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High molecular weight polylactic acids by polyesterification using diisopropylcarbodiimide (DIPC) and 4-(dimethylamino) pyridinium ptoluene sulfonate (DPTS)


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High molecular weight poly(l-lactic acid) has been prepared from the corresponding functionally terminated oligomers employing a polyesterification method at room temperature using diisopropylcarbodiimide and 4-(dimethylamino) pyridinium-p-toluene sulfonate. Self-condensation of hydroxyl and carboxylic acid-terminated poly(l-lactic acid) oligomers (M n : ~1,000) resulted in polymers with high molecular weights (M n > 45,000) under mild conditions. End-group analysis by MALDI-TOF provided evidence for N-acylurea formation in the product under the reaction conditions employed.
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High Molecular Weight Poly (L-lactic acid) s by
Polyesterification using Diisopropylcarbodiimide (DIPC) and
4-(Dimethylamino) pyridinium-p-toluene sulfonate (DPTS)
S. Shyamroy, B. Garnaik and S. Sivaram*
Polymer Science and Engineering Division
CSIR-National Chemical Laboratory,Pune-411008, India
Phone no- +9120-25902304
Fax no- +9120-25902615
High molecular weight poly (L-Lactic acid) have been prepared from the corresponding
functionally terminated oligomers employing a polyesterification method at room
temperature using diisopropyl carbodiimide (DIPC) and 4-(dimethylamino) pyridinium-p-
toluene sulfonate (DPTS). Self-condensation of hydroxyl and carboxylic acid-terminated
poly (L-Lactic acid) oligomers (Mn: ~1000) resulted in polymers with high molecular
weights (Mn> 45,000) under mild conditions. End group analysis by MALDI-TOF
provided evidence for N- acylurea formation in the product under the reaction conditions
Poly (L-lactic acid) (PLLA) is a synthetic and biodegradable aliphatic polyester.
Increasing concerns regarding sustainability of polymer materials have resulted in
greater focus on renewable and biodegradable polymers derived from biomass. PLLA is
a thermoplastic which has useful properties and are derived from sugars. PLLA is being
explored as a potential alternative to petroleum derived polymers, such as, poly (olefin)
s, poly (ester) s and poly (styrene) [1-4]. PLLA has good mechanical properties and can
be either rigid or elastic in nature. The polymer finds many applications, especially, as
fibers, films and transparent thermoplastics in packaging, consumer goods and many
articles of short life-cycle use.
Synthesis of high molecular weight PLLA can be accomplished by two methods, namely,
1) dehydrative melt polycondensation of L (+)-lactic acid (LA) [5] and 2) ring opening
polymerization (ROP) of a cyclic dimer (lactide) [6]. The method of choice is ROP of
cyclic lactide and is used for the large scale production of PLLA [7-8] . This method
involves the polymerization of LA to low molecular weight oligomers which is then
depolymerized to a cyclic dimer (lactide), purified by melt crystallization followed by
ROP to PLLA (Mw>50,000 ) using a suitable catalyst. Melt phase polycondensation of
LA has been widely studied [9-12]. Melt phase polycondensation methods have several
drawbacks, such as inability to achieve high molecular weights due to competitive
formation of cyclic oligomers at higher temperatures, undesirable side reactions leading
to broad polydispersity color, racemization and difficulties in removal of water
effectively from a high viscosity melt. Solid state polycondensation is another alternative
method for the synthesis of high molecular weight PLLA. However, the low melting point
of lactic acid oligomers and the slow reaction rate have limited the scope of this method
for the synthesis of PLLA [13-15].
