Full text of DOI:10.1016/S0032-3861(01)00484-0

Polymer 42 ꢀ2001) 9319±9327
www.elsevier.com/locate/polymer
Polyꢀa-alkyl g-glutamate)s of microbial origin:
I. Ester derivatization of polyꢀg-glutamic acid) and thermal degradation
*
  Â
Jose Melis, Margarita Morillo, Antxon Martõnez de Ilarduya, Sebastian MunÄoz-Guerra
 Á
Departament d' Enginyeria Quõmica, Universitat Politecnica de Catalunya, ETSEIB, Diagonal 647, Barcelona 08028, Spain
Received 19 April 2001; received in revised form 26 June 2001; accepted 3 July 2001
Abstract
A series of polyꢀa-alkyl g-glutamate)s ꢀPAAG-n with n ˆ 1±10) were prepared from polyꢀg-glutamic acid) ꢀPGGA) of microbial origin
by a two-steps synthesis procedure. The polyꢀa-ethyl g-glutamate) was ®rstly obtained by esteri®cation of PGGA and then transesteri®ed
with the selected alkanol to obtain the corresponding PAAG-n. The modi®cation reactions were found to occur without alteration of the
original enantiomeric composition but with a signi®cant reduction in molecular weight. The thermal degradation of PAAG-n was examined
by thermogravimetric analysis combined with IR and NMR spectroscopy. At low temperatures ꢀ,3008C† decomposition was found to take
place by depolymerization with releasing of the corresponding alkyl pyroglutamate. Decarboxylation of PAAG-n to an unsaturated poly-
amide seemed to be the main process occurring at high temperatures ꢀ400±4508C†: q 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Polyꢀg-glutamic acid); Polyꢀa-alkyl g-glutamate)s; Thermal degradation
1. Introduction
PGGA is highly hydrophilic, readily soluble in water and
its physical behavior is largely affected by the presence of
humidity. Esteri®cation of PGGA is usually considered to
be the most adequate method to modify the polymer in order
to render materials with suitable processing and handling
properties. With regards to their potential applications in the
biomedical ®eld, a good thermal stability compatible with a
fair biodegradability is a combination of properties highly
desired for these PGGA derivatives.
The synthesis and properties of a number of polyꢀg-gluta-
mate)s derived from natural PGGA have been described so
far. The preparation procedures used by different authors
[6±11] are slight variations in the application of the reaction
originally developed by Bocchi et al. [12] for the esteri®ca-
tion of aminoacids in polar aprotic solvents. Essentially, the
method consists of treating the sodium salt of PGGA with
alkyl or aryl halides in the suitable solvent and under the
appropriate conditions of time and temperature. Polyꢀg-
glutamate)s with alkyl side chains ranging from methyl
up to dodecyl, in addition to the benzyl derivative, have
been thus prepared from PGGA attaining conversions
between 0.9 and 1.0. The reliability of the method and the
accomplishment of the esteri®cation reaction are however
depending on the size of the alkyl side group, with better
results being seemingly reached for short esters. Thus, Shah
et al. [6] reported that esteri®cation of PGGA to polyꢀa-
dodecyl g-glutamate) had to be carried out in successive
Polyꢀg-glutamic acid) ꢀPGGA) is a biosynthetic polymer
consisting of a nylon 4 main chain bearing a carboxyl group
attached to the g-carbon atom of every repeating unit. The
naturally produced polymers may have molecular weights
of up to millions and usually contain nearly equal amounts
of d- and l-units. In laboratory fermentation cultures, the
d/l ratio may be partially controlled so that polymers
with varying degree of stereoregularity may be yielded
[1±3]. An industrial process using Bacillus subtilis F-201
has been recently established for the production of race-
mic PGGA on a large scale [4]. The research interest
presently renewed for this biopolymer is due to its poten-
tial as a biocompatible material with biodegradable prop-
erties. In fact, PGGA is known to be degraded by
proteases at a rate that is depending on the degree of
esteri®cation and length of the alkyl side chain [5].
* Corresponding author.
E-mail address: munoz@eq.upc.es ꢀS. MunÄoz-Guerra).
