Biomacromolecules 2000, 1, 275-281
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A Novel Route To Poly(E-caprolactone)-Based Copolymers via Anionic Derivatization S. Ponsart, J. Coudane,* and M. Vert Centre de Recherche sur les Biopolyme` res Artificiels, Faculty of Pharmacy, 15, Avenue Charles Flahault, 34060 Montpellier Cedex 2, France Received February 16, 2000; Revised Manuscript Received March 24, 2000
Poly(-caprolactone) (PCL) is known to biodegrade under composting or water sewage plant conditions. However, as compared with poly(R-hydroxy acids) derived from lactic and glycolic acids, PCL is much more resistant to chemical hydrolysis and is achiral, a feature that limits very much the possibility of property modulation through the configurational structure of polymer chains. For the sake of enlarging the family of PCL-type polymers, a novel method is proposed which is based on the anionic activation of PCL chain by the removal of a proton from the methylene group in R-position of the ester carbonyl present in the main chain, using a nonnucleophilic base such as lithium diisopropyl amide (LDA). This activation leads to a polycarbanion onto which various electrophile groups can be attached. The feasibility of the process was first shown on poly(methyl acrylate), (PMA), whose polyacrylic main chain is resistant to strong bases. The PMA polycarbanion was modified by various electrophiles, namely benzaldehyde, naphthoyl chloride, benzyl chloroformate, and iodomethane. In a second stage, the same reactions were performed successfully on PCL. The degree of substitution depended on the experimental conditions. PCL underwent main chain degradation during the formation of the polycarbanion whereas the reaction with the electrophiles did not cause any further main chain cleavages. The degradation of PCL chains can be limited enough to give access to novel functional PCL polymers. Introduction Nowadays, increasing attention is paid to degradable and biodegradable biocompatible polymers for applications in the biomedical and pharmaceutical fields, primarily because after use they can be eliminated from the body via natural pathways. Poly(lactic acid) (PLA) and its copolymers with glycolic acid (PLAGA) have been used for many years as matrices for the controlled release of drugs or devices for osteosynthesis.1-6 Poly(-caprolactone) (PCL: structure 1)
1
is another member of the aliphatic polyester family which is biocompatible and of interest for biomedical and pharmaceutical applications. PCL has been proposed to make sustained release drug delivery systems because of its high permeability to drugs.7-9 In the field of biomedical applications, examples such as pleural and pericardial post operating adhesions are mentioned in the literature.10 In contrast to the PLAGA polymer family, PCL is known to undergo microbial and enzymatic degradations under outdoor conditions, a property which is referred to as biodegradation.11-13 Whether PCL biodegrades in the human body is still a matter of controversial discussion. Anyhow, PCL is highly hydro* Corresponding author. E-mail:
[email protected].
phobic and crystalline and degrades very slowly in vitro in the absence of enzymes, and in vivo as well.7,9,14-17 Because of the absence of chirality, PCL is far from offering the range of properties and thus the versatility of PLA-type polymers.18 Many authors took advantage of copolymerization to modulate the properties inherent to the PCL main chain. PCL was copolymerized with various comonomers, including lactide, glycolide, and ethylene oxide to generate polymeric compounds with different properties.19-23 Copolymerization of L-lactide and DL-lactide with -caprolactone was considered as a means to increase mechanical strength in orthopedic applications such as fixation of prosthetic devices.19 Terpolymers of glycolide (60%), DL-lactide (30%) and -caprolactone (10%) were synthesized to obtain materials with a half-life in the 15-20 days range.23 However the synthesis of copolymers by ring-opening polymerization is difficult and has to be performed at high temperature.21,23 The resulting properties of the copolymers were not always as good as expected. Basically, enlarging the property range offered by PCL can also be achieved by functionalization of the PCL backbone. However functionalized poly(caprolactone)s are not available yet, although the synthesis of low molar mass poly(bromo--caprolactone) was recently reported after copolymerization of -caprolactone with γ-bromo--caprolactone.24 In this paper, we wish to report a novel and versatile route to functional copolymers of the PCL type, based on the chemical modification of the PCL main chain using anionic activation with a nonnucleophilic base according to processes
