Polymerization mechanism of trimethylene carbonate carried out with zinc(II) acetylacetonate monohydrate

Article abstract of DOI:10.1002/pola.24683

The proposed mechanism of initiation and course of ring-opening polymerization of cyclic trimethylene carbonate (TMC) involving zinc(II) acetylacetonate is in accordance with the mechanism of monomer activation. At the first stage of the process, coordination of carbonate to Zn(Acac) ₂·H₂O complex occurs with the release of weakly coordinated water molecules. This free water molecule reacts with active TMC-Zn(Acac)₂ complex. The reaction results in the formation of propanediol and CO₂ emission. During further stages of the investigated process, the formed propanediols, or later the oligomeric diols produced with polymerization, are cocatalysts of the chain propagation reaction. The chain propagation occurs because of repeating activation of the TMC monomer through the creation of an active structure resulting in the exchange/transfer reaction between the zinc complex and the monomer, with its following attachment to the hydroxyl groups, carbonate ring opening, and formation of the carbonic unit of polymer chain.

Full text of DOI:10.1002/pola.24683

Polymerization Mechanism of Trimethylene Carbonate Carried Out with
Zinc(II) Acetylacetonate Monohydrate
MALGORZATA PASTUSIAK,1 PIOTR DOBRZYNSKI,1,2 BOZENA KACZMARCZYK,1 JANUSZ KASPERCZYK1,3
1Polish Academy of Sciences, Centre of Polymer and Carbon Materials, 34 Sklodowska-Curie Street, 41-800 Zabrze, Poland
2Institute of Chemistry, Environmental Protection and Biotechnology, Jan Dlugosz University in Czestochowa,
13 Armii Krajowej, 42-218 Czestochowa, Poland
3Department of Biopharmacy, School of Pharmacy, Medical University of Silesia, 1 Narcyzow Str., 41-200 Sosnowiec, Poland
Received 15 February 2011; accepted 19 March 2011
DOI: 10.1002/pola.24683
Published online 11 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The proposed mechanism of initiation and course of
ring-opening polymerization of cyclic trimethylene carbonate
(TMC) involving zinc(II) acetylacetonate is in accordance with
the mechanism of monomer activation. At the first stage of the
process, coordination of carbonate to Zn(Acac)2ꢀH2O complex
occurs with the release of weakly coordinated water molecules.
This free water molecule reacts with active TMC–Zn(Acac)2
complex. The reaction results in the formation of propanediol
and CO2 emission. During further stages of the investigated
process, the formed propanediols, or later the oligomeric diols
produced with polymerization, are cocatalysts of the chain
propagation reaction. The chain propagation occurs because
of repeating activation of the TMC monomer through the crea-
tion of an active structure resulting in the exchange/transfer
reaction between the zinc complex and the monomer, with its
following attachment to the hydroxyl groups, carbonate ring
opening, and formation of the carbonic unit of polymer chain.
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2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
49: 2504–2512, 2011
KEYWORDS: catalysts, initiator; ring-opening polymerization; tri-
methylene carbonate; zinc(II) acetylacetonate
INTRODUCTION Significant progress and development in
many fields of medicine require various new materials with
special properties, including bioresorbable polymeric materi-
als. This generates a need to develop perfect polymer materi-
als with functional microstructure and with desired mechani-
cal and physicochemical properties. Among the bioresorbable
materials, aliphatic polyesters and polyester-co-carbonates
synthesized from initial cyclic monomers such as lactide, gly-
colide, caprolactone, and TMC are of particular importance.
The most important requirement appeared to this type of
material is biocompatibility with body tissue. Due to the fact
that complete elimination of the initiator used in the poly-
merization process is practically impossible, biocompatibility
of materials prepared conventionally with toxic tin com-
pounds is somewhat controversial. Therefore, the initiators
are sought based on bioelements with low toxicity, which
could completely replace the previously widely used tin com-
pounds. Such initiators include, among others, zinc com-
pounds. Obviously, compounds and complexes of this metal
have been and are still the object of studies, as in many
cases, they proved excellent initiators of polymerization and
copolymerization of lactides, lactones, and cyclic carbo-
nates.1–3 However, most often these are complexes with a
complicated structure, difficult to synthesize, hygroscopic
compounds, and easily undergo hydrolysis, and thus, highly
problematic in practical and industrial applications.