0032-3861/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0032-3861ꢀ01)00484-0

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J. Melis et al. / Polymer 42 62001) 9319±9327
Table 1
Esteri®cation of PGGA to PAAG-2
PGGA
Reaction conditions
PAAG-2
a
a
h ꢀdl g21
)
d/lb
Solvent
t ꢀdays)
T ꢀ8C)
C ꢀ%)c
Yield ꢀ%)
h ꢀdl g21
)
Mvd
d/lb
ꢀDMSO)
DMS
DCA
PG ꢀDL)GA
1.77
0.86e
59:41
56:44
NMP
1
60
45
45
45
100
88
92
PAAꢀDL)G-2
PAAꢀD)G-2
0.70
0.81
0.52
0.48
1.32
1.41
0.82
0.75
88,000
60:40
52:48
52:48
52:48
DMSO
7
.98
.98
.98
96,000
46,000
41,000
11
13
PGꢀD)GA
5.00
89:11
89:11
85:15
NMP
NMP
1
1
60
60
45
100
100
100
80
88
1.40
0.92
0.38
2.36
1.64
0.57
192,000
118,000
28,000
88:12
88:12
87:13
1.70e
0.30e
DMSO
11
a
Intrinsic viscosity measured at 25 ^ 0:18C in dimethyl sulfoxide ꢀDMSO) or dichloroacetic acid ꢀDCA).
Enantiomeric ratio determined by HPLC.
b
c
d
1
Conversion degree measured by H-NMR.
Viscosity-average molecular weight estimated by using the Mark±Howink±Sakurada equation [24] h ˆ 2:9 £ 1024Mv0:74 established for polyꢀg-methyl
a,l-glutamate) in DCA.
e
PGGA samples subjected to microwave irradiation for molecular weight reduction.
steps using n-dodecyl iodide in hexamethylphosphoramide
in order to obtain conversions higher than 95%. The short-
coming of the esteri®cation reaction carried out in solution
is that the initially solubilized PGGA becomes insoluble as
esteri®cation proceeds. This fact restricts the accessibility of
the partially modi®ed polymer to the nucleophylic attack by
the alkyl halide and prevents from reaching complete
conversions. The chemical synthesis of optically pure
PGGA and some of their esters, i.e. polyꢀg-glutamate)s,
was reported long time ago by Hungarian researchers
[13±16] and recently revisited by Sanda et al. [17].
In this paper, we wish to report on a synthetic route that
may be alternatively used for the systematic preparation of
PGGA esters, as well as on the thermal degradation of these
compounds. This new method of synthesis is based on the
transesteri®cation of the polyꢀa-ethyl g-glutamate) with
alkanols. By this means, a series of polyꢀa-alkyl g-gluta-
mate)s, henceforth abbreviated as PAAG-n with n standing
for the number of carbon atoms in the alkyl side chain, are
prepared for n ranging between 1 and 10. The method is of
general application yielding good results regardless the
length of the alkyl side chain. The stability to heating of
polyꢀg-glutamate)s are comparatively examined and the
molecular mechanism involved in the thermal degradation
is investigated.
analytical grade and used as received. Two polyꢀg-glutamic
acid)s differing in the enantiomeric composition were used
in this work. PGꢀDL)GA with a nearly racemic d/l ratio and
a weight average molecular weight of about 4 £ 105 was
kindly supplied by Dr Kubota of Meiji ꢀJapan). PGꢀD)GA
with a highly enriched d-composition and a weight average
molecular weight of about 106 was prepared speci®cally for
the purpose of this work by fermentation of B. licheniformis,
as reported in detail elsewhere [18]. For evaluating the in¯u-
ence of the polymer size on the polymer modi®cation reac-
tions, some original samples were degraded by irradiation
with microwaves according to the method developed by us
for reducing the molecular weight of polypeptides [19]. The
main characteristics of all the PGGA samples used in this
work are shown in Table 1.
2.2. Esteri®cation and transesteri®cation reactions
Polyꢀa-ethyl g-glutamate) ꢀPAAG-2) was obtained by
either of the two methods described in the literature [6±
11]. Method A: to a solution of PGGA ꢀ5 g, 38.8 mmol)
in dimethyl sulfoxide ꢀDMSO) ꢀ125 ml) containing 6.5 g
ꢀ77.6 mmol) of NaHCO3, ethyl bromide ꢀ22 ml,
294 mmol) was added dropwise under vigorous stirring.