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already prospected in organic chemistry.25-28 Substitution reactions via the R hydrogen of an ester are well-known in organic chemistry, but they have not been extended to aliphatic polyesters, so far, probably because of the sensitivity of these macromolecules to lysis. The feasibility of the method was demonstrated first using poly(methyl acrylate) (PMA: structure 2) as a hydrolysis-resistant model of PCL
Ponsart et al. Scheme 1. Chemical Modification of PMA via Anionic Route
2
chains. Indeed PMA bears a methyne-type hydrogen atom in the R-position of carbonyl group of the ester-type and thus resembles PCL. Lithium diisopropylamide was selected as nonnucleophilic strong base to obtain a carbanion by temporary removal of the methyne-type proton and its replacement by another electrophilic group as recommended in the literature.25 Various electrophilic reagents such as carbonyl-containing compounds, namely benzaldehyde, naphthoyl choride, and benzyl chloroformate, or alkyl halide compounds such as iodomethane, were assessed to check the versatility of the reaction. For the sake of showing the simplicity of the method, all reagents were used as received except the tetrahydrofuran solvent (THF), which had to be made anhydrous to perform the first stage properly. The same reactions were finally applied to PCL and the results were discussed with respect to substitution yields and simultaneous main chain degradation. Experimental Section Chemicals. PCL (Mn ) 53 700; Mw ) 80 000) and PMA (Mn ) 20 000; Mw ) 36 000) (toluene solution at 400 g/L) were purchased from Aldrich (St. Quentin Fallavier, France). LDA (2 M/l in THF/n-heptane) was from Acros Organics (Noisy-Le-Grand, France). Benzaldehyde, iodomethane, benzyl chloroformate, and acetophenone were purchased from Aldrich and naphthoyl chloride was purchased from Acros. NH4Cl and anhydrous MgSO4 were from Prolabo (Paris, France), CH2Cl2 from Riedel de Hae¨n (Seelze, Germany), and CH3OH from Carlo Erba (Milano, Italy). All the previous compounds were used as received whereas THF from BDH Laboratories Supplies (Poole, England) was distilled on benzophenone/sodium until a deep blue color was obtained. Substitution Reaction. Typically, PCL (0.01 mol, 1.14 g) was dissolved in 50 mL of anhydrous THF in a carefully dried reactor equipped with a mechanical stirrer and maintained at -78 °C under an argon atmosphere. A solution of LDA (0.01-0.03 mol) was injected with a syringe through a septum, the low-temperature being maintained for 30 min. Typically, benzaldehyde (0.02 mol, 2.12 g) was added, and the reaction was allowed to proceed for 30 min. The mixture was then poured into 200 mL of NH4Cl aqueous solution (20 g of NH4Cl/L). The copolymer was extracted with dichloromethane, the aqueous layer being extracted three more times with the same solvent. The combined organic layers were washed three times with distilled water, dried on anhydrous MgSO4, and filtered, and the solvent was partially evaporated under reduced pressure. The concentrated solution of copolymer was treated with
an excess of methyl alcohol. The precipitated polymer was washed with methyl alcohol until the washing solution was clear. The recovered copolymer was finally dried under vacuum for several hours. The same procedure was used for all the substituting reactants and for PMA. Methods. Molecular weights were determined by SEC using a Waters equipment fitted with a 60 cm long 5 µm mixed C PLgel column as the stationary phase and a Waters 410 refractometric detector. THF at 1 cm3/min flow rate was used as the mobile phase. For the naphthoyl-modified polymers, a Waters 470 fluorimetric detector (λex ) 298 nm, λem ) 365 nm) was coupled. Typically, 10 mg of polymer were dissolved in 2 mL of THF and the resulting solution was filtered through a 0.45 µm Millipore filter before injection of 20 µL of sample solution. Mn and Mw data were referred to polystyrene standards. 1H nuclear magnetic resonance (1H NMR) spectra were recorded at room temperature using Bruker spectrometers operating at 250 and 400 MHz. Deuterated chloroform was used as solvent, and chemical shifts were expressed in ppm from the tetramethylsilane (TMS) resonance. Infrared (IR) spectra were recorded on a Perkin-Elmer 1760 FTIR spectrometer, the polymers being dissolved in THF before casting onto a NaCl plate. Thin layer chromatography (TLC) was performed on Merck 60F254 silica gel-coated aluminum sheets, UV detection being achieved at 254 nm. CHCl3 was used as the mobile phase for the copolymers grafted with benzaldehyde, benzyl chloroformate and acetophenone, and CHCl3/CH3OH (8/2 v/v) for the substituted polymers bearing naphthoyl groups. Dialyses were carried out using Spectra/Por membrane tubing (cutoff: 3500 Da).