Direct room temperature unactivated polymerization of a carboxylic acid with phenol
using diisopropyl carbodiimide (DIPC) and 4-(dimethylamino) pyridinum-p-toluene
sulfonate (DPTS) has been suggested as a mild method for the synthesis of high
molecular weight polyesters by Moore and Stupp [16]. They examined the self-
condensation of several hydroxyl acid monomers, wherein, both the hydroxyl and the
carboxyl group are attached to an aromatic ring. The reagent DPTS was found to be an
effective catalyst resulting in polymers with DP > 50 at room temperature. More
significantly, N-acyl urea could not be detected as a terminal group. Typically,
carbodiimide promoted esterification reactions are known to result in the formation of
unreactive N-acyl urea (Scheme 1) [16-17]. In a polymer analogous reaction, N-acyl
urea formation will limit the chain growth process. Use of DPTS is believed to depress
the formation of N-acyl urea. Hyperbranched poly(ε-caprolactone)s have been prepared
by condensation of AB2 polyesters at room temperature using 1,3-
dicyclohexylcarbodiimide (DCC) and DPTS[18]. Hyperbranched block polymers have
been prepared by the co-condensation of AB2 macromonomers in presence of DCC and
DPTS [19]. The AB2-type macromonomers have been condensed into long chain
branched poly(Lactide)s at room temperature using DCC and DPTS [20].
Surprisingly, little attention has been paid to polyesterification methods for the synthesis
of PLLA. Akutsu et al studied the effect of condensing agents, such as 1, 1-carbonyl-
diimidazole, N, N, N’, N’-tetramethylchloroformamidium chloride and N, N’-
dicyclohexylcarbodiimide / 4-dimethylaminopyridine in direct polyesterification of LA [21].
Aqueous solution of LA was first azeotropically distilled followed by polyesterification at
room temperature for 24 hours. PLLA with a maximum Mn of 15,000 was produced.
Neither the nature of starting oligomers nor the presence of terminal N-acyl urea groups
was reported.
A direct polyesterification method for synthesizing PLLA has certain advantages. Direct
polyesterification can be used to synthesize model polymers of LA or copolymers with
other naturally occurring α, ω- hydroxyl carboxylic acids. Such copolymerization
reactions are not possible with cyclic lactides. Furthermore, room temperature
polyesterification is free of complications, such as, trans-esterification, cyclization and
racemization normally observed in high temperature polymerization processes.
Moreover, this reaction is insensitive to traces of moisture.
In view of above, we undertook a careful study of polyesterification of well characterized
LA oligomers to high molecular weight PLLA using DIPC/DPTS system. A 1:1 adduct,
namely 4-(dimethylamino)-pyridinium-p-toluene sulfonate (DPTS) was prepared using a
mixture of a strongly basic, super-acylation catalyst, 4-(dimethylamino)-pyridine (DMAP)
and a strongly acidic (protonic) catalyst p-toluene sulfonic acid (PTSA). The diisopropyl
urea by-product derived from DIPC is claimed to be non-toxic and FDA approved [22].
The objective of the present study was to explore the scope of this reaction and to
assess the importance of N-acyl urea formation during this reaction.
Materials and Methods
Materials. L (+) -Lactic acid was obtained from Purac, USA as 88 % (w/w) aqueous
solution and was used without further purification. Tetraphenyltin and 4-
dimethylaminopyridine (DMAP) were obtained from Aldrich, USA and used as such. p-
Toluene sulfonic acid (PTSA) obtained from Aldrich, USA was dried azeotropically using
benzene and recrystallized from petroleum ether. Benzene, p-xylene, dichloromethane
(DCM), ethylene dichloride (EDC), chlorobenzene and toluene were obtained from
Aldrich, USA. EDC and chlorobenzene were dried over CaH2, whereas, toluene was
dried over metallic sodium. Diisopropyl carbodiimide (DIPC) was obtained from Aldrich,
USA and was used without further purification. DPTS was synthesized as per reported
procedure [16].