The mixture was left to react for 5 days at 458C and then
precipitated with cool water. The partially ethylated PGGA
was recovered by ®ltration, washed with water and then
subjected to further esteri®cation for 3 days under the
same reaction conditions used in the ®rst stage. Method B:
2 g ꢀ15.5 mmol) of PGGA suspended in N-methyl pyrroli-
done ꢀNMP) ꢀ200 ml) were left overnight under stirring at
808C. The mixture was cooled down to 608C and added with
NaHCO3 ꢀ5.25 g, 62.5 mmol). Then ethyl bromide ꢀ6 ml,
80 mmol) was slowly added for 2 h and left to react for a
period of time between 20 and 30 h. After removing the
NaBr precipitate, the reaction solution was poured into
2. Experimental
2.1. Materials
All chemicals were obtained commercially from either
Aldrich or Merck. They were analytical grade or higher,
and used without further puri®cation. Solvents to be used
under anhydrous conditions were dried by standard
methods. Alcohols for transesteri®cation reaction were of

J. Melis et al. / Polymer 42 62001) 9319±9327
9321
Once the reaction was ®nished the ®nal solution was poured
into methanol or ether and the mixture left to cool down to
room temperature. The precipitated polymer was separated
from the supernatant solution by decantation. Puri®cation
was accomplished by dissolving the product either in
chloroform or in a mixture of chloroform±tri¯uoroacetic
acid ꢀCHCl3±TFA) and reprecipitating it with methanol or
ethanol.
Scheme 1.
2.3. Measurements
Enantiomeric compositions of PGGA and PAAG-n were
determined according to the method developed by Crom-
wick and Gross [20]. The hydrolyzed polymer was made to
react with the Marfey's reagent ꢀ1-¯uoro-2,4-dinitrophenyl-
5-l-alanine amide, Pierce, Rockford, IL) to form the
diastereoisomeric dipeptides which were then analyzed by
HPLC. Viscosities were measured at 25:0 ^ 0:18C using an
Ubbelohde viscometer. FT-IR spectra were recorded on a
Perkin±Elmer FT-2000 instrument from ®lms prepared by
acidi®ed cool water ꢀ1.5 l, pH 1.5) to precipitate the poly-
mer, which was then separated by ®ltration. In both cases,
the PGGA-2 was obtained as a white powder that was
repeatedly washed with cool water, ethyl ether and ®nally
dried under vacuum at 508C.
PGGA-n for n ˆ 1±10 ꢀexcept for n ˆ 2) were obtained
according to the following general procedure: PGGA-2 ꢀx
moles) were suspended in ꢀ10±50) x moles of the alcohol of
choice containing 5 mol% of TiꢀOBu)4 at a temperature of
1808C and magnetically stirred under a nitrogen atmo-
sphere. In the case that lower boiling point alcohols were
used, reactions were conducted in a pressurized vessel. The
advancement of the reaction was monitored by comparing
1
casting from chloroform solution. H and 13C-NMR spectra
were recorded on a Bruker AMX-300 NMR spectrometer
with samples dissolved in DMSO-d6, CDCl3 or CDCl3±TFA
mixtures and using TMS as reference. DSC thermograms
were recorded in a Pyris I Perkin±Elmer calorimeter at a
heating rate of 108C min21 under a nitrogen atmosphere.
1
the areas of the appropriate signal in the H-NMR spectra.
1
Fig. 1. Evolution of the transesteri®cation reaction of PAAꢀDL)G-2 with isobutanol followed by H-NMR. The inset shows that full replacement of the ethyl
methylene protons ꢀ,4:3 ppm† by the butyl methylene protons ꢀ,4:0 ppm† happened after 25 min of reaction.

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J. Melis et al. / Polymer 42 62001) 9319±9327
Table 2
Transesteri®cation of PAAꢀDL)G-2 with alcohols
c
Polymer
t ꢀmin)a
C ꢀ%)b
Yield ꢀ%)
h ꢀdl g21
)
Mvd
d/le
Tm
DMSO
DCA
PAAꢀDL)G-2
PAAꢀDL)G-1
PAAꢀDL)G-3
PAAꢀDL)G-4
PAAꢀDL)G-4f
PAAꢀDL)G-6f
PAAꢀDL)G-8f
PAAꢀDL)G-10f
±
85
70
20
25
40
45
50
±
100
100
100
100
100
.98
.98
±
85
91
59
84
88
78
73
0.81
0.39
0.26
0.43
0.37
±
1.41
0.9
0.65
0.94
0.81
±
96,000
50,000
38,000
55,000
45,000
±
58:42
261
228
±
60:40
±
236
260
230
168
±
58:42
±
±
±
±
±
±
±
±
55:45
a
Reaction time at 1808C.