Results and Discussion Modification of the PMA Model. PMA was first activated under the form of a carbanion-bearing polymer chain by the removal of the proton located on the tertiary carbon atom on the main chain which is close to the pendent ester carbonyl group (Scheme 1). Activation was achieved by using LDA in an anhydrous organic medium. Preliminary investigations showed that 30 min were convenient to form the polycarbanion intermediate (step 1, Scheme 1). The resulting polycarbanion was thus allowed to react with different electrophiles such as benzyl chloroformate, iodomethane, benzaldehyde and naphthoyl chloride. The reaction (step 2, Scheme 1) was performed for 120 min, the temperature being kept at -78 °C during the first 30
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Poly(-caprolactone)-Based Copolymers Table 1. Experimental Conditions Used to Modify PMA after Activation by LDA
no. LDA/M 1 2 3 4 5 6 7 8b 9
1 2 3 4 1 1 1 1 3
reacn temp (°C)
electrophile reagent
-78 f RTa -78 f RT -78 f RT -78 f RT 1 h at -78 2 h at -78 -78 -78 -78 f RT
benzaldehyde benzaldehyde benzaldehyde benzaldehyde naphthoyl chloride naphthoyl chloride benzaldehyde benzyl chloroformate iodomethane
Scheme 2. Chemical Modification of PCL via Anionic Route
deg of substitution yield (%) (%) 24 20 34 23 18 19 13 15 -c
90 60 80 94 100 100 81 60 27
a Room temperature. b Reaction time: 5 min for the formation of carbanion and 15 min for the substitution reaction. c cf. text.
min and then allowed to reach room temperature over the next 90 min. Table 1 shows the results obtained with the various substituting reagents. For each compound the degree of substitution was determined from 1H NMR spectra by comparing the areas of the peaks related to the substituent to that of the PMA methyl ester taken as an internal standard. In the case of methylation by iodomethane, the singlet reflecting the pendent methyl group was located at ≈1.5 ppm and thus partially overlapped by the methylene protons of the polymer chain. In this particular case the determination of the degree of substitution was less accurate than for the other substituting groups. The effects of different experimental factors, namely LDA/ repeating unit ratio (LDA/M), reaction temperature, and reaction time, on the degree of substitution were investigated. LDA was used in excess and the resulting copolymers were purified by precipitation using methyl alcohol. The influence of the LDA/M ratio was assessed in the case of the reaction of the polycarbanion with benzaldehyde (nos. 1-4 in Table 1). The degree of substitution did depend on the LDA/M ratio. However the substitution leveled off at ca. 34% substitution for LDA/M ) 3. Steric hindrance and electronic repulsion of the negative charges on the polyanionic chain were supposed to be possible sources of the limitation. Treating a second time a copolymer already bearing 34% benzyl units was used to test the contribution of steric hindrance. As the degree of substitution did not increase, it was concluded that steric hindrance was a major factor in substitution limitation. The influence of temperature conditions on the second step was evaluated at constant LDA/M ratio ) 1 (no. 1 and no. 7 in Table 1). The degree of substitution increased from 13 to 24% when the temperature was raised from -78 °C to room temperature. The influence of the reaction time at step 1 was evaluated in the case of naphthoyl chloride for LDA/M ) 1 (nos. 5 and 6 in Table 1). The formation of the polycarbanion was performed at -78 °C using two reaction times, namely 1 and 2 h. The substitution step was then allowed to proceed under similar conditions, namely 30 min at -78 °C followed by 90 min at room temperature. There was no significant effect on the degree of substitution.