Synthesis of PLLA oligomer [13]
Using a reactor vessel equipped with a Dean and Stark type condenser, 40.2 g of 88 %
L-lactic acid was azeotropically dehydrated using 40 mL of p- xylene for 6 h at the reflux
temperature of the azeotrope. The reaction vessel was cooled to 50°C, tetraphenyltin
(0.0707g) was added followed by heating the reaction mixture slowly to the reflux
temperature (143oC) of the solvent under mild stirring for 15 h. The reaction mixture was
cooled to room temperature and 100 mL of chloroform was added. The resultant
solution was poured into 400 mL of n-hexane for precipitating the oligomer. The
oligomer was collected by filtration and further purified by repeated dissolution and
precipitation. The yield of the isolated oligomer was 97%. Mn, Mw and polydispersity of
the obtained oligomer was 900, 2100 and 2.3, respectively, as determined by SEC. The
Mn determined by VPO was 1200. In the 13C spectrum of PLA oligomer (Figure 1), the
peaks appearing at 169.4 to 169.9 ppm are due to ester carbonyl groups and at 173.0
to 173.4 ppm are due to carboxylic acid end functional group. The DPn was estimated
based on the relative integral ratio of these signals. Reproducibility was established
using two consecutive NMR experiments [23]. The value of Mn and degree of
polymerization (DPn) were calculated independently from 13C NMR and was found to be
1100 and 15 respectively (Figure 1). Carboxylic acid equivalent of PLLA was also
determined using quantitative 13C NMR and found to be 0.909 meq/g, which
corresponds to an average of one equivalent of carboxylic group per chain of the
oligomer. The rotation of PLLA oligomer (1g/dl in CHCl3) was measured at 20 oC and
was found to be -157o (98-100% optical purity). The Tg and Tm as determined by DSC
were 44 and 146°C, respectively.
General procedure for polyesterification of PLLA oligomer using DIPC - DPTS
To a glass reactor fitted with septum adaptor, PLLA oligomer, obtained as above and
DPTS were added. To this was added a dry solvent (DCM, EDC, toluene or
chlorobenzene as the case may be) in 10 mL / 100 mg PLLA proportion, followed by
addition of DIPC (1.2 mole equivalent of carboxylic acid present in the PLLA oligomer).
The reaction mixture was stirred for a defined period in the temperature range of 30-
80°C. After completion of the reaction, the viscous solution was diluted with
dichloromethane and the byproduct DIPC-urea filtered off. The organic layer was
washed with a 10 wt% solution of aqueous acetic acid followed by two washings with
distilled water, dried over MgSO4 and evaporated in vaccuo. The product (a white solid)
was further purified by dissolution in CH2Cl2, and precipitating from petroleum ether
[20]. The polymer was characterized by SEC and VPO for weight average and number
average molecular weights, respectively.
Molecular weights (Mn and Mw) and polydispersity were determined with respect to
polystyrene standards by SEC on a Waters 150 C machine at 25°C by eluting PLLA
solutions (10 mg/ mL in CHCl3) with toluene as internal standard, through a series of μ-
Styragel columns (30 cm length) of pore sizes 105, 104, 103, 500, and 100 A°,
respectively. Chloroform was used as the mobile phase (flow rate 1 mL/ min) and a
refractive index detector was used for detection. Number average molecular weight was
determined using a Knauer K-7000 Vapor Pressure Osmometer. Samples for 1 H NMR
were prepared in chloroform-d in 5 mm dia NMR tubes at room temperature. The
sample concentration for 13C NMR measurements was 10 % by weight. Proton
decoupled 13C NMR spectra with NOE were recorded on a Bruker DRX 500 MHz NMR
spectrometer working at 125.577 MHz for Carbon-13. 13C-NMR spectroscopy was also
performed on the Bruker DRX 500 MHz NMR spectrometer in 10 mm o.d. sample tubes.
CDCl3 served as solvent and TMS as internal standard for all 13C-NMR measurements.
Relative peak areas were proportional to the number of carbon atoms. Peak areas were
calculated by deconvolution method using WIN-NMR software.
MALDI-TOF MS analysis was performed on a Kratos Kompact MALDI -IV spectrometer
equipped with 0.7 m linear and 1.4 m reflection flight tubes as well as 337 nm nitrogen
LASER of pulse width 3 ns. All experiments were carried out at an accelerating potential
of 20 kV. In general, mass spectra from 200 shots were accumulated to produce a final
spectrum. The samples were dissolved in tetrahydrofuran (1 mg/ mL) and mixed with the
matrix (15 mg/ mL of THF) before drying on the sample plate. 2, 4, 6-
trihydroxyacetophenone (THAP) was used as the matrix. The sample plate was inserted
into the apparatus under high vacuum (~ 10 5 Pa).
