrene oligomers, 2
Azo-acetoxystyrene oligomers, 2, capped with TEMPO
were prepared under SFRP reaction conditions via addition
of 1 to acetoxystyrene, AS, under bulk and chlorobenzene
(PhCl) solvent conditions.
General bulk procedure
Mixtures of 1 and AS were introduced into silicone-septum-
capped ampoules (10 mL), swept with N2, and these were
immersed in an oil bath thermostatted to 130 8C. After the
Published by NRC Research Press

918
Fig. 5. H NMR spectra for azo-naphthalene oligomers, 3, experiment T (n = 5) and experiment S (n = 10).
Can. J. Chem. Vol. 88, 2010
1
set reaction times, each mixture was poured into excess cold
MeOH (10 mL) to quench the reaction and the precipitate,
in each case, was collected by filtration. The precipitates
were dried in vacuo (<1 torr, 50 8C) (1 torr = 133.322 Pa)
overnight to give yellow coloured powders.
gave a colourless solution, after filtration. This filtrate was
cooled (~ –70 8C) from which 4-vinylphenol (4-hydroxys-
tyrene) crystallized as colourless crystals (4.5 g, 45%);
mp 59–60 8C (lit. 68–69 8C,34 73.5 8C35). IR (KBr, cm–1) n:
3249, 2977, 1656, 1541, 1410, 1110, 992, 834, 647. 1H
NMR ([CD3]2CO, 400 MHz) d: 8.45 (1H, s, OH), 7.34 (2H,
d, J = 6.8, H-4,8), 6.98 (2H, d, J = 6.8, H-5,7), 6.67 (1H,
d,d, 17.6, 10.4, H-2), 5.60 (1H, d, Jtrans = 17.6, H-1), 5.01
(1H, d, Jcis = 10.4, H-1). 13C NMR (CD3OD, 100 MHz) d:
157.0, C-6; 136.4, C-5,7; 129.4, C-3; 127.0, C-4,8; 114.9,
C-1; 109.4, C-2. EI-MS (M + H)+ m/z: 120.0532 (calcd.
for C8H8O: 120.0531 g/mol).
General solvent procedure
Experimental series B–F (cf. Table 1) were carried out at
120 8C. Mixtures of the reagents and PhCl solvent were pre-
pared in ampoules, as given above, and the temperature
maintained by immersion of the vials in the thermostatted
oil bath. The mass ratios of AS to PhCl are listed in Table 1.
Three aliquots were withdrawn via gas-tight syringe from
each experimental series at different time intervals to yield
a range of oligomeric mixtures, 2, that were analyzed by
MALDI-TOF/MS, 1H NMR spectrometry, IR spectropho-
tometry, and GPC (UV–vis detector), as outlined in the pre-
vious text and in the Results and discussion section.
Table 3 shows molar ratios and amount of PhCl solvent
used in each experimental run.
Preparation of azo-naphthalene oligomers, 3
4-(1-methoxynaphthyl)styrene
Saponification of 4-acetoxystyrene (10 g, 0.061 mol in
2.5 g MeOH) with KOH (0.25 g, 0.0045 mol), under N2 at-
mosphere, followed by acidification (glacial AcOH) and ad-
dition of toluene to precipitate any poly(4-hydroxystyrene)
Weighed quantities of 4-vinylphenol (1.04 g, 8.6 mmol),
1-chloromethylnaphthalene (1.2 g, 6.8 mmol), K2CO3
(2.05 g, 14.7 mmol), and 18-crown-6 polyether (0.29 g,
Published by NRC Research Press

Ali et al.
919
Fig. 6. ESI-MS spectrum for oligomer 3 with repeating unit ‘‘n’’ equal to 1 after separation with column chromatography (fraction H-5).
Table 3. Reaction conditions for azo-acetoxystyrene oligomers 2.
Experimental series
Azo unimer, 1 mg (mmol)
10 (0.02)
10 (0.02)
10 (0.02)
10 (0.02)
Acetoxystyrene, 5 mg (mmol)
251 (1.54)
230 (1.41)
267 (1.65)
250 (1.54)
Chlorobenzene (PhCl) mg (mmol)
Bulk (monomer solvent)
Bulk (monomer solvent)
Bulk (monomer solvent)
Bulk (monomer solvent)
170 (1.51)
A-1a
A-2
A-3
A-4
B
28 (0.06)
170 (1.05)
C
31 (0.06)
162 (1.05)
324 (2.90)
D
30 (0.06)
162 (1.05)
487 (4.34)
E
31 (0.06)
162 (1.05)
648 (5.78)
F
31 (0.06)
162 (1.05)
1600 (14.2)
aExperiments A-1 through A-4 were performed at 130 8C. All other runs were performed at 120 8C.
Table 4. Azo-naphthalene oligomers 3. Reaction conditions: 120 8C.
Experiment
1 mg (mol)
12 (0.027)
12 (0.027)
13 (0.025)
13 (0.028)
11 (0.024)
6 mg (mmol)
500 (1.92)
500 (1.92)
50 (0.20)
110 (0.42)
50 (0.20)
PhCl mg (mmol)
1500 (13.3)
1500 (13.3)
288 (2.57)
600 (5.30)
1 mL
Ratio 6:PhCl (w/w)
Time (min)
Q-1
Q-2
R
S
T
1:3
1:3
1:4
1:5
1:49
15
40
10
30
60
1.1 mmol) were mixed with acetone (25 mL) in a round-bot-
tom flask and the reaction mixture was brought to reflux for
36 h. The acetone was removed under reduced pressure (ro-
tary evaporator) and the residue added to water and di-
chloromethane. Separation, drying, and evaporation of the
organic layer yielded crude product as an oil. This product
was purified by column chromatography (CH2Cl2/hexane,
1:1 v/v) to yield after evaporation of solvent colourless crys-
tals of 4-(1-methoxynaphtyl)styrene (1.7 g, 75%); mp 48–
1
50 8C. H NMR (CD2Cl2, 400 Mz) d: 8.23 (1H, d, J = 7.6,
H-16), 7.99 (1H, d, J = 8.8, H-13), 7.95 (1H, d, J = 8.0, H-
11), 7.66 (1H, m, H-15), 7.61 (2H, m overlapping signals,
H-10.14), 7.56 (1H, d, J = 8.4, H-9), 7.45 (2H, d, J = 8.0,
H-4,4’).
Published by NRC Research Press

920
Can. J. Chem. Vol. 88, 2010
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with N2 for 5 min prior to immersion in an oil bath stabi-
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reaction was quenched by addition of copious cold MeOH to
give a gummy material that was dissolved upon addition of
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Supplementary data
1
Supplementary data (MALDI-MS traces, H NMR spec-
tra, GPC profiles for oligomers) for this article are available
on the journal Web site (canjchem.nrc.ca).
Acknowledgements
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We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for funding of this research
(E.B.) and the Higher Education Commission of Pakistan
(HECP) for support (D.A.). Dildar Ali especially thanks
HECP for a Fellowship under the Split Ph.D. Program. The
authors thank Thomas M. Kraft for editorial assistance and
Dr. Julian M. Dust for discussion.
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