From these preliminary experiments, it was concluded that (i) the formation of the PMA polycarbanion is easy, rapid and efficient, (ii) the resulting polyanion can react with various electrophiles, and (iii) the substitution is limited, steric hindrance being at least partly responsible for the limitation. Modification of PCL. In the case of PCL, the two-step reaction corresponded to the formula shown in Scheme 2. In contrast to PMA, PCL chains bear two hydrogen atoms per repeating units that can be removed because of the action of LDA. However, it is likely that when one of those two hydrogen atoms is removed, the second is stabilized as part of the carbanion. Therefore, only one site of activation per repeating units was considered to determine the values of the degree of substitution by NMR. Initially the molar mass and the polydispersity of the PCL sample were 53 700 g‚mol-1 and 1.5, respectively. Preliminary attempts showed that the molar mass of the substituted polymer decreased dramatically when reaction time, temperature and LDA/M ratio increased. Deeper investigation was carried out to measure the influence of the reaction time on the molar mass of the resulting copolymer. The molar masses were evaluated by SEC in THF and the degrees of substitution were determined from the integration of NMR peaks specific of the substituent and of PCL as described for PMA. The two steps of the reaction were considered separately. The temperature and the LDA/M ratio were first
Figure 1. Variations of the molar masses of PCL with time as determined by SEC during the formation of the polycarbanion by reaction with LDA (step 1 of Scheme 2): (×) Mn experimental; (O) Mw experimental, (-)Mn theoretical from eq 4 with k/M0 ) 5 × 10-4 min-1 g-1 mol.
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Scheme 3. Intramolecular Autocondensation Reaction of PCL
set to -78 °C and 1, respectively. These rather low values were selected to limit PCL chain degradation. Samples were periodically withdrawn from the reaction medium and hydrolyzed with NH4Cl-containing aqueous solution. The copolymer aliquots were then extracted with dichloromethane and finally precipitated in methyl alcohol. Figure 1 shows that the cleavage of polymer chains was very fast as the decrease of molar mass occurred dramatically within the first 3 min. The molar masses of the recovered copolymers were then ca. half the initial molar masses. Beyond three minutes, no more change was detected. The rapid decrease of Mn can be assigned to a very few cuts of the main chain. Let us define “n0” and “nj” as the average numbers of ester junctions in a polymer chain at time zero and at time t, respectively. One can write DPn ) “nj” + 1 and DPn0 ) n0 + 1, where DPn0 and DPn stand for the polymerization degrees of initial and final polymers, respectively. If “R ) nc/n0” stands for the average fraction of cleaved ester bonds, “nc” being the average number of cleaved ester bonds/chain, nc ) DPn0/DPn - 1, one can write R ) nc/n0 )
1
( ) DPn0
DPn0 - 1 DPn
-1 ≈
1
-
DPn
1 DPn0
if n0 . 1
Table 2. Variations of the Molar Mass of PCL during the Electrophilic Substitution on the PCL Polycarbanion time (min)
Mn × 10-3 (g/mol)
time (min)
Mn × 10-3 (g/mol)
2.5 7.6 18
20 15 15
24.5 45 90
19 19 22
Figure 2. Chemical structures of the various PCL copolymers synthesized by reaction of the following electrophiles with the PCL polycarbanion: R ) methyl, iodomethane; R ) 1-methyl-1-hydroxybenzyl, benzaldehyde; R ) benzyloxycarbonyl, benzyl chloroformate; R ) 1-hydroxybenzyl, benzaldehyde; and R ) naphthoyl, naphthoyl chloride.
