NOVEL SYNTHESIS OF (CARBOXYLIC ACID)-TELECHELIC
POLY(-CAPROLACTONE)
Robson F. Storey and John W. Sherman
Department of Polymer Science
University of Southern Mississippi
Southern Station Box 10076
Hattiesburg, MS 39406-0076
polymerization (Figure 2) clearly show that conversion of the monomer was
complete by 3.5 h. However, the final molecular weight (2700 g/mol) was
higher than theoretical (2,000 g/mol), which was attributed to the
condensation polymerization of the -hydroxyl--(carboxylic acid)
oligomers.
Additional evidence for the occurrence of condensation
polymerization was the appearance of water vapor on the walls of the flask
during the quenching process.
O
Introduction
There has been recent interest in using end functionalized low molecular
weight polyester prepolymers for the synthesis of biodegradable networks. 1,2,3
These syntheses involve the initial ring-opening polymerization of a cyclic
ester, such as lactide, glycolide and/or -caprolactone, with a polyol initiator
to produce hydroxyl-telechelic polyesters. The latter have been used in
various post-polymerization reactions to produce a variety of polymers
containing end groups suitable for cross-linking. In particular, it has been
reported that (carboxylic acid)-telechelic polyesters can be synthesized using
a two step synthesis4 consisting of isolation of the hydroxyl-telechelic
prepolymer followed by reaction with succinic anhydride. The focus of this
paper is the synthesis of (carboxylic acid)-telechelic poly(-caprolactone)
polymers using a new, one-step method that eliminates isolation of the
hydroxyl-telechelic prepolymer intermediate.
Experimental
Materials. All reagents were used without further purification. Glycolic
acid (99%), and succinic anhydride (97%) were purchased from the Aldrich
Chemical Co. Stannous 2-ethyl-hexanoate (stannous octoate, 95%) was
purchased from Sigma Chemical Co. -Caprolactone (high purity) was
donated by Union Carbide Co.
Instrumentation. Gel permeation chromatography (GPC) was used to
determine molecular weights and molecular weight distributions, M w/Mn, of
polymer samples with respect to polystyrene standards (Polysciences
Corporation). The system configuration has been described previously.4
13C NMR spectra of the polymers were obtained on a Bruker AC-200
spectrometer using 5 mm o.d. tubes. Sample concentrations were about 25%
(w/v) in CDCL3 containing 1% TMS as an internal reference.
Synthesis of -Hydroxyl--(Carboxylic Acid) Poly(-Caprolactone).
Glassware and stir bar were dried at 145-155C for 24 h, fitted with rubber
septa, and cooled under a flow of dry nitrogen. To a 40mL test tube equipped
with a 24/40 ground glass joint and magnetic stir-bar were added glycolic
acid (5.1 x 10-3 mol, 0.39g), -caprolactone (8.8 x 10-2 mol, 10g) and
stannous octoate catalyst (1.4 x 10 -4 mol/mol monomer). The tube was
purged with dry N2 gas, sealed with a glass stopper, and placed in a 140C
constant temperature oil bath. The polymerization was carried out for 3.5 h
with continuous stirring, and then quenched by immersing the tube in an icewater bath. The product was characterized by 13C NMR with no purification.
Synthesis of (Carboxylic Acid)-Telechelic Poly(-Caprolactone). To
a 40mL test tube equipped with a 24/40 ground glass joint and magnetic stirbar were added glycolic acid (5.4 x 10 -3 mol, 0.41g), -caprolactone (8.8 x 102 mol, 10g), succinic anhydride endcapper (5.4 x 10 -3 mol, 0.55g), and
stannous octoate catalyst (1.4 x 10-4 mol/mol monomer). The tube was then
purged with dry nitrogen gas, sealed, and placed in a 140C constant
temperature oil bath. The polymerization was carried out for 12 h with
continuous stirring, and then quenched by immersing the tube in an ice-water
bath. The product was characterized by 13C NMR with no purification
Results and Discussion
-Hydroxyl--(Carboxylic Acid) Poly(-Caprolactone).
The
synthesis of -hydroxyl--(carboxylic acid) poly(-caprolactone) depicted in
Figure 1, involved the reaction of glycolic acid with -caprolactone in the
presence of stannous octoate catalyst. In view of the reported role of hydroxyl