Our team researches on the ring-opening polymerization
(ROP) of cyclic esters and carbonates and have shown that
very effective initiators proved to be relatively simple and
susceptible to the synthesis of acetylacetonates of low-toxic-
ity metals such as calcium, magnesium, iron, zinc, and zirco-
nium.4–8 For technological reasons, these compounds have
some very essential advantages, namely, they are easy to
store and use, most importantly, highly soluble in molten
monomers, and thus provide full homogeneity of the ongoing
process. Acetylacetonates of other metals such as aluminum9
or tin10 have also been described as effective initiators/cata-
lysts for ring-opening homopolymerization and copolymer-
ization of cyclic esters. Until now, the mechanism of initia-
tion of lactide and glycolide with zirconium (IV)
acetylacetonate polymerization5 has been studied. This initia-
tor has not been practically active in the polymerization of
cyclic TMC and is active only in the presence of a small
amount of coinitiator; the compound capable of deprotona-
tion (which was lactone) followed the exchange of acetylacet-
onate ligand with formation of metal–oxygen bond, which
finally enabled the initiation of TMC polymerization.6 In
Correspondence to: P. Dobrzynski (E-mail: piotr.dobrzynski@cmpw-pan.edu.pl)
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another work,9 a slightly different mechanism of initiation of
lactide polymerization in the presence of aluminum acetyla-
cetonate and alcohols was proposed. In that case, according
to the authors, the active complex was the complex formed
as a result of exchange of acetylacetonate group by the alco-
holate group.
obtain poly(trimethylenecarbonate) and poly(dimethyl trime-
thylenecarbonate) with high molar mass.4 As opposed to
previously described TMC polymerization with the presence
of the zirconium (IV) acetylacetonate,6 this reaction did not
require the use of a coinitiator. This fact suggested that ini-
tiation reaction of polymerization of cyclic carbonates using
Zn(Acac)2 had to occur in accordance with a mechanism
completely different from that previously described for the
polymerizations initiated with zirconium acetylacetonate. In
this ROP of cyclic carbonates, the process of proton transfer
from monomer to acetylacetonate ligand of Zn(Acac)2 com-
plex could not have taken place because the deprotonation
of cyclic carbonate is very difficult due to the chemical struc-
ture of this compound. Also, a potential exchange of acetyla-
cetonate ligand of this complex by alcohols or water traces,
which was reported as the cause of initiating lactide poly-
merization with aluminum(III) acetylacetonate,9 could be
excluded because of the known relatively high stability of
Zn(Acac)2ꢀH2O in aqueous and alcoholic solutions in a large
range of pH and temperature.18,19 Our tests confirmed the
stability of this compound in benzene solution containing
about 10 wt % ethyl alcohol.
Zinc acetylacetonate is a compound that has been known
since 1960s and was widely used in laboratories in various
fields of science and industry.11 Description of compounds
derived from zinc acetylacetonate, obtained from monohy-
drate zinc acetylacetonate, in which the water ligand was
replaced by the compounds containing atoms with donor
character, such as N,N-dimethylaminoethanol or 2-phenyl-2-
oxazoline ligands, might be found in the literature as
well.12,13
Zinc acetylacetonate is used essentially in the form of com-
plexes that besides acetylacetonate ligands contain much less
coordinated, one or two water molecules. The structure of
anhydrous form of this compound is also known. Monohy-
drate is a complex built from one atom of Zn and three
ligands: two chelating acetyloacetonate rings and one water
ligand. In the presence of monohydrate, the TMC polymeriza-
tion process in the melt proceeded at an unexpectedly high
rate and efficiency. The resulting polymers were character-
ized by high-molecular weight similar to the presumed.4
To explain the process of initiating the TMC polymerization,
a series of samples was obtained by changing the quantita-
tive composition of the starting reaction mixture (Zn(A-
cac)2:TMC molar ratio as 1:1, 1:3, 1:10, 1:15, and 1:50). The
resulting products present in the final solid phase as well as
in distilled received benzene solution were analyzed by NMR
and FTIR spectroscopy.
