Synthesis of new telechelic polymers containing two OH groups at the same chain end using 8-boraindane/oxygen controlled radical initiator

Article abstract of DOI:10.1021/ma0495341

The synthesis of telechelic polymers which contains two OH groups using 8-boraindane/oxygen controlled radical initiator was analyzed. It was observed that process is centered on bicycloborane compound which serve as functional initiator. In the polymeriz

Full text of DOI:10.1021/ma0495341

6260
Macromolecules 2004, 37, 6260-6263
In this paper, we discuss a novel route for prepar-
ing new telechelic polymers, including poly(methyl
methacrylate) (PMMA) and poly(trifluoroethyl acrylate)
(PTFEA), containing two reactive OH groups at the
same polymer chain end. The process is centered on a
bicycloborane compound (i.e., 8-boraindane) serving as
a functional initiator that not only initiates the con-
trolled radical polymerization but also produces two
functional groups at the beginning of the polymer chain,
as illustrated in Scheme 1.
Syn th esis of New Telech elic P olym er s
Con ta in in g Tw o OH Gr ou p s a t th e Sa m e
Ch a in En d Usin g 8-Bor a in d a n e/Oxygen
Con tr olled Ra d ica l In itia tor
H. Hon g a n d T. C. Ch u n g*
Department of Materials Science and Engineering,
The Pennsylvania State University,
University Park, Pennsylvania 16802
Received March 8, 2004
Scheme 1 only explains the reaction mechanism;
however, oxidation reactions on both 5- and 6-member
rings are possible. Ideally, one of the three cyclic C-B
bonds in 8-boraindane (I) is oxidized under a controlled
oxidation condition to form a peroxide compound (C-
O-O-BR2) (II) that initiates controlled radical polym-
erization.13 In the presence of MMA monomers at
ambient temperature, the C-O-O-BR2 species is
further decomposed to an alkoxyl radical (C-O*) and a
borinate radical (*O-BR2). The alkoxyl radical is active
in initiating the polymerization of MMA. On the other
hand, the borinate radical is believed to be too stable
to initiate polymerization due to the back-donating of
electrons to the empty p-orbital of boron. However, this
“stable” borinate radical may form a reversible bond
with the radical at the growing chain end to form
dormant species and prolong the lifetime of the propa-
gating radical. In the whole polymerization process, the
monooxidized bicycloborane residue remains bonded to
the beginning of the polymer chain (III), despite the
continuous growth of the polymer chain. After complet-
ing the polymerization, the two unreacted C-B bonds
in the borane residue can be completely converted to
two OH groups14 by H2O2/NaOH reagent. The resulting
telechelic PMMA polymer (IV) has two OH groups
located at the same chain end as well as a controlled
molecular weight and relatively narrow molecular weight
distribution.
Revised Manuscript Received J uly 2, 2004
Telechelic polymers,1 possessing two terminal func-
tional groups, are an important class of polymeric
materials. They find applications as prepolymers for
inclusion in final products (linear polymers, networks,
or surfaces)2 with well-specified properties.3 A well-
known example is polyol, such as telechelic poly-
(ethylene oxide) and poly(propylene oxide) having two
terminal OH groups (one on each chain end), which
becomes part of polyurethane4 after coupling with
diisocyanates or/and triisocyanates. They can be used
along or mixed with other diols or triols to control the
final polymer architecture. In general, the prepolymers
provide flexibility to the polymer design in order to
prepare materials with a wide array of physical proper-
ties. However, the purity of prepolymer, with a func-
tionality of 2 (f ) 2) in each polymer chain, is essential
for successful construction of the final polymer struc-
ture.
In chemistry, the ability to synthesize telechelic
polymers containing exactly two terminal functional
groups (f ) 2) is very limited. Most commercial telechelic
polymers, such as aliphatic polyesters5 (with two op-
posing terminal acid or alcohol groups) and poly-
(ethylene oxide) and poly(propylene oxide)6 (with two
opposing terminal alcohol groups), are prepared by
polycondensation reaction and ring-opening polymeri-
zation with suitable initiators, respectively. The prepa-
ration of telechelic vinyl polymers (f ) 2) is very difficult
and usually requires the combination of living polym-
erization, difunctional initiators7 or functionally sub-
stituted initiators,8 and selective termination (function-
alization) chemistry. Although this methodology has
been applied to practically all vinyl polymerization
techniques, including anionic,9 cationic,10 free radical,11
and metathesis,12 only a few examples have been
demonstrated to achieve a functionality of 2 in each
polymer chain. The complete end-capping of the active
polymer chain end with a functional group is particu-
larly difficult due to the low concentration of active sites
and the limited number of suitable reagents that can
achieve 100% functionalization.
Table 1 summarizes several comparative polymeri-
zation sets in the preparation of telechelic PMMA and
PTFEA polymers, including both bulk and solution
reaction conditions. After mixing 8-boraindane with
MMA or TFEA monomers (with or without solvent) a
controlled amount of oxygen was slowly introduced into
the system to start the polymerization at ambient
temperature. The 8-boraindane/O2 mole ratios were
controlled between 3/1 and 3/2 to minimize the potential
for multiple-oxidation reaction on each 8-boraindane.
Comparing the polymer yield vs reaction time in two
bulk polymerization sets (runs 1-5 and 6-9) shows that
there was almost double the productivity in the 3/2 case
vs the 3/1 case, which indicates the formation of active
sites linearly proportional to the oxygen quantity under
controlled oxidation conditions. As expected, the poly-
mer molecular weight also showed a significant reduc-
tion in the higher active site environment. After a
designated reaction time, all polymers were treated with
NaOH and H2O2 to ensure complete oxidation of the
borane moieties. The resulting polymers were charac-
It is scientifically interesting to develop a new syn-
thesis route for preparing vinyl-based telechelic poly-
mers as well as new telechelic polymer structures that
have two terminal functional groups located at the same
chain end. Upon subsequent condensation reaction, this
telechelic prepolymers can be incorporated as the long
side chains in the final polymer structure (branch,
network, supermolecular structures) to provide specific
properties.
1
terized by H and 13C NMR-DEPT measurements and
gel permeation chromatography (GPC) using a standard
polystyrene calibration curve.
Figure 1 (top) compares the GPC curves of three
telechelic PMMA samples (runs 1, 3, and 5 in Table 1).
* To whom all correspondence should be addressed.
10.1021/ma0495341 CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/27/2004

