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Synthesis of new statistical andblock co-polyesters by ROP of α,α,β-trisubstituted β-lactones and theircharacterizationsFarid Ouhib , Solo Randriamahefa , Philippe Guérin & ChristelBarbaudPublished online: 02 Apr 2012.

To cite this article: Farid Ouhib , Solo Randriamahefa , Philippe Guérin & Christel Barbaud(2005) Synthesis of new statistical and block co-polyesters by ROP of α,α,β-trisubstituted β-lactones and their characterizations, Designed Monomers and Polymers, 8:1, 25-35

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Designed Monomers and Polymers, Vol. 8, No. 1, pp. 25–35 (2005) VSP 2005.Also available online - www.vsppub.com

Synthesis of new statistical and block co-polyesters by ROPof α,α,β-trisubstituted β-lactones and theircharacterizations

FARID OUHIB, SOLO RANDRIAMAHEFA, PHILIPPE GUÉRINand CHRISTEL BARBAUD ∗

Laboratoire de Recherche sur les Polymères, UMR C7581 CNRS-Université Paris XII, 2-8 Rue HenriDunant, F-94320 Thiais, France

Abstract—The preparation of α,α,β-trisubstituted β-lactones has opened the route to living anionicring-opening polymerization of these monomers. Therefore, transfer reactions due to hydrogenabstraction were limited and it is now possible to strickly control sample molecular weight. Moreover,living end-groups lead to the preparation of block co-polymers with block controlled lenghts. Inthe same way, high-molecular-weight statistical co-polymers have been synthesized. Hydrolyzablemicelles and nanoparticles will be prepared from these co-polymers after chemical modifications toadjust the hydrophilic/hydrophobic balance.

Keywords: 4-Alkyloxycarbonyl-3,3-dimethyl-2-oxetanones; living anionic polymerization; poly(alkyl(R,S)-3,3-dimethylmalate); statistical co-polyesters; block co-polyesters.

1. INTRODUCTION

The development of functionalized macromolecules with the strict adjustment oftheir structures and their properties leads to complex architecture and puts on thediversification of hydrolyzable polymers. These new biodegradable, bioassimilableand biocompatible poly((R,S)-3,3-dimethylmalic acid) derivatives and their co-polymers could be used in temporary therapeutic applications such as carrier or drugdelivery system. Indeed, we have demonstrated in a recent paper that the hydrolysisof poly((R,S)-3,3-dimethyl malic acid) (PDMMLA, Fig. 1) gave the corresponding(R,S)-3,3-dimethylmalic acid [1]. This diacid is a natural and non-toxic compound

∗To whom correspondence should be addressed. Fax: (33-1) 4978-1208;e-mail: barbaud@glvt-cnrs.fr

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Figure 1. Structure of poly((R,S)-3,3-dimethylmalic acid) (PDMMLA) and poly(alkyl (R,S)-3,3-dimethylmalate) (PDMMLAalkyl).

Scheme 1. Biosynthesis of ketovaline by β,β-dimethyldeshydrogenase (EC.1.1.1.84).

involved in the biosynthesis of pantothenate and Coenzyme A catalyzed by β,β-dimethyldehydrogenase (EC. 1.1.1.84, Scheme 1) [2–4].

This paper reports the synthesis and characterization of new statistical and blockco-polyesters by living anionic ring-opening polymerization [5–8]. These new co-polymers can contain several types of pendant groups (neutral, hydrophilic, reactive,etc.) which can be arranged at different compositions and in various distributionsalong the macromolecular backbone in order to modulate solubility and degradationrate [9–11]. The synthesis of such polymeric materials will permit, in a forthcomingstudy, the preparation of nanoparticles and micelles.

2. EXPERIMENTAL

2.1. Materials and methods

1H-NMR spectra were recorded on a Bruker AC-200 (200 MHz) and 13C-NMRspectra on a Bruker Avance 300 (75 MHz), chemical shifts in ppm (δ), couplingconstants were expressed in Hz. IR spectra were recorded on a Bruker-Tensor27 FT-IR. The glass transition temperature (Tg) is determined by differentialscanning calorimetry using a DSC-2010 TA Instrument (heating rate 10◦C/min)under nitrogen. Our Tg values correspond to the calculated inflection point. Size-exclusion chromatography (SEC) was performed on a chromatograph equipped witha P100 pump (Spectra Physics), a Rheodyne injector, different sets of columns and adifferential refractometer RI 71 (Shodex) coupled with a MiniDawn light scatteringdetector (Wyatt technology). Polymers were analyzed in THF with a set of twoPL-gel Mixed C columns (Polymer Laboratories). THF was freshly distilled fromNa/benzophenone; in all other cases, commercially available reagent-grade solventswere employed without purification. All reactions in anhydrous organic solvents

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were performed under argon atmosphere. All the glass apparatuses were kept onenight in a drying-oven at 100◦C before use.