The anhydrous form of this complex is a compound with
much more complicated three-core structure.15–17 Because of
this reason, as expected, preliminary studies conducted by
us showed that the TMC polymerization proceeded much
slower in the presence of anhydrous acetylacetonate when
compared with the use of monohydrated complex. Polycar-
bonates obtained in this reaction presented a much higher
molecular weight than was calculated, which indicated the
complexity of the process and proves that not all the zinc
atoms of the complex took an active part in the process of
propagation of the polycarbonate chain. These results
prompted us to use a more active monohydrate form of this
zinc complex for further studies. Unprecedented activity of
zinc(II) acetylacetonate monohydrate in the reaction of cyclic
carbonates polymerization was particularly interesting and
previously observed. This aspect is more important because
the mechanism of this type of reaction has not been
studied yet.
The 1H NMR spectra of the obtained compounds are shown
in Figure 1. It was revealed that for the spectra of obtained
reaction products, the chemical shifts and the intensities of
the starting signals of the CH and CH3 groups of acetylaceto-
nate ligands (signals 1 and 2) did not change. Performed cal-
culations (Table 1) showed that molecule of the final product
still contained started two Acac ligands. Stability of the ace-
tylacetonate ligands was also confirmed by the analysis of
distilled solvent, where the presence of free acetylacetonate
Hacac was not observed.
Detailed analysis of NMR spectra of the product obtained by
reacting 1 mol of zinc(II) acetylacetonate monohydrate with
1 mol of TMC [Fig. 1(A)] revealed the presence of a series of
signals beside those connected with the Acac groups men-
tioned above (signals 1 and 2), which arose from the reac-
tion products formed as a result of the ring-opening reaction
of TMC. These signals include signals from protons of the
A practical aspect of this research is nonetheless important,
because understanding of the presented mechanisms of poly-
merization initiation allows us to further optimize the struc-
ture and composition of the initiator used, and thus, it will
be possible to increase the efficiency of such initiators, as
well as to control the entire course of the ongoing process.
CH2 groups forming
a polymer chain, whose intensity
increased along with the M/I ratio (Fig. 1, signals e and d).
For the purpose of facilitation of the assignment of signals to
appropriate group, additional studies were carried out, moni-
toring the course of TMC polymerization by recording the
changes in 1H NMR spectra of TMC and Zn(Acac)2 solution
(with M/I ratio as 1:1) with time. The process was per-
formed in deuterated benzene solution at room temperature,
under conditions that significantly slowed down the investi-
gated reaction rate. At the beginning of the reaction (up to 2
RESULTS AND DISCUSSION
The purpose of this study was to investigate the mechanism
of polymerization of cyclic carbonates initiated by the zinc
(II) acetylacetonate, for example, polymerization of TMC.
From our previous studies, it was known that this reaction
occurred with high efficiency, at a rapid rate, and allows to
POLYMERIZATION MECHANISM OF TMC, PASTUSIAK ET AL.
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FIGURE
1
1H NMR spectra (in
benzene-d6) of reaction products
of Zn(Acac)2ꢀH2O with TMC con-
ducted with I/M molar ratio as;
(A) 1:1, (B) 1:10, and (C) 1:15.
TABLE 1 Dependence of the Composition of the Reaction Mixture on the Concentration Acetylacetonate Groups, Unsaturated
Bonds, and Lactidyl (Carbonate) Chains Length in the Obtained Products
No
Monomer
Ratio M/I
Conv. (%)
Coacac (%)
Ckacac (%)
Ctacac (%)
Cchain (Average Number of Units)
VCO (cm3)
2
1
2
3
4
5
TMC
TMC
TMC
TMC
TMC
1
3
ꢁ100
ꢁ100
ꢁ100
ꢁ100
ꢁ100
66.7
40.0
16.7
11.8
66.7
38.1
16.6
9.4
50
1.0
–
25
1.9
20
22
20
–
10
15
50
9.1
6.3
8.5
15.9
56 (based on GPC)
48 (based on OH
groups measurement
with assumption 2
OH groups in chain)
Where; Conv., TMC conversion; Coacac, initial content of acetylacetonate groups ¼ [acac]o/([acac]o þ [TMC]); Ckacac, content of acetylacetonate groups
in obtained complex ¼ [acac]/([acac] þ [TMC origin]); Ctacac, theoretical calculated content of acetylacetonate groups in obtained mixture when one
ligand will be changed; Cchain, average amount of carbonate units ¼ [CH2 signal e/2]/ [CH2 signal a]; VCO , volume of emitted CO2 (normal conditions)
2
calculated on the 1 mmol of used Zn(Acac)2ꢀH2O.