Results and Discussion
Polyesterification reaction of PLLA oligomer using DIPC - DPTS
PLLA oligomer prepared by a procedure reported earlier [13] was subjected to
polyesterification using DIPC-DPTS in DCM solvent at 30°C to establish the initial
feasibility of the reaction. The results are shown in Table 1.
The carboxyl group equivalent: DPTS: DIPC for the PLLA oligomer was 1:1:1(entry no.
1) and 1:0.1:1.2 (entry nos. 2 to 26, Table 1). It was found that 1.2 equivalent of DIPC
with respect to the carboxyl-equivalent of the oligomer gave best results. This ratio was
used in all subsequent experiments. Chlorinated and aromatic solvents were used with
a view to achieve different reaction temperatures. Several catalyst concentrations were
also examined.
Reaction in DCM at room temperature and reduced DPTS concentration (0.1 equivalent)
resulted in high molecular weight PLLA (Mn = 45,600, entry no. 4, Table 1). The reaction
is favored by lowering of the solvation efficiency of the solvent. The weakly solvated
oligomer, either in, monomeric or dimeric form exhibits higher reactivity.
Reaction at 30 °C resulted in highest observed molecular weights (Mn = 45,600 entry
no.4, Table 1). Mn tends to decrease at higher reaction temperature. It was observed
that molecular weight leveled off after a certain period of time in all solvents and at all
temperatures. Polyesterification (activated acylation) involves a number of equilibria
between conjugate protonic acid-base pairs. Therefore, a non-polar aprotic solvent
should be most suitable since it will not interfere with these equilibria, either, by acting
as proton donor or acceptor [24]. Non-polarity and low dielectric constant are also
important to ensure that the ion-pairs are tightly caged. Reduction in DPTS
concentration from a stoichiometric equivalent to 0.1 equivalent resulted in higher
molecular weights of the resulting polymers. A similar observation was made by Moore
and Stupp who reported that DTPS is very effective in low concentrations [16].
Both DMAP and PTSA play a crucial role in this acylation mechanism as shown in
Scheme 1[16]. DMAP forms adducts with different protonated intermediate species,
thereby, accelerating the conversion of O-acyl urea to alkyl urea and the ester as well as
acylation by the anhydride once that is formed. On the other hand, PTSA causes a
depression of an important side reaction, namely, conversion of the O-acyl urea to an N-
acyl urea, which does not take part in the final acylation. This conversion is
thermodynamically facile, particularly, in a weakly acidic system. The conversion of O-
acyl urea to N-acyl urea needs migration of the lone pair of the imine nitrogen to the
carbonyl carbon. But the spatial disposition of the lone pair is away from the carbon; so
a rotational inversion of the already existing C-N double bond is necessary. A weak acid
causes a protonation-deprotonation equilibrium with the basic imine nitrogen, such that
sp3 character of the C-N double bond increases. This renders rotation around the C-N
bond more facile and the rearrangement easier.
However, in presence of a strong acid like p-toluene sulfonic acid (PTSA) the nitrogen of
the DMAP is cationated, whereas the nitrogen of the carbodiimide is protonated.
Therefore, the rearrangement is inhibited. A 1:1 molar ratio of DMAP and PTSA has
been found to be the best catalyst composition [16].
The effect of variation of DMAP and PTSA with respect to DPTS was studied and the
results are shown in Table 2. The results show that a small excess of DMAP along
with DPTS (entry no.1 to 4) caused a reduction in the molecular weights ( compare with
entry no.2 to 5 in Table 1). An excess of PTSA along with DPTS (entry no.5 to 8, Table
2) had little effect on the molecular weight (compare with entry no.2 to 5 in Table 1).