Macromolecules, Vol. 37, No. 17, 2004
Communications to the Editor 6261
Sch em e 1
The molecular weight of obtained polymer continuously
increased throughout the polymerization process, from
18 000 g/mol in run 1 to 100 000 g/mol in run 5. The
broader molecular weight distribution at the beginning
of the polymerization may be attributed to the slow
formation of active sites that are prepared in situ by
reacting two very dilute reagents, including borane
reagent in solution and oxygen in gas phase. Similar
results were also observed in the PTFEA polymers (runs
10-14) that were prepared in THF solution. With longer
reaction time, the polymer increases its molecular
weight and the overall molecular weight distribution
gradually reduces. In the second reaction set (runs 6-9),
both the polymer molecular weight and the molecular
weight distribution increase with reaction time. The
broadening in the molecular weight distribution may
indicate the diffusion problem in the bulk condition,
especially after high monomer conversions (runs 7 and
8).
Figure 1 (bottom) plots polymer molecular weight vs
monomer conversion in the first bulk polymerization set
(runs 1-5 in Table 1) using a 1/3 [oxygen]/[borane] mole
ratio. Compared with a theoretical line based on the [g
of monomer consumed]/[mole of initiator], a good match
with the straight line through the origin supports the
presence of controlled radical polymerization in the
reaction. The linear increase of the molecular weight
with a relatively narrow molecular weight distribution
implies a “stable” propagating chain end without sig-
nificant termination and chain transfer reactions. Ap-
parently, a constant number of active sites are formed
after the oxidation reaction, which maintained reactivity
throughout the polymerization process. This controlled
radical polymerization was also evidenced in the prepa-
ration of diblock copolymers, such as poly(methyl meth-
acrylate-b-trifluoroethyl acrylate), by sequential mono-
mer addition (discussed later).
F igu r e 1. (top) GPC curves of telechelic PMMA samples (runs
1, 3, and 5). (bottom) Plots of polymer molecular weight vs
monomer conversion (runs 1-5).
The two terminal OH groups located at the beginning
of the polymer chain were examined by a combination
of 1H and 13C NMR-DEPT measurements. Figure 2
at 0.15 ppm corresponding to -O-Si(CH3)3 is clearly
observed with a reasonable intensity for qualitative
analysis. The peak intensity ratio between two peaks
(3.58 and 0.15 ppm) and the representing protons
indicate about two OH groups per PMMA chain.
To provide direct experimental evidence for the exist-
ence of both primary and secondary OH groups in the
telechelic polymer (as shown in eq 1), the resulting
polymers were also examined by the 13C NMR (DEPT-
135) technique. Figure 3 shows the 13C NMR (DEPT-
135) spectrum of a relatively low molecular weight
telechelic PTFEA polymer (Mn ) 7 × 103; Mw/Mn ) 2).
In addition to three expected chemical shifts at 25.4-
35.6 ppm (negative), 41.1 ppm (positive), and 60.1 ppm
(negative), corresponding to the CH2, CH, and O-CH2
1
shows the H NMR spectra of a telechelic PMMA (Mn
) 11 × 103; Mw/Mn ) 1.6) and its corresponding
trimethylsilane derivative. In addition to the expected
chemical shifts between 0.9 and 1.9 ppm, corresponding
to the CH3 and CH2 groups in the PMMA backbone,
there is a strong singlet chemical shift at 3.58 ppm
(-O-CH3 group in PMMA) overlapping with several
weak peaks that indicate the existence of OH groups.
To determine the concentration of the OH groups, the
telechelic PMMA polymer was quantitatively converted
to the corresponding silane derivative by reacting with
Cl-Si(CH3)3 reagent. In Figure 2b, a new chemical shift