(1) and
and DPn ) DPn0/(RDPn0 + 1)
Applied to the PCL sample used in this study, eq 2 shows that Mn drops from ca. 50 000 down to 20 000, the value obtained after 3 min in Figure 1, for R ) 3.7 × 10-3, i.e., for the cleavage of less than 1% of the ester groups initially present in the polymer chain. Such a decrease of the molar mass depends on the rate of ester bond cleavage. Assuming the rate of hydrolysis is proportional to the number of ester bonds, one can write: dN/dt ) -kN, and, after integration N ) N0 exp(-kt)
from which one can write ln(DPn0/nj0) ) ln(DPn/nj) -kt, and
ln
DPn0 - 1
) ln
DPn0
DPn - 1
+ kt
DPn
If DPn0 . 1 and DPn . 1 ln
DPn0 - 1 DPn0
DPn0 DPn
DPn0 *nj ) p0*nj0 exp(-kt) DPn
DPn0/nj0 ) (DPn/nj) exp(-kt)
(3)
where k stands for the hydrolysis rate constant, N and N0 being the total numbers of ester bonds at time t and time 0, respectively. If p0 and p are the initial and final numbers of macromolecular chains, n0 ) N0/p0 and nj ) N/p as p/p0 )
N ) p*nj ) p0*
(2)
and
≈ -1/DPn0
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Figure 3. 1H NMR in deuterated chloroform: (a) PCL; (b) PCL-co-(R-methylcaprolactone) 11%; (c) PCL-co-R-(methylhydroxybenzyl)caprolactone 11%; (d) PCL-co-R-(benzyloxycarbonyl)caprolactone 11%; (e) PCL-co-(R-(hydroxybenzoyl)caprolactone) 11%; (f) PCL-co-(R-naphthoylcaprolactone) 10%.
ln
DPn - 1 DPn
≈ -1/DPn
and thus (1/DPn) ) (1/DPn0) + kt If M0 stands for the molar mass of the caprolactone unit (M0 ) 114), and Mn0 and Mn stand for the average molar masses at time zero and time t, respectively 1/Mn ) 1/Mn0 + (k/M0)*t
(4)
Equation 4 fitted experimental data for k/M0 ) 5 × 10-4 min-1 g-1 mol during the first 3-min period. However,
beyond 3 min, the value of the molar mass departed from the theoretical values and remained constant unexpectedly. This behavior could have been due to the fact that the polymer aliquots were precipitated in methyl alcohol where small PCL oligomers are soluble, thus leading to fractionation and molar mass limitation. Actually, beyond 3 min the yield in polymer remained constant, thus excluding a fractionation according to molar mass. Therefore, we concluded that the cleavage of PCL chains during the formation of the polycarbanion is a side reaction occurring in parallel to the proton extraction. This interpretation was further supported by the fact that increasing the amount of LDA initially introduced within the reaction medium led to a greater decrease of the molar mass and a lowering of the plateau. A transesterification by intramolecular autocondensation, leading to a
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Table 3. Experimental Conditions Used to Modify PCL after Activation by LDA run no. LDA/M 1 2 3 4 5
1 1 1 1 0.25
electrophile reagent iodomethane acetophenone benzyl chloroformate benzaldehyde naphthoyl chloride
deg of substitution yield (%)a (%)b 11 11 11 11 10
a Percent of substituted hydrogen/repeating unit tated copolymer/weight of initial PCL
84 64 76 81 71 b
Mn, Ip 36 000, 2.2 50 000, 1.8 25 000, 4.7 41 000, 1.9 24 000, 8.3
Weight of precipi-
macrocycle and an alcoholate could explain the decrease of molecular weight (Scheme 3). The alcoholate beeing totally unreactive on a PCL chain in this conditions, that can justify the occurrence of a plateau of molecular weight. Anyhow, the presence of two simultaneous reactions, namely proton extraction and chain cleavage, led to small PCL molecules activated under a polycarbanion form since the step 2 attack by electrophiles was feasible. The time dependence of the second step was evaluated for the reaction of PCL polycarbanion with naphthoyl chloride. Table 2 shows that time had no further influence on the molecular weights obtained at the end of the first step. Therefore, it was concluded that chain scission occurred exclusively during the polycarbanion formation. As in the case of PMA, various electrophiles were allowed to react with PCL polycarbanions under the following standard conditions: LDA/M ) 1, temperature ) -78 °C and reaction times are 30 min (step 1) and 30 min (step 2). The resulting copolymers were recovered after hydrolysis by adding an ammonium chloride aqueous solution followed by extraction with methylene chloride and precipitation in methyl alcohol. The co-repeating units introduced in PCL chains are presented in Figure 2. The structure of the copolymers and the degree of substitution were determined by 1H NMR. The spectra of the genuine PCL and of the derived copolymers are shown in Figure 3, parts a-f. Molar masses and molar mass polydispersity for the various substitution reactions were determined by SEC. Analytical data are listed in Table 3. The various reactions led to significant substitution regardless of the electrophilic reagent (alkyl halide, ketone, aldehyde, acyl chloride, chloroformate). The values of the degree of substitution remained in the 10% range and were much lower than those observed in the case of PMA. In the case of the PCL methylation by iodomethane (Figure 3b) the degree of substitution was calculated from the ratio of the area of the doublet corresponding to methyl protons at 1.1 ppm (e) to the area of the OCH2 resonance (a). For the substitution by acetophenone (Figure 3c), the degree of substitution was determined from the ratio of the aromatic proton (e) peak area at 7.1-7.4 ppm (or doublet of the methyl group (f) at 1.4 ppm), to the area of the OCH2 resonance (a). In the case of the benzyloxycarbonyl group (Figure 3d), the OCH2 peaks (f) at 5.25 ppm and aromatics signals (e) at 7.35 ppm that differ from those of the chloroformate precursor were used to determine the degree of substitution.