This implies that DMAP causes either degradation or some other side reaction that is
responsible for the reduction of molecular weight. An increase in the concentration of
DMAP relative to PTSA available in DPTS results in the generation of more conjugate
base of PTSA; therefore, less PTSA was available for trapping the O-acyl urea
intermediate by protonation. This, in turn, allows more O-acyl urea to be converted to N-
acyl urea, which did not take part in further reaction. This could be one possible reason
for reduced molecular weights. The reduction of molecular weight due to increase in the
DMAP concentration can be explained based on a degradation mechanism involving
traces of moisture in the system. The DMAP being a very strong base catalyzes
hydrolytic degradation of the PLLA chain. Such side reactions result in many different
types of chains as was evidenced by the emergence of shoulders towards lower
molecular weight regions in the SEC elugrams (Figure 2).
Mn increased up to 12 h after which it levels off (entry no.5, Table 1). This leads to
tailing of peaks and emergence of new peaks in the SEC eluegram, in the lower
molecular weight region, indicating occurrence of more side reactions or hydrolytic
degradation. Emergence of new peaks in the lower molecular weight side of the elugram
was very prominent when an increased amount of DPTS (1.0 equivalent) was used.
Under these conditions the molecular weight achieved was also lower.
The occurrence of lower molecular weight shoulders in the SEC elugram was also
examined further by MALDI-TOF MS. The MALDI-TOF MS of the sample at entry no.1,
Table 1, is shown in Figure 3. The spectrum (Figure 3) is dominated by a series of
intense peaks in the region ranging from 1000 to 3000 Da corresponding to an empirical
formula of (CH3)2CH-NH-CO-N(CH(CH3)2)-(CO-CH(CH3)-O)n-H-----Na+, and formula
weight (72n + 143 +23), where n ranges from 15 to 41. A particular peak at 1318 Da
corresponds to an empirical formula of (CH3)2CH-NH-CO-N(CH(CH3)2)-(CO-CH(CH3)-
O)n-H-----Na+ and formula weight (72n+143+23),where n is 16. The most intense peaks
belonging to this series, corresponding to oligomers with n = 15 to 25 in the region from
1000 to 2000 Da are reported enlarged in Figure 3a. In this region, the MALDI spectrum
of corresponding potassium adducts were also seen (mass = 72n + 143 + 39). There
was another set of peaks in the region ranging from 2000 to 3600 Da are reported in
Figure 3b, which corresponded to an empirical formula (CH3)2N-C5H5N+-(CO-CH(CH3)-
O)-H-----Na+ and formula weight (72n + 122 + 23), where n ranged from 26 to 34.
It is, therefore, evident that in presence of large excess of DMAP, the carboxylic terminal
group of the PLLA chains were largely capped as either N-acylurea derivative or salt of
DMAP, both of which prevented the chain from further growth. Presence of significant
amount of N-acylurea derivative meant that under such conditions PTSA failed to trap
the O-acylurea derivative by protonation. The detrimental effect of DMAP on the
molecular weight is supported by MALDI-TOF spectral evidence. The shoulders in the
SEC elugram of the corresponding sample were also attributed to such end-capped,
low molecular weight PLLA chains.
PLLA with a Mn ~45,000 was obtained by polyesterification of a preformed hydroxyl-
carboxyl telechelic oligomer using diisopropylcarbodiimide as a condensing agent and
DPTS as the activator. It was observed that DPTS prepared from a 1:1 ratio of DMAP
and PTSA when used in low concentration (0.1 equivalents) was most effective. Use of
PTSA in slight excess had little effect on polyesterification; however, use of excess
DMAP caused a reduction in molecular weight. This was attributed to reduced
availability of protons from PTSA for trapping the O-acylurea by protonation and
inhibiting its conversion to N-acylurea. MALDI TOF provided support for this hypothesis.
The effect of solvent on the mechanism of the polyesterification reaction appears more
complicated and warrants further studies.