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FIGURE 2 1H NMR spectra (in benzene-d6) of reaction products of Zn(Acac)2ꢀH2O with TMC conducted with I/M molar ratio as 1:1,
at 25 ꢂC after (A) 2 h and (B) 4 h.
h), the appearance of two multiples of weak signals of vary-
ing intensity was recorded [Fig. 2(A) signals c and g]. Based
on NMR studies, carried out under identical conditions on a
series of standards, the appearing signals were assigned to
the CH2 group protons of propylene glycol (Table 2). It sug-
gested that at an early stage, the reaction propylene glycol
was an observed product, which, in turn, had to arise from
decarboxylation of the TMC molecules. Expected release of
CO2 has also been noted than experimentally—which defi-
nitely confirmed proposed first stage of TMC polymerization.
bonds of electron pairs of carbonyl oxygen of that com-
pound. This reaction proceeded with the release of a weakly
coordinated water molecule (Scheme 1), analogous to the
reaction of formation of hydrated Zn(Acac)2 complexes with
TABLE 2 Chemical Shifts in the 1H NMR Spectra of TMC
Reaction (Benzene-d6)
Signal Origin
d (ppm)
1
2
CH in Zn(Acac)2
CH3 in Zn(Acac)2
5.10 (s)
At the next step of reaction, after the next 2 h, appearance of
signals associated with a growth of the chain with carbonate
group was detected (Fig. 2B, signals; a0, b0, and f0—see also
Table 2). Based on the conducted investigations and detailed
analysis of the received NMR and FTIR spectra, a mechanism
of initiating the polymerization was proposed as shown in
Scheme 1. The proposed initiation mechanism of the TMC
polymerization reaction as well as the chain propagation is
induced with activation of the monomer, similar to the previ-
ously described TMC polymerization reaction catalyzed by
selected metal salts, with the presence of alcohol,20 or cata-
lyzed by protonic acids and alcohols.21,22
1.75 (s)
TCM1 AOCH2A in unreacted TMC
3.30 (m)
0.75 (m)
4.15 (m)
4.15 (m)
3.40 (m)
3.35 (m)
3.75 (m)
1.63 (m)
1.55 (m)
TCM2 ACH2ACH2ACH2 in unreacted TMC
a
a0
b
b0
c
AO(CO)OCH2CH2CH2OH ends of chain
AOA(CO:Zn(acac)2)AOCH2CH2CH2OH
AO(CO)OCH2ACH2OH end of chain
AOA(CO:Zn(acac)2)AOCH2CH2CH2OH
HOCH2CH2CH2OH
f
f0
AO(CO)OCH2CH2CH2OH ends of chain
AOA(CO:Zn(acac)2)AOCH2CH2CH2OH
d
e
g
AO(CO)AOCH2ACH2ACH2OA in PTMC chain 1.65 (m)
AO(CO)AOCH2ACH2ACH2OA in PTMC chain 4.00 (m)
The first stage of the investigated process was the formation
of an adduct: Zn(Acac)2ꢀTMC, as a result of the TMC mole-
cule coordination to the central metal by creating donor
HOCH2CH2CH2OH
1.70 (m)
POLYMERIZATION MECHANISM OF TMC, PASTUSIAK ET AL.
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SCHEME 1 Proposed mechanism of TMC polymerization catalyzed by zinc(II) acetylacetonate monohydrate—numbers for protons
correspond to 1H NMR signals given in Table 2.
other organic molecules as: amines, ketons, oxazolines, or
thiosemicarbazones.12,13,23 The resulting compound was not
possible to separate and to characterize due to a very high
rate of consecutive reactions. This adduct quickly underwent
a very fast reaction with the released water molecule leading
to the opening of the TMC carbonate ring and forming of an
unstable derivative of carbonate acid, which also underwent
a rapid decarboxylation with the emission of CO2 and with
the formation of propylene glycol. As a result of these reac-
tions, the reconstruction of Zn(Acac)2 complex followed.