groups as initiators of the ring-opening polymerization,5,6 this reaction was
expected to produce an oligomer (A) containing a carboxylic acid group on
one end, derived from a single, terminal unit of glycolic acid, and n units of caprolactone, and terminating in a primary hydroxyl group at the other end of
the chain. GPC chromatograms of aliquots taken at various times from the
O
O
n
O
+
SnOct
HO
OH
HO
O H
O
140 C
A
O
n
Figure 1. Synthesis of -Hydroxy--(Carboxylic Acid) End Functional
Poly(-Caprolactone).
1h
754 g/mol
toluene flow marker
-caprolactone
2h
2259 g/mol
3.5 h
2754 g/mol
800
1000
1200
Time (seconds)
Figure 2. GPC traces showing the incorporation of -caprolactone during the
synthesis of oligomer (A).
Figure 3 shows the 13C NMR spectrum of oligomer (A) with structural
assignments. The spectrum shows evidence of a mechanism involving the
ring-opening polymerization of -caprolactone initiated by glycolic acid
followed by condensation polymerization and transesterification. This
combination of reactions created a polymer structure containing slightly more
than one glycolic acid unit per chain, one at the head of the chain and an
occasional additional unit at some random position along the rest of the
chain. In the carbonyl carbon region of the spectrum are six peaks designated
1-6. The largest (1) at 173.6 ppm was attributed to the ester carbonyl carbons
derived from internal -caprolactone repeat units that are adjacent to other
caprolactone units. Only one peak, at 177.2 ppm (2), has a chemical shift
characteristic of carboxylic acid carbonyl carbons, and this peak was logically
assigned to the carboxylic acid end groups derived from glycolic acid.
Peaks 3 and 5 were identified according to the assignments for caprolactone/glycolide copolymers given by Kricheldorf and coworkers, 7 and
peak 6, barely discernible as a shoulder on the downfield side of peak 1, was
attributed to the carbonyl carbons of -caprolactone units at the hydroxyl end
of the chain.5 Peak 4 was assigned to the carbonyl carbons of terminal
glycolic acid units at the hydroxyl end of the chain, i.e., (glycolic
acid)/hydroxyl end groups. Condensation polymerization alone cannot
produce such end groups; however, it was reasoned that in-chain ester
linkages of the type ~Gly-O-CO-Capro~ are particularly susceptible to
transesterification under the conditions employed in the polymerization, and
this reaction resulted in the creation of the observed end groups.
In Figure 3, the methylene carbons of -caprolactone are defined as ,
, , , and , moving away from the carbonyl carbon, while the glycolic acid
methylene carbon is defined as g. In the  region, there are five peaks of
interest: , , OH, g, and g. The largest peak at 64.1 ppm () was assigned to
the -carbons of internal caprolactone units that are adjacent to other caprolactone units. The smaller companion peaks at 65.2 ppm () and 62.5
ppm (OH) were attributed to -carbons of caprolactone units adjacent to
glycolic acid units and terminal caprolactone units at the hydroxyl end of the
chain, respectively.
Two peaks due to glycolic acid methylene residues were observed at
60.2 (g) and 60.5 ppm (g). The former was assigned to glycolic acid units
surrounded on both sides by caprolactone units. 7 The g peak was assigned to
the methylene carbons of terminal glycolic acid units at the carboxylic acid
end of the chain. However, a third peak was expected in this region due to
the methylene carbon of glycolic acid/hydroxyl end groups. It must be
assumed that this peak (gOH) is coincident with either g or g. Later, it will be
seen that gOH becomes resolved into a separate peak when the terminal
hydroxyl end groups are esterified.
In the -carbon region of Figure 3, the main internal caprolactone peak
() is flanked on the upfield side by a smaller peak at 33.6 ppm. This peak
was attributed to -carbons of -caprolactone adjacent to glycolic acid units.
The small peak located at 32.2 ppm, OH, was attributed to the -carbon
located at the hydroxyl endgroup.5 The remainder of the , , and  regions
were insensitive to the presence of adjacent glycolic acid residues. 7
Capro