Conversion degree measured by H-NMR.
b
c
d
1
Intrinsic viscosity measured at 25 ^ 0:18C measured in dimethyl sulfoxide ꢀDMSO) or dichloroacetic acid ꢀDCA).
Viscosity-average molecular weight estimated by using the Mark±Howink±Sakurada equation [24] h ˆ 2:9 £ 1024 Mv0:74 established for polyꢀg-methyl
a,l-glutamate) in dichloroacetic acid ꢀDCA).
e
Determined by HPLC.
f
Non-soluble in the solvents used for viscosity measurements.
Thermogravimetric analyses were performed with a Perkin±
Elmer TGA-6 thermobalance under ¯owing nitrogen.
The transesteri®cation reaction was carried out at 1808C
with PAAG-2 suspended in a large excess of the alcohol of
choice and in the presence of TiꢀBuO)4 as catalyst. The
advance of the reaction was followed by 1H-NMR by
comparing the areas of the CH2 signal of the ethyl leaving
group with a conveniently selected signal of the replacing
alkyl side group. The evolution of transesteri®cation of
PAAG-2 with isobutanol is shown in Fig. 1. The conversion
was considered to reach 100% when the ethyl CH2 signal
completely disappeared. A good advantage offered by this
reaction is that the polymer passes into solution as transes-
teri®cation proceeds. Thus, the reaction mixture becomes
homogenized when a certain degree of conversion is
reached making easier the subsequent attack by the
replacing alcohol ꢀwhich is the reaction solvent) and possi-
ble to attain almost total conversion. The results obtained in
the transesteri®cation step are shown in Table 2 for the
whole set of alcohols examined in this work. Conversions
attained between 98 and 100%, the larger alcohols being
less ef®cient as substituting reagents. Puri®cation of the
products was feasible by simple precipitation from the reac-
tion mixture by addition of methanol. The purity of PAAG-n
thus obtained was assessed by IR, 1H and 13C-NMR spectro-
scopy, as illustrated in Fig. 2 for the case of PAAG-6. Simi-
larly to what happened in the esteri®cation stage, a certain
decreasing in viscosity was found to accompany the trans-
esteri®cation reaction whereas no appreciable change in the
enantiomeric composition was detected.
3. Results and discussion
3.1. Preparation and characterization of poly6a-alkyl
g-glutamate)s
Esteri®cation of PGGA to PAAG-n was performed in two
stages, as indicated in Scheme 1 ꢀreaction scheme for the
preparation of PAAG-n by esteri®cation in two steps). The
ethyl ester of PGGA ꢀPAAG-2) was obtained in ®rst place
by treating the polyacid with ethyl bromide using the two
methods available in the literature for the synthesis of this
compound. These methods essentially differ to each other in
the solvent and temperature used for reaction: DMSO at
458C in method A and NMP at 608C in method B. Two
PGGA, one almost racemic, PGꢀDL)GA, and the other
highly enriched in d-units, PGꢀD)GA, were used for ester-
i®cation. Reaction conditions and characteristics of the
resulting PAAG-2 are compared in Table 1. In both meth-
ods, results were found to be essentially independent of both
molecular weight and enantiomeric composition of the
polymer and no signi®cant alteration of the d/l ratio was
observed to occur. Although the conversion attained by the
two methods was nearly complete, PAAG-2 obtained in
NMP distinguished in showing no traces of unreacted
carboxilic groups. Comparison of viscosities for PGGA
and PAAG-2 ꢀno strictly comparable) revealed that a
considerable reduction in molecular weight took place
upon esteri®cation. Such a reduction is though to be due
to uncontrolled hydrolysis of the polyamide chain, a process
that appears to take place much easier for high molecular
weight species and, according to expectations, to increase
considerably with reaction time and temperature.
It can be concluded from these results that the character-
istics of the ®nal polyꢀa-alkyl g-glutamate)s obtained by
transesteri®cation do not differ signi®cantly from those
obtained by the single-step esteri®cation methods described
in the literature. The same can be said about yields, as far as
short alkyl groups are concerned. Since the solubility of the
polymer increases with the advancement of transesteri®ca-
tion, almost complete conversions are attained regardless

J. Melis et al. / Polymer 42 62001) 9319±9327
9323
1
Fig. 2. Infrared ꢀa), H-NMR ꢀb) and 13C-NMR ꢀc) spectra of PAAG-6.