These studies demonstrated, through the measurement of
the volume of evolved gas, that a process of releasing carbon
dioxide actually occurred and that it proceeded in accord-
ance with expected stoichiometry. Number of moles of the
emitted gas was practically the same as in the case where
monohydrate zinc acetylacetonate was used, irrespective of
the TMC amount held in the starting reaction mixture (Table
1, row 2–4). This fact constitutes another proof that the
reaction in question proceeds in the observed way due to
the amount of water molecules in the zinc complex (about 1
mol of H2O in 1 mol of the zinc complex) rather than any
other compound present in or introduced into the reaction
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FIGURE 3 FTIR spectra of Zn(Acac)2ꢀH2O, products of reaction Zn(Acac)2 with TMC in I/M ratio as; 1:1, 1:15, TMC and PTMC in the
region of 1850–1500 cmꢃ1 (a) and 1500–800 cmꢃ1 (b).
mixture. The 1H NMR spectra of reaction mixtures also con-
firmed the presence of signals that are assigned to the sig-
nals of propylene glycol (Fig. 1, signals c and g), which, simi-
larly the excreted CO2 could only formed as a result of TMC
decarboxylation. The zinc acetylacetonate complex formed in
that process reacted with another molecule of TMC in the
presence of formed glycol, in accordance with the process
proposed in Scheme 1. As a result, a linear derivative of car-
bonate was formed, already stable, ending with hydroxyl
groups and coordinated to the zinc atom of the acetylaceto-
nate complex through the carbonyl oxygen of carbonate
group (Scheme 1). In the spectrum of the reaction product
of 1 mol of TMC with 1 mol of zinc acetylacetonate appropri-
ate signals were observed, which have been assigned to CH2
groups of such compound [Figs. 1(A) and 2(B), signals a0, b0,
and f0 ]. Analysis of the results [Table 1, row 1, Fig. 1(A)],
and previously presented observations of the reaction con-
ducted in deutered benzene solution at room temperature,
shows that although with the equimolar composition of the
initial reaction mixture, the part of carbonate underwent the
oligomerization reaction, as evidenced by the typical signals
of CH2 groups of carbonate units of the chain [Fig. 1(A), the
signals e and d]. Chain formation also explains the appear-
ance of signal b attributed to the ending CH2 group in the
chain of forming carbonate oligomer noticed in the NMR
spectrum. Signals a and b, partly overlapped, also originate
from the two CH2 ending groups of the same chain. In the
case of the reaction in the presence of monomer excess, a
disappearance of the signals caused by propylene glycol,
which in these conditions reacted in the aforementioned
reaction, is observed [Fig. 1(B,C)], which also confirms the
proposed mechanism. Coordination of oligomeric molecules
created on this way to zinc (by carbonyl group oxygen in
forming chain) was so weak that the exchange/transfer reac-
tion with the next TMC monomer molecule could proceed.
As a final result, the renewal of an active structure 2
occurred (which is associated with the observed reduction of
signals b0 and f0 ), which means that the next reaction with
hydroxyl groups of formed oligomeric molecules was possi-
ble. In the case of a large excess of monomer, cyclically
repeating process caused as a result rapid propagation of
polycarbonate chain, as evidenced by an increase in the in-
tensity of the signals characteristic for CH2 groups forming
the chain [Fig. 1(B,C), signals e and d]. In the spectra for
oligomers obtained in this way, two signals from protons of
the CH2 groups with practically the same intensity [Fig.
1(B,C), signals a and b] were observed. They have been
assigned to groups present at the ends of the propagated
chain. Calculations of the average number of carbonate units
in forming oligomer chain, based on measurements of signal
intensities of protons corresponding to the chain end groups
(signal a or b—with the same intensity) and signals charac-
teristic of protons of CH2 groups of the chain (e; group
COOCH2A), were directly proportional to the molar ratio of
M/I, in which the reaction mixture of TMC and acetylaceto-
nate zinc was prepared (Table 1, row 2–4) as expected. This
fact indirectly confirmed the accuracy of signal assignments
we had performed. The proposed mechanism also confirmed
the results of measurements of amount active hydroxyl
group in obtained TMC, low-molecular weight polymer (Ta-
ble 1, row 5), which are consistent with the expected pres-
ence of two OH groups in the one molecule.
The course of the proposed mechanism of cyclic carbonates
polymerization is also confirmed by FTIR results obtained
for model compounds and oligomers. Generally, it can be
noticed that for the products of TMC polymerization, rela-
tively minor changes are observed in the FTIR spectra (Fig.
3).