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Table 1. Polyesterification of Lactic acid oligomersa
(Mole ratio)
( h)
a PLLA oligomer carboxyl group equivalent: DPTS: DIPC was 1:1:1(entry no-1) and 1:0.1:1.2 (entry no-2
to 26)
Table 2. Effect of concentration of DMAP and PTSA on the Polyestrification of
Oligomers (DCM,30 °C)
(mol/mol of oligomer)
Figure Captions:
Figure 1. Quantitative 13C-NMR (500 MHz) spectrum around carbonyl (ester), carbonyl
(acid) and carbonyl (lactone) areas of PLLA oligomer.
Figure 2. SEC of PLLA from polyesterification reaction: (a) entry no.2 (b) entry no.3 (c)
entry no.4 (d) entry no.5 and (e) entry no.1, Table 1.
Figure 3. MALDI-TOF MS of PLLA sample entry no.1,Table 1 obtained after
polyesterification using DIPC and 1.0 equivalent DPTS, solvent DCM, 30 °C, 12 h. (
entry no.1, Table 1)
Scheme Captions:
Scheme 1. Polyesterification of oligomers of L(+)-lactic Acid
Scheme 1. Polyesterification of oligomers of L(+)-lactic Acid
Figure 1. Quantitative 13C-NMR (500 MHz) spectrum around carbonyl (ester), carbonyl
(acid) and carbonyl (lactone) areas of PLLA oligomer samples
Figure 2. SEC of PLLA from polyesterification reaction: (a) entry no.2 (b) entry no.3 (c)
entry no.4 (d) entry no.5 and (e) entry no.1, Table 1.
Figure 3. MALDI-TOF MS of PLLA sample entry no.1,Table 1 obtained after
polyesterification using DIPC and 1.0 equivalent DPTS, solvent DCM, 30 °C, 12 h. (
entry no.1, Table 1),a: expanded region (1000-2000Da) and b: expanded region (2000-
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Biomass-based copolymers with alternating ricinoleic acid and 4-hydroxycinnamic acid derivatives (p-coumaric acid, ferulic acid, and sinapinic acid) exhibit a repeating structure based on soft and hard segments, derived from ricinoleic and 4-hydroxycinnamic acids, respectively. To achieve this alternating sequence, copolymers were synthesised by the self-condensation of hetero-dimeric monomers derived by the pre-coupling of methyl ricinolate and 4-hydroxycinnamic acid. The glass transition temperature (Tg) was observed to increase as the number of methoxy groups on the main chain increased; the Tg values of poly(coumaric acid-alt-ricinoleic acid), poly(ferulic acid-alt-ricinoleic acid), and poly(sinapinic acid-alt-ricinoleic acid) are −15 °C, −4 °C, and 24 °C respectively, 58 °C, 69 °C, and 97 °C higher than that of poly(ricinoleic acid). The polymers were processed into highly flexible, visually transparent films. Among them, poly(sinapinic acid-alt-ricinoleic acid) bearing two methoxy groups on each cinnamoyl unit, is mechanically the strongest polymer, with an elastic modulus of 126.5 MPa and a tensile strength at break of 15.47 MPa.
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The development of synthetic biodegradable polymers using solvent free polymerization has a unique potential to be used as sustainable polymers in biomedical applications. The aim of this work was to synthesize and characterize a sustainable class of poly(lactic acid) (PLA) under different operating conditions via direct polycondensation of lactic acid (LA). Several parameters were tested including the absence of solvents and catalysts on the polymerization, in addition to polymerization temperature and time. Polymerization conditions were evaluated using response surface method (RSM) to optimize the impact of temperature, time, and catalyst. Results showed that molecular weight (Mw) of PLA increased with increasing polymerization time. Highest Mw of 28.4 kD with relatively a broad polydispersity 1.9 was achieved at polymerization temperature 170 °C at 24 h in the free solvent polymerization. This led to a relevant inherent viscosity of 0.37 dl/g. FTIR spectra exhibited a disappearance of the characteristic peak of the hydroxyl group in LA at 3482 cm⁻¹ by increasing the intensity of carbonyl group. The ¹H nuclear magnetic resonance (NMR) exhibited the main chain at 5.22 ppm and the signal of methyl proton at 1.61 ppm as well as a signal at 4.33 and 1.5 assigned to the methane proton next to the terminal hydroxyl group and carboxyl group respectively. Meanwhile, the PLA synthesized with a catalyst [Sn(Oct)2] in a free solvent demonstrated comparatively high thermal transition properties of glass transition, melting, and crystallinity temperatures of 48, 106, and 158 °C, respectively. These results are of significant interest to further expand the use of PLA in biomedical applications.