Figure 4. Refractometry-detected and fluorimetry-detected SEC chromatograms of naphthoyl substituted PCL in THF: (-) refractometry as recovered after precipitation in methanol; (-) fluorimetry after precipitation in methanol; (- - -) fluorimetry after precipitation in methanol and dialysis.
Similarly the chemical shifts of the aromatic protons in benzaldehyde-substituted PCL (7.25 ppm) were different as compared with the chemical shift of free benzaldehyde, showing that the aromatic ring was covalently bound to PCL (Figure 3e). The absence of small molecules entrapped in the polymer matrix was shown by TLC analysis. In the case of the naphthoyl-substituted copolymer, the degree of substitution was estimated from the ratio of the area of naphthoyl protons peaks between 7.3 and 9 ppm (e) to the area of the OCH2 signal (a). The binding of the fluorophore was checked by SEC. The chromatograms of the naphthoylsubstituted PCL recovered after precipitation in methanol were analyzed using fluorometric detection (λex ) 298 nm; λem ) 365 nm) (Figure 4 middle) and refractometric detection (Figure 4 bottom). Data showed that the polymer moiety was fluorescent, but a small amount of fluorescent small molecules was also detected. Therefore, the copolymer was further purified by dialysis against THF to yield a copolymer almost free of fluorescent small molecules (Figure 4 top). This conclusion was born out by TLC analysis performed with chloroform/methyl alcohol as the mobile phase that led to a fluorescent mark at Rf ) 0, a behavior typical of polymer-bonded naphthoyl moieties. Therefore, dialysis appeared as a convenient method for the ultimate purification of the copolymers. Conclusion The activation of ester-bearing polymer chain by creating carbanions using the LDA method was successful for the preparation of PMA-based copolymers containing up to ≈30% modified repeating unit, depending on the reagent. Several electrophiles were attached to the PMA chains but the list is far for being exhaustive. The method appears as a powerful and versatile tool to modify adequate polymers. The same method, applied to PCL, led to some degradation by main chain cleavage and provided copolymers containing up to 10% co-units, depending on the substituting group and the reaction conditions. The method presents the advantage of starting from preformed polymers, and thus does not
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require any polymerization reaction nor synthesis of new comonomer. The process is based on a rapid one pot reaction which respects, at least partly in the case of PCL, the macromolecular chain of the starting polymer. The formation of copolymers was checked by 1H NMR analyses and the covalent binding of the substituting group on the macromolecular chain was shown by SEC or TLC analyses. The study of the properties of the copolymers and the synthesis of other copolymers are underway. Acknowledgment. Financial support from the French MESR (Ministry of Superior Education and Research) for the fellowship thesis is gratefully acknowledged. References and Notes (1) Mauduit, J.; Bukh, N.; Vert, M. J. Controlled Release 1993, 23, 209220. (2) Mauduit, J.; Bukh, N.; Vert, M. J. Controlled Release 1993, 23, 221230. (3) Hutchinson, F. G.; Furr, B. J. A. Biochem. Soc. Trans. 