Two bands of the same intensity recorded for TMC monomer
for the C¼¼O ester stretching vibrations mode can suggest
that in crystalline form (samples are analyzed in solid state),
TMC molecules form associates or it may be a result of inter-
actions in the TMC crystal lattice. Similarly, it can be
explained with the presence of relatively strong bands at
1193, 1144, and 1116 cmꢃ1 in the region due to the
POLYMERIZATION MECHANISM OF TMC, PASTUSIAK ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
in the spectrum of the product of a reaction carried out with
considerable excess of TMC and can be assigned to the OAH
group stretching vibrations in a longer chain of ester glycol.
Additional band at a higher frequency indicate that some free
OH groups are also present in that case. In the spectrum
obtained for the products of reaction conducted under 15:1
TMC excess, the bands similar to that arising from the PTMC
chain are mainly seen. Relatively weak bands associated with
Zn(Acac)2ꢀH2O complex are also detected. Their shapes are the
same as those detected for the equimolar reaction products.
The subsequently performed research of the model TMC
polymerization carried out in benzene solution and catalyzed
by the hydrated Zn(Acac)2 showed that this process pro-
ceeded in a relatively short time and had the character of a
living polymerization. The conducted reaction was incompa-
rably faster than the previously described lactide or lactones
polymerization with acetylacetonates use (Fig. 5) and was
notably faster than most ROP reactions of this type in ac-
cordance to insertion–coordination mechanism, which can
also prove its rather ionic nature. The polymer was obtained
with almost 100% efficiency (Fig. 5). The number-average
molecular weight of the product dependence of the TMC
conversion showed a linearity (Fig. 6), and the resulting mo-
lecular weights were very close to the masses calculated the-
FIGURE 4 FTIR spectra of Zn(Acac)2 (a), product reaction of
TMC with Zn(Acac)2 molar ratio as 1:1 (b), 15:1 (c), TMC (d),
and PTMC (e) in the region of 3850–2600 cmꢃ1
.
oretically [calculated according to the formula, Mn
¼
stretching vibrations of OAC(¼¼O) group in TMC monomer.
In the case of reaction products of 1 mol of TMC with 1 mol
of Zn(Acac)2 complex, one sharp band at 1745 cmꢃ1 is
detected. Simultaneous disappearance of the other three
bands proves that these interactions are no longer present.
It can also be noticed that although the C¼¼O ester groups
absorb at the same frequency as the poly(trimethylene car-
bonate) - PTMC, the absence of a broad band at 1250–1200
cmꢃ1, characteristic for the OAC(¼¼O)AO groups in polymer,
indicates that there are no polymer or oligomeric chains in
that sample. Compared to the spectrum of Zn(acac)2, only
slight changes are observed on spectra of reaction products
in the region of 1650–1500 cmꢃ1 (C¼¼O and C¼¼O stretching
vibration) as well as 1490–1200 cmꢃ1 (attributed mainly to
the CH and CH3 groups deformation.23(a) These small
changes could indicate that in the case of reaction of 1 mol
of TMC with 1 mol of Zn(Acac)2ꢀH2O complex, both Acac
ligands have been preserved and only weak changes in the
interactions between Acac ligand with Zn atom take place,
not excluding that due to detachment of the water molecule.
Elimination of water molecules can also indicate significant
changes observed in the regions attributed to the stretching
vibrations of OAH groups (Fig. 4). Certainly, spectra of reac-
tion products of TMC with Zn(Acac)2ꢀH2O strongly differ
from the initial Zn complex in this region. For TMC reaction
products, a broadband with two maxima at 3300 and 3390
cmꢃ1 is recorded. The band at 3300 cmꢃ1 as well as the
bands at 1050 and 980 cmꢃ1 also present in the spectrum,
originating from the stretching vibrations of CAOH and de-
formation of OAH groups, respectively, appear at the same
wavenumber as it is observed for 1,3-propanediol. This con-
firms the 1,3-propanediol formation during monomer activa-
tion stage. The second band at 3390 cmꢃ1 is also observed
(([TMC]/[I] ꢄ 102)/100) ꢄ conversion %]. Particularly, good
fitting of the experimental data points to theoretical numbers
was observed in the case of the values calculated on the ba-
sis of actual measured water content in the zinc complex
(Fig. 6, gray line).