Present paper deals with the preparation and characterization of carboxyl-functionalized poly(lactic acid) (PLA) of non-linear, star-shaped architecture prepared by direct melt polycondensation of lactic acid and pentetic acid - applied as a star polymer core. Influence of the optical purity of the lactic acid (L or DL) on the structure and properties of the product is investigated in close details. The influence of pentetic acid core on molar mass, as analysed by the means of gel permeation chromatography using triple detection (refractive index, light scattering and viscometric detectors) is performed as a detailed survey. Furthermore FTIR and NMR spectra, end group analysis, and water contact angle measurements were analysed to receive detailed product characteristics. Differential scanning calorimetry was carried out to examine thermal properties. Obtained results confirm the possibility of the starshaped structure preparation using a simple polycondensation reaction. The properties of the prepared polymers were found to be dependent on optical purity of the lactic acid precursor.
Recently, we reported a new class of biodegradable, thermoresponsive polyesters (TR-PEs) inspired by polyacrylamides and elastin-like peptides (ELPs). The polyesters exhibit tunable cloud point temperatures (Tcp) and thermoresponsive coacervation in aqueous solution as shown via UV-vis spectroscopy, 1H NMR, and DLS. However, the Tcp of all TR-PEs remained low (<15 °C), and higher thermoresponsivity would be beneficial for many applications. This study examines the synthesis, polymerization, and analysis of a new monomer bearing a more hydrophilic pendant group, bis-2-methoxyethylamine (bMoEtA). The resulting TR-PE, TR-bMoEtAPE, displays a threefold increase in Tcp (ca. 50 °C) that is affected by solution (DI water vs. phosphate buffered saline), concentration (1–40 mg mL−1) molecular weight (20–130 kDa), and cosolutes (Hofmeister salts and urea). The Tcp and Tg of random TR-bMoEtAPE copolymers can be tuned via comonomer feed. Variable temperature 1H NMR indicated a cooperative coacervation mechanism above Tcp, further reinforced by DLS measurements. As evidenced by UV-vis and SEC analysis, TR-bMoEtAPE underwent rapid degradation over a period of 7 days in DI water and PBS. Finally, cytotoxicity studies suggested that TR-bMoEtAPE is non-cytotoxic even at high concentrations (ca. 1000 μg mL−1). The increased Tcp and tunability suggests TR-bMoEtAPE as a potential candidate for future functionalized TR-PE therapeutic-delivery systems.
This comprehensive, truly one-stop reference discusses monomers, methods, stereochemistry, industrial applications and more. Chapters written by internationally acclaimed experts in their respective fields cover both basic principles and up-to-date information, ranging from the controlled ring-opening polymerization methods to polymer materials of industrial interest. All main classes of monomers including heterocyclics, cyclic olefins and alkynes, and cycloalkanes, are discussed separately as well as their specificities regarding the ring-opening polymerization techniques, the mechanisms, the degree of control, the properties of the related polymers and their applications. The two last chapters are devoted to the implementation of green chemistry in ring-opening polymerization processes. Of much interest to chemists in academia and industry.
Poly(lactic acid) as a completely biodegradable plastics has considerable properties, but the high prices prevent it from being used as normal materials. A low cost poly(lactic acid) may be synthesized by the direct polycondensation.The poly(lactic acid) synthesized by the solution polycondensation, direct melt polycondensation, melt polycondensation/chain extending and melt poly condensation/solid polymerization is reviewed.