9324
J. Melis et al. / Polymer 42 62001) 9319±9327
Fig. 3. TGA analysis of PAAꢀDL)G-4. Trace A: weight loss at a heating rate
of 108C min21 Trace Ap ꢀdotted line): derivative curve of trace A. Trace B:
weight loss as a function of time at a ®xed heating temperature of 2758C.
the length of the alkylating alcohol. As a result, yields
obtained for PAAG-n signi®cantly improve for longer
alkyl groups with regards to the single-step method. For
instance, PAAG-10 is obtained with an overall yield of
65% whereas a value of 50% is reported for this polygluta-
mate when prepared by direct esteri®cation [9]. Such differ-
ences are expected to be more accentuated as the length of
the alkyl group increases. Evidence for this was obtained by
an exploratory essay in which PGꢀDL)GA was subjected to
conversion to PAAꢀDL)G-18 by the two methods. Whereas
a complete conversion was attained by transesteri®cation of
PAAG-2 with 1-octadecanol with a overall yield of 60%,
direct esteri®cation with octadecyl bromide failed to afford
conversions higher than 20%. A further advantage of the
two-steps esteri®cation method is the possibility of using
alcohols instead of alkyl halides as alkylation reagents, a
feature that expands the esteri®cation options and makes
more reliable the puri®cation of the resulting PAAG-n. In
principle any short PAAG-n could be selected as precursor
Fig. 5. ꢀa) Evolution of the infrared spectra of PAAꢀDL)G-4 upon heating
at 2758C for the indicated periods of time. ꢀb) Infrared spectrum of the
remaining PAAꢀDL)G-4 after heating at 4508C.
for transesteri®cation, our preference for PAAG-2 obeying
exclusively to reliability and simplicity reasons.
3.2. Thermal degradation
PAAG-n with n # 6 are semicrystalline polymers show-
ing DSC traces with melting peaks that tend to decrease in
both temperature and enthalpy as n increases. Thermal
decomposition of PAAG-n is a property transcendent for
technical processing of these compounds. TGA curves of
PAAG-4, in both the isothermal and dynamical modes, are
represented in Fig. 3 to illustrate the general behavior
observed for the whole series. Dynamic TGA curves
revealed that thermal degradation in PAAG-n happens
well above Tm with decomposition onsets usually appearing
around 3008C. Apparently, decomposition takes place in a
single step with a maximum rate at a temperature ꢀTd)
located between 350 and 4008C. The residue left at 4508C
ꢀW450) decreases from ,30% down to ,10% of the initial
mass as n increases from 1 up to 10. Td and W450 for the
whole PAAG-n series are plotted against n in Fig. 4, which
clearly shows the trends displayed by these parameters with
the variation in size of the alkyl side chain. On the other
hand, isothermal TGA curves showed that, despite the
apparent stability displayed by the dynamical TGA trace
below 3008C, heating of the polymer at temperatures
Fig. 4. Decomposition temperature ꢀTd) and remaining weight at 4508C
ꢀW450) for PAAG-n as a function of n.

J. Melis et al. / Polymer 42 62001) 9319±9327
9325
1
Fig. 6. H-NMR ꢀa) and 13C-NMR ꢀb) spectra of the condensate yielded at the thermal degradation of PAAꢀDL)G-4 at 2758C.
between 250 and 3008C for long periods of time implies a
substantial weight loss.
temperatures below Td takes place mainly by depolymeriza-
tion of the chain with production of the corresponding n-
alkyl-pyroglutamates.