The dispersity of molecular weights of the polycarbonate
was relatively low and did not exceed the value of 1.2 during
the whole process. Only after complete conversion of mono-
mer, after additional 2-h incubation of the reaction product
(Fig. 6), a slight effect of decrease in molecular weight was
found with a simultaneous increase in mass dispersion from
the Mw/Mn value equal to 1.2 to about 1.8. This effect was
FIGURE 5 Conversion of TMC as a function of polymerization
time, reaction carried out in benzene at 70 ꢂC, with M/I molar
ratio as 50:1.
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cetonate at a pressure of 0.1 mm and a bath temperature of
ꢂ
110 C, according to the method given in ref. 23(b).
Polymerization Procedure
The model oligomerization of TMC was carried out under ar-
gon, in sealed glass Erlenmayer flask (50 mL) equipped with
a magnetic stirrer. Twenty milliliter of dry benzene was added
into the reaction vessel, and 1 mmol of TMC was added, and
then the reaction vessel was immersed in a thermostatically
ꢂ
controlled oil bath at 70 C. Then, the initiator, zinc(II) acety-
loacetonate monohydrate, in amount depending on the type
of experiment was added (Table 1). The reactions were con-
ducted for about 3 h, until the total conversion of TMC was
achieved. In some of the conducted synthesis reactions, the
volume of gaseous CO2 released as a result of the reaction
was controlled. In this case, the upper stub of the reaction
container was equipped with a gas supply to the bottom of
the gas receiver—graduated gas burette with leveling bulb,
filled with lime water at ambient temperature. Simultane-
ously, when the reaction was started, the argon supply was
cut off, and the decrease of the solution content in the burette
was observed and thus measuring the gas volume.
FIGURE 6 Relationship between Mn, Mw/Mn, and the degree of
TMC conversion, reaction carried out in benzene at 70 ꢂC, with
I/M molar ratio as 1:50. Top line: theoretical molecular weight
calculated based on the amount of Zn(acac)2ꢀH2O, bottom line:
molecular weight calculated based on the actual amount of
water in the Zn(acac)2ꢀH2O (H2O/M ratio as 1.08:50).
most likely related to the known phenomenon of thermal
degradation of aliphatic polycarbonates in the presence of
more amount of active OH groups.24,25
The solution polymerization of TMC was conducted in a
larger Erlenmayer flask (150 mL) equipped with the same
devices under the same conditions for 6 h. Dry benzene (60
mL), TMC (50 mmol), and Zn(Acac)2ꢀH2O (1 mmol) were
added into the reaction vessel. In the kinetics investigations,
several samples were taken out via syringe at various times
(Figs. 5 and 6). These samples were treated with aqueous
acetic acid solution to stop the reaction and to extract the
initiator. The obtained mixtures were washed with cold
water, and the organic phases, including the polymer, were
separated and concentrated by evaporation for NMR and gel
permeation chromatography analysis.
The proposed mechanism explains a relatively high resist-
ance of presented catalytic system to monomer pollutions
observed during experimental work. It is determined by the
phenomenon of selective activation of the monomer.20 Poly-
carbonate, obtained as described by the monomer activation,
contains active hydroxyl groups on both ends of the chain.
Further leading research attempting to use this mechanism
for the copolymerization of TMC with lactide (results of
these studies will be the subject of a separate publication)
showed that the course of this reaction in accordance with
the activation mechanism in this case was impossible. Even
the smallest amount of lactide or cyclic lactone contained in
the starting reaction mixture caused the initiation of copoly-
merization and its subsequent course to occur in accordance
with the other mechanism of ROP of lactide, rather analo-
gous to the polymerization of TMC initiated by Zr(Acac)4
with the participation of the lactone coinitiator, and thus
with forming the initiating complex in the acetylacetonate
ligand exchange reaction.6
Measurements
The conversion of the reaction and the structure of the
obtained products were determined by 1H NMR spectros-
copy. The 1H NMR spectra ꢂwere recorded at 600 MHz with
Avance II Bruker TM at 25 C. Dried benzene-d6 was used as
the solvent, and tetramethylsilane was applied as the inter-
nal standard. The spectra were obtained with 64 scans, 2.65-
s acquisition time, and 11-ls pulse width.