The possibility of the preparation of aliphatic polyesters from dicarboxylic acids and diols by polycondensation in the presence of carbodimides under mild conditions was investigated. The following three possible routes were checked: the polycondensation of dicarboxylic acids with diols in the presence of 4-dimethylaminopyridine, the polycondensation of dicarboxylic acids with diisourea derivatives and the polycondensation in the presence of pyridine and p-toluenesulfonic acid. The last method was found to give polyesters with high molecular weights in good yields. This carbodiimide method can be utilized in the synthesis of biodegradable and surface active aliphatic polyesters.
A series of long-chain branched poly(d-/l-lactide)s is synthesized in a two-step protocol by (1) ring-opening polymerization of lactide and (2) subsequent condensation of the preformed AB2 macromonomers promoted by different coupling reagents. The linear AB2 macromonomers are prepared by Sn(Oct)2-catalyzed ROP of D- and L-lactide with 2,2-bis(hydroxymethyl)butyric acid (BHB) as an initiator. Optimization of the polymerization conditions allows for the preparation of well-defined macromonomers (Mw/Mn = 1.09–1.30) with adjustable molecular weights (760–7200 g mol−1). The two-step approach of the synthesis comprises as well the coupling of these AB2 macromonomers and hence allows precise control over the lactide chain length between the branching units in contrast to a random polycondensation.
We propose here a significantly improved process for the preparation of a lactide-clay intercalated mixture, which yields a high molecular weight (MW, Mn ∼ 126,000 Da) poly(L-lactic acid) (PLLA) clay nanocomposite (PLACN) in short times of in situ ring opening polymerization (ROP). In situ ROP using such a lactide-clay mixture enables ROP in the “nano-sized reactors” formed by the clay galleries. Cloisite®Na clays have been modified in-house with two different surfactant modifiers, hexadecyltrimethylammonium bromide and dioctadecyl dimethyl ammonium bromide, and these modified clays are compared with Cloisite®20A and Cloisite®30B. Interlayer spacings of modified clays are correlated with the resulting PLACN morphology and polymer MW growth. The optical purity of PLACNs is found similar, whereas thermal stability is significantly superior to that of neat PLLA. XRD/SAXS and TEM analyses confirm that PLACN can be prepared either with intercalated or exfoliated morphology when using either nonfunctionalized or functionalized modified clay, respectively. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers.
Aliphatic polyesters derived from lactides of various configurations (LL, DD, and DL) are promising as materials not only for packaging and for production of many other commonly used polymer products but also for unique medical applications. This paper describes current situation in synthesis and major applications of polylactides. Results of basic studies of “classical” polymerization of lactides prepared with stannous carboxylates or alkoxides, allowing to obtain polymers with molar masses (Mn) up to 106 g/mol and with the low content of tin are also presented. There are discussed advantages and disadvantages of catalyzing/initiating systems that contain zinc, magnesium, calcium, and other metal carboxylates and alkoxides as well as strong organic base initiators. Syntheses of polylactides with well defined microstructure are described. Characteristic features of bulk, solution, and dispersion polymerizations are compared. An outlook for development of polylactide production and applications is presented. Copyright © 2014 John Wiley & Sons, Ltd.
The direct polycondensation of racemic lactic acid using condensing agents, such as 1,1′-carbonyldiimidazole (CDI), N,N,N′,N′-tetramethylchloroformamidinium chloride (TMCFAC), and N,N′-dicyclohexylcarbodiimide/4-dimethylaminopyridine (DCC/DMAP), was investigated. Reactions using CDI proceeded slightly. Polycondensation using TMCFAC with pyridine gave a polymer in 83% yield, but the number-average molecular weight (M̄n) was 3700. Thus, TMCFAC is not effective in the direct polycondensation of lactic acid. However, DCC/DMAP is effective, and polycondensation in dichloromethane at room temperature for 24 h afforded poly(lactic acid) having M̄n of 15400 in 89% yield.