O

C
CH2 CH2 CH2 CH2 CH2 O Capro



O
Capro
''
O
O
C CH2 CH2 CH2 CH2 CH2 O
HO
3
O
Capro
caprolactone
succinic anhydride
toluene flow marker
2h
1397 g/mol
3.5 h
2496 g/mol
gOH
C CH2 OH
5h
2508 g/mol
9h
2611 g/mol
4
1
Gly
attributed to the ultimate -carbon adjacent to the terminal succinyl group.
The ultimate methylene carbon of the succinyl end groups () was observed
at 29.0 ppm. The penultimate succinyl methylene carbons, () and (), were
assigned to peaks at 28.8 and 29.1 ppm, respectively. The existence of two,
distinct -methylene carbon peaks lent additional support for the presence of
the two types of end groups depicted in Figure 6.
C
g'
O
CH2 O C
'
CH2 CH2 CH2 CH2 CH2 O
12 h
2538 g/mol
2
g
O
C
C CH2 O Capro
6
5
800

CH2 CH2 CH2 CH2 CH2 OH
Time (seconds)

''
O
 

Capro

OH
180
', ''
g'
'
2 345
60
CH2 O C
40
O
30
20
O
OH
O
' 
7
O

CH2 CH2 C OH
9
7

O
SnOct
 

50
ppm
1 39
(Carboxylic Acid)-Telechelic Poly(-Caprolactone). The synthesis of
(carboxylic acid)-telechelic poly(-caprolactone) is depicted in Figure 4.
This polymerization involved ring-opening of -caprolactone initiated by
glycolic acid, with termination by reaction with succinic anhydride.
O
+
O

HO
O
O
O
140 C
O
B
OH
nO
Figure 4.
The Synthesis of (Carboxylic Acid)-Telechelic Poly(Caprolactone).
GPC was used to monitor the conversion of -caprolactone and the
incorporation of succinic anhydride onto the polymer chain end. Figure 5
depicts the GPC chromatograms of aliquots taken at various times, and it
clearly shows that by 12 h there is complete conversion of monomer and
incorporation of succinic anhydride into the polymer.
Figure 6 shows the 13C NMR spectrum of oligomer (B) and suggests
that succinic anhydride has been incorporated onto the chain-end and perhaps
occasionally into the interior of the chain as a result of condensation
reactions. The spectrum revealed a pattern of disappearance and appearance
of certain peaks consistent with the reaction of succinic anhydride with the
hydroxyl chain ends. For example, two new peaks arose at 178.0 and 172.2
ppm which were attributed to the succinyl carbonyls, 7 and 8, respectively.
Closer inspection revealed that peak 8 actually consists of two peaks. This
third carbonyl peak at 172.4 ppm (9) and a new peak in the g-region at 60.9
ppm (g) were assigned to succinyl units formed by reaction of succinic
anhydride with glycolic acid/hydroxyl end groups.
Additional strong evidence of succincic acid incorporation was
observed in the - and -carbon regions of the spectrum. The two peaks
associated with the hydroxy/caprolactone endgroup, depicted in Figure 3 as
OH at 62.5 ppm and OH at 32.2 ppm were absent in Figure 6. The
incorporation of the succinyl group caused these carbons to fall in line with
the other main-chain caprolactone carbons. The peak at 64.6 ppm () was
 ''
8
5
72
180
O
 
CH2 CH2 C OH
g
70
+ HO
C
g''
Figure 3.
NMR spectrum and structural assignments of -hydroxy-(carboxylic acid) end functional poly(-caprolactone).
O
O

13C
n
O
OH
1
O
C CH2 CH2 CH2 CH2 CH2 O C
8
Capro
6
1200
Figure 5. GPC traces showing the incorporation of both -caprolactone and
succinic anhydride during the synthesis of oligomer (B).
OH

1000
170
'
70
g''
'
g'
60
ppm

', ''
g
50
40
30
20
Figure 6. 13C NMR spectrum and additional structural assignments of
(carboxylic acid)-telechelic poly(-caprolactone).
Conclusions
Initiation of -caprolactone with glycolic acid yields a polymer structure
with carboxylic acid head groups derived from glycolic acid and occasional
additional glycolic acid units placed randomly along a poly(-caprolactone)
chain terminating with a hydroxyl tail. This structure was attributed to
initiation by glycolic acid, followed by condensation polymerization and
transesterification reactions. When the same polymerization was carried out
in the presence of sufficient succinic anhydride to cap the hydroxyl ends of
the chains, a (carboxylic acid)-telechelic polymer was produced via a one step
synthesis. This method demonstrated good incorporation of the monomer
into the polymer in a reasonable amount of time and enabled the elimination
of a separate reaction of the polymer with succinic anhydride.
Acknowledgements. The authors of this paper would like to thank
Boehringer-Mannheim for financial support of this research.
References
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Polymer 1993, 34, 4365.
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