EXPERIMENTAL
The number-average and weight-average molar masses (Mn
and Mw, respectively) and dispersity indexes (Mw/Mn) of the
oligomers were determined by gel permeation chromatogra-
phy with a Viscotek RImax chromatograph. Chloroform was
used as the eluent, and the temperature and the flow rate
Materials
The monomer 1,3-TMC was purchased from (Boehringer,
Ingelheim, Germany), and it was recrystallized from dry
ethyl acetate and then dried in a vacuum oven at room tem-
perature. Zinc(II) acetylacetonate monohydrate (Zn(a-
cac)2ꢀH2O; Alfa Aesar) was used as received. The actual
water content in this complex was determined by means of
the TGA measurements (Metler – Toledo, TGA/DSC Stare).
The measured amount constituted 6.6% of the complex,
which equals to 1.08 mol of H2O for 1 mol of the complex.
Anhydrous benzene (Aldrich) was dried over CaH2 before
distillation. The anhydrous form of zinc acetylacetonate is
obtained by sublimation of the hydrated form of zinc acetyla-
ꢂ
were 35 C and 1 mL/min, respectively. Two PL Mixed E col-
umns with a Viscotek model 3580 refractive index detector
and injection volume equal to 100 lL were used. The molec-
ular weights were calibrated with polystyrene standards.
The correct Mn value for PTMC was determined according to
the method described by Kricheldorf et al.26
To determine the actual amount of hydroxyl groups in the
synthesized polycarbonates, a method based on ASTM D
2849-69 was used,27 consisting of the hydroxyl groups’
POLYMERIZATION MECHANISM OF TMC, PASTUSIAK ET AL.
2511

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
3 Chen, P.; Liu, Y.; Lin, Ch.; Ko, B. J Polym Sci Part A: Polym
esterification with phthalic anhydride, in the environment of
pyridine, in the presence of imidazole as initiator, and later
hydrolysis of the excess of anhydride. The resulted acid
groups were titrated with sodium hydroxide solution.
Chem 2010, 48, 3564–3572.
4 Dobrzynski, P.; Pastusiak, M.; Bero, M. J Polym Sci Part A:
Polym Chem 2005, 43, 1913–1922.
Infrared spectra were acquired on a DIGILAB FTS-40A Fou-
rier transform infrared spectrometer in the range of 4000–
400 cmꢃ1 at a resolution of 1 cmꢃ1 and for an accumulated
32 scans. Samples were analyzed in a form of pellets in po-
tassium bromide.
5 Dobrzynski, P. J Polym Sci Part A: Polym Chem 2004, 42,
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CONCLUSIONS
8 Hua, J.; Gebarowska, K.; Dobrzynski, P.; Kasperczyk, J.;
Wei, J.; Li, S. J Polym Sci Part A: Polym Chem 2009, 47,
3869–3879.
The mechanism of polymerization of the cyclic TMC carbon-
ate involving zinc(II) acetylacetoniate proved to be radically
different from previously proposed lactide, e-caprolactone, or
TMC polymerization mechanisms with use of acetylaceto-
nates of zirconium(IV) or aluminum(III). The reaction
described in this article is much more rapid than previously
studied polymerization and is in accordance with the mecha-
nism of monomer activation. At the first stage of the process,
coordination of carbonate to Zn(Acac)2ꢀH2O complex occurs,
followed by ligand exchange and release of weakly coordi-
nated water molecules, which react with active TMC and
zinc complex formed in this process. The reaction results in
the formation of 1.3-propanediol, reproducing a zinc acetyla-
cetonate complex and emission of carbon dioxide. During
further stages of the investigated process, the formed pro-
panediol and the oligomeric diols produced later with poly-
merization are cocatalysts of the chain propagation reaction.
The chain propagation occurs because of repeated activation
of the TMC monomer through the creation of an active struc-
ture resulting in the exchange/transfer reaction between the
zinc complex and the monomer, with its following attach-
ment to the hydroxyl groups, with carbonate ring opening
and formation of another carbonic unit of the growing chain.
The reaction has the living polymerization character and is
of very high efficiency, reaching 100%. Monohydrate zinc(II)
acetylacetonate, simple in structure, stable, and relatively
inexpensive zinc compound, proved to be a very effective
catalyst for homopolymerization of cyclic carbonates, effec-
tive than many other zinc complexes presented earlier,1
which are difficult to synthesize because of their complicated
chemical structure, and in many cases, very unstable.
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This research was supported by Centre of Polymer and Carbon
Materials of Polish Academy of Sciences and partially was
investigated in the frame of project MEMSTEND UDA-
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