1985, 13, 520-523. (4) Hecquet, B.; Chabot, F.; Delatorre Gonzalez, J. C.; Fournier, C.; Hilali, S.; Cambier, L.; Depadt, G.; Vert, M. Anticancer Res. 1986, 6, 1251-1256. (5) Vert, M. Angew. Makromol. Chem. 1989, 166, 155-168. (6) Zimmerman, M. P. J. R.; Alexander, A. J. Biomed. Materi. Res. 1987, 21, 345-361. (7) Pitt, C. G.; Schindler, A. Capronor-A biodegradable delivery system for levonorgestrel. In Long-acting contraceptiVe deliVery systems; Zatuchni, G. L., et al., Eds.; Harper and Row, publishers: Philadelphia, PA, 1984; pp 48-63. (8) Calvo, P.; Thomas, C.; Alonso, M. J.; Vila-Jato, J. L.; Robinson, J. R. Int. J. Pharm. 1994, 103, 283-291. (9) Lemmouchi, Y.; Schacht, E.; Kageruka, P.; De Deken, R.; Diarra, B.; Diall, O.; Geerts, S. Biomaterials 1998, 19, 1827-1837. (10) Nakamura, T.; Hitomi, S.; Shimamoto, T.; Hyon, S. H.; Ikada, Y.; Wanabe, S.; Shimizu, Y. Surgical application of biodegradable films
(11) (12)
(13) (14) (15) (16)
(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28)
prepared from lactide--caprolactone copolymers. In Biomaterials and clinical applications; Pizzoferrato, A., et al., Eds.; Elsevier science publishers B. V.: Amsterdam, 1987; pp 759-764. Lefebvre, F.; David, C. Polym. Degrad. Stab. 1994, 45, 347-353. Jarrett, P.; Benedict, C. V.; Bell, J. P.; Cameron, J. A.; Huang, S. J. Mechanism of the biodagradation of the polycaprolactone. In Polymers as Biomaterials; Shalaby, S. W., et al., Eds.; Plenum Pub1. Corp.: New York, 1985; pp 181-192. Rutkowska, M.; Dereszewska, A.; Jastrzebska, J. H. Macromol. Symp. 1998, 130, 199-204. Bei, J. Z.; Li, J. M.; Wang, Z. F.; Wang, S. G. Polym. AdV. Technol. 1997, 8, 693-696. Li, S. M.; Espartero, J. L.; Foch, P.; Vert, M. J. Biomater. Sci., Polym. Ed. 1996, 8, 165-187. Pitt, C. G. Nonmicrobial degradation of polyesters: mechanisms and modifications. In Biodegradable polymers and plastics; Vert, M., et al., Eds.; The Royal Society of Chemistry: Cambridge, England, 1992; pp 7-17. Wu, C.; Gan, Z. H. Polymer 1998, 39, 4429-4431. Chabot, F.; Vert, M. Polymer 1983, 24, 53-59. Zhang, X.; Wyss, U. P.; Pichora, D.; Goosen, M. F. A. J. Macromol. Sci.sPure Appl. Chem. 1993, A30, 933-947. Petrova, T.; Manolova, N.; Rashkov, I.; Li, S. M.; Vert, M. Polym. Int. 1998, 45, 419-426. Cerrai, P.; Guerra, D.; Lelli, L.; Tricoli, M. J. Mater. Sci., Mater. Med. 1994, 5, 33-39. Li, S. M.; Garreau, H.; Vert, M.; Petrova, T.; Manolova, N. J. Appl. Polym. Sci. 1998, 68, 989-998. Sawhney, A. S.; Hubbell, J. A. J. Biomed. Mater. Res. 1990, 24, 1397-1411. Detrembleur, C.; Mazza, M.; Halleux, O.; Lecomte, P.; Mecerreyes, D.; Hedrick, J.-L.; Je´roˆme, R. Macromolecules 2000, 33, 14-18. Petragnani, N.; Yonashiro, M. Synthesis 1982, 521-578. Downham, R.; Kim, K. S.; Ley, S. V.; Woods, M. Tetrahedron Lett. 1994, 35, 769-772. Murta, M. M.; de Azevedo, M. B. M.; Greene, A. E. Synth. Commun. 1993, 23, 495-503. Sun, X. F.; Kenkre, S. L.; Remenar, J. F.; Gilchrist, J. H.; Collum, D. B. J. Am. Chem. Soc. 1997, 119, 4765-4766.
BM005521T