It can be reasonably expected from TGA results that
different molecular mechanisms must be operating in the
low and the high temperature thermal decomposition of
PAAG-n. To investigate the nature of these mechanisms,
the products resulting from the thermal decomposition of
PAAG-4 were analyzed by IR and NMR spectroscopy. A
sample of this polymer heated at 2758C showed weight
losses of about 25 and 75% after 10 and 60 min of treatment,
respectively. The IR spectra of the sample before and after
heating are compared in Fig. 5a, which proves that no signif-
icant changes have happened in the chemical structure of the
polymer after treatment for 10 min. The spectra taken after
heating for 60 min continues to be essentially the initial one
although some changes indicative of side chain loss are
observed. The NMR spectra of the condensed gases that
A comparison of the decomposition mechanisms occur-
ring at low temperatures ꢀbelow 3008C) in polyꢀa-peptide)s,
polyꢀb-peptide)s and polyꢀg-peptide)s with regard to their
chemical structures is depicted in Scheme 2 ꢀscheme of the
thermal degradation at low temperatures of polyꢀa-alkyl g-
glutamate)s ꢀPAAG-n), polyꢀa-alkyl b-aspartate)s ꢀPAAA-n)
and polyꢀg-alkyl a-glutamate)s ꢀPGAG-n)). In the three
cases, the nucleophylic attack of the NH onto the CO
ester carbon is intramolecular and leads to formation of a
®ve-membered amide ring. According to Kojima et al. [21],
the thermal decomposition of polyꢀa-glutamate)s involves
the scission of the ester side group with formation of a N-
acyl pyrrolidone and releasing of the corresponding alkanol.
The same process is known to occur in polyꢀb-aspartate)s
with formation in this case of stable cyclic aspartimide [22].
In the case of polyꢀg-glutamate)s, the occurrence of such a
mechanism would lead to either a six-membered cyclimide
or a three-membered amide ring, both structures being
energetically disfavored. On the contrary, a stable ®ve-
membered 5-alcoxycarbonyl-2-pyrrolidone will be produced
if the nucleophilic substitution reaction takes place on the
1
were generated upon heating are shown in Fig. 6. Both H
and 13C-NMR spectra evidenced that n-butyl pyroglutamate
is the only product present in the condensate. Similar results
were obtained when PAAG-10 was subjected to a similar
investigation but using a heating temperature of 2908C in
the isothermal treatment. It can be concluded therefore that
the thermal decomposition of polyꢀa-alkyl g-glutamate)s at

9326
J. Melis et al. / Polymer 42 62001) 9319±9327
Scheme 2.
Scheme 3.
ester main chain. The process should happen preferentially
at the chain ends in order to account for the observe releas-
ing of the n-alkyl-pyroglutamate. A similar mechanism was
proposed for the thermal decomposition of nylon 4 with
concomitant generation of 2-pyrrolidone [23].
polyꢀa-glutamate)s and polyꢀg-glutamate)s, as schemati-
cally indicated in Scheme 3 ꢀcompared schemes of the ther-
mal degradation of PGAG-n and PAAG-n at high
temperatures). In the former case, the process results in
the formation of a vinyl side group, whereas in the latter,
the double bond becomes forming part of the polyamide
backbone. In both cases, the mechanism would imply the
occurrence of an intermediate cyclic conformer, which may
be anticipated to be arranged more easily in polyꢀa-gluta-
mate)s due to the ¯exibility of the side chain. This is in
agreement with the lower temperatures that are observed
to be required for the decomposition of these compounds.
When PAAG-4 was further heated above 4008C, the
remaining residue amounts less than 10% of the original
sample weight. The IR spectrum of this residue is shown
in Fig. 5b. This spectrum does not contain ester bands indi-
cating that the alkoxycarbonyl side chain has completely
disappeared. In fact, the most characteristic absorption of
this spectrum is a couple of bands at 1638 and 1618 cm21 in
addition to a broad peak appearing around 3250 cm21
.
According to antecedents reported on the thermal degrada-
tion of polyꢀg-methyl a,l-glutamate) above 3008C, such
spectra could be interpreted as arising from the unsaturated
polyamide that is formed upon decarboxylation of PAAG-4.
The decomposition process would be therefore the same for
Acknowledgements
The authors are greatly indebted to Dr Kubota for his
kindly gift of PGGA samples. Financial support to this

J. Melis et al. / Polymer 42 62001) 9319±9327
9327
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[10] Borbely M, Nagasaki J, Borbely J, Fan K, Bhogle A, Sevoian M.
Polym Bull 1994;32:127.
Â
work was given by CICYT ꢀMinisterio de Educacion y
Cultura, grant no. MAT99-0578-CO2-02). One of the
authors ꢀJMV) acknowledges the grant given by AECI to
carry out his PhD Thesis in Spain.
[11] Gonzales D, Fan K, Sevoian M. J Polym Sci, Polym Chem
1996;34:2019.
[12] Bocchi V, Canasti G, Dossena A, Marchelli R. Synthesis 1979:961.
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[14] Kovacs J, Kovacs HN, Ballina R. J Am Chem Soc 1963;85:839.
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