6262 Communications to the Editor
Macromolecules, Vol. 37, No. 17, 2004
Ta ble 1. A Su m m a r y of Telech elic P MMA a n d P TF EA P olym er s P r ep a r ed by 8-Bor a in d a n e/O2
reaction conditionsa
PMMA polymer
monomer
(mL)
borane/O2
(mmol/mmol)
solvent
(mL)
time
(min)
yield
(%)
Mn
(g/mol)
Mw
(g/mol)
PDI
(Mw/Mn)
run no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
MMA/10
MMA/10
MMA/10
MMA/10
MMA/10
MMA/10
MMA/10
MMA/10
MMA/10
TFEA/5
TFEA/5
TFEA/5
TFEA/5
TFEA/5
1.2/0.4
1.2/0.4
1.2/0.4
1.2/0.4
1.2/0.4
1.2/0.8
1.2/0.8
1.2/0.8
1.2/0.8
0.6/0.4
0.6/0.4
0.6/0.4
0.6/0.4
0.6/0.4
0
0
0
0
0
0
0
0
10
20
40
60
90
20
40
60
90
120
240
360
480
600
7
11
19
24
40
22
42
63
80
12
19
40
52
60
18 000
28 000
45 000
60 000
100 000
15 300
18 600
20 300
b
42 000
60 000
76 000
110 000
160 000
34 400
51 200
68 500
b
14 000
23 500
49 000
52 500
56 000
2.3
2.1
1.8
1.7
1.6
2.2
2.8
3.4
b
2.0
2.0
1.9
1.7
1.6
0
THF/40
THF/40
THF/40
THF/40
THF/40
7000
12 000
25 000
30 500
33 000
a
b
Reaction temperature: 25 °C. Double peaks.
F igu r e 2. 1H NMR spectra of (bottom) a telechelic PMMA
(Mn ) 11 × 103; Mw/Mn ) 1.6) and (top) its corresponding
trimethylsilane derivative.
F igu r e 3. (top) 1H and (bottom) 13C NMR (DEPT-135)
spectrum of a telechelic PTFEA polymer (Mn ) 7 × 103; Mw/
Mn ) 2).
groups, respectively, in the PTFEA backbone, there are
two distinctive chemical shiftssone negative peak at
68.2 ppm, corresponding to the primary CH2-OH group,
and one positive peak at 77.6 ppm, corresponding to the
secondary CH-OH group.
It is interesting to take advantage of the controlled
radical polymerization mechanism to broaden the mo-
lecular structure, especially for block copolymers. In
addition, the preparation of diblock copolymers, such
as poly(methyl methacrylate-b-trifluoroethyl acrylate)
by sequential monomer addition, provides strong evi-
dence for the controlled radical polymerization. Figure
4 compares the 1H NMR spectra and GPC curves of
poly(methyl methacrylate-b-butyl methacrylate) diblock
copolymer and the corresponding poly(methyl meth-
acrylate) homopolymer. The molecular weight almost
doubles from the homopolymer (Mn ) 12 400 and Mw )
24 000 g/mol) to the diblock copolymer (Mn ) 32 800 and
Mw ) 58 000 g/mol) without a broadening in the
molecular weight distribution. Basically, the copolymer
composition is controlled by the monomer feed ratio and
reaction time.
Overall, the combined experimental results strongly
indicate a clean and effective route for preparing tele-
chelic polymers, including PMMA, PTFEA, and PMMA-

Macromolecules, Vol. 37, No. 17, 2004
Communications to the Editor 6263
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MA0495341