2.2. Synthesis of homo-polymers

The synthesis of two different lactones used for all these polymerizations was doneaccording to the procedure described in the literature [12].

2.2.1. Poly(benzyl (R,S)-3,3-dimethylmalate) (PDMMLABz) (3). Poly(benzyl(R,S)-3,3-dimethylmalate) was synthesized by anionic ring-opening polymerizationof (R,S)-4-benzyloxycarbonyl-3,3-dimethyl-2-oxetanone (1, 208 mg, 0.9 mmol)in the presence of tetraethylammonium benzoate (0.9 µmol) as initiator, undernitrogen at 37◦C in bulk for 5 days. Polymerization was controlled by IR. The cruderesulting material was dissolved in acetone, neutralized with one drop of acetic acidand purified by precipitation in cyclohexane. Polymeric material 3 was separatedand dried under vacuum. 100% conversion (m = 187 mg). Tg = 45◦C. SEC(light scattering, THF): Mw = 340 kg/mol, Mn = 320 kg/mol, pI = 1.05. IR:1750 cm−1: νC O. 1H-NMR (200 MHz, CD3COCD3): δ = 1.00 (s, 3H, CH3), 1.10(s, 3H, CH3), 5.00 (s, 2H, CH2), 5.20 (s, 1H, CH), 7.10 (m, 5H, C6H5). 13C-NMR(75 MHz, CD3COCD3): δ = 21 (2CH3), 46 (C main chain), 67 (O CH2), 77 (CHmain chain), 129, 136 (C6H5), 168, 173 (2C O).

2.2.2. Poly(hexyl (R,S)-3,3-dimethylmalate) (PDMMLAHe) (4). Poly(hexyl(R,S)-3,3-dimethylmalate) was synthesized by anionic ring-opening polymerizationof (R,S)-4-hexyloxycarbonyl-3,3-dimethyl-2-oxetanone (2, 214 mg, 0.94 mmol) inthe presence of tetraethylammonium benzoate (0.94 µmol) as initiator, under ni-trogen at 37◦C in bulk for 5 days. Polymerization was controlled by IR. Thecrude resulting material was dissolved in acetone, neutralized with one drop ofacetic acid and purified by precipitation in ethanol. Polymeric material 4 was sep-arated and dried under vacuum. 100% conversion (m = 198 mg). Tg = −9◦C.SEC (light scattering, THF): Mw = 200 kg/mol, Mn = 177 kg/mol, pI = 1.10.IR: 1750 cm−1: νC O. 1H-NMR (200 MHz, CD3COCD3) δ = 0.80 (s, 3H,CH3 hexyl), 1.20 (m, 12H, 2CH3 main chain and (CH2)3 CH3), 1.50 (m, 2H,

(CH2) (CH2)3 CH3), 4.00 (m, 2H, O CH2 ), 5.20 (s, 1H, CH main chain).13C-NMR (75 MHz, CD3COCD3): δ = 14 (CH3 hexyl), 22 (2CH3), 25, 28, 31(4CH2), 45 (C main chain), 66 (O CH2), 76 (CH main chain), 168, 173 (2C O).

2.3. Synthesis of statistical co-polymers

2.3.1. Poly(benzyl (R,S)-3,3-dimethylmalate-co-hexyl (R,S)-3,3-dimethylmalate)(PDMMLABz10-co-He90) (5). Polymer 5 was synthesized by anionic ring-openingpolymerization from a mixture of (R,S)-4-benzyloxycarbonyl-3,3-dimethyl-2-ox-etanone (1, 196 mg, 0.83 mmol) and (R,S)-4-hexyloxycarbonyl-3,3-dimethyl-2-oxetanone (2, 1.77 g, 7.78 mmol) in the presence of tetraethylammonium benzoate

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(8.61 µmol) as initiator under nitrogen at 37◦C in bulk for 5 days. Polymerizationwas controlled by IR. The crude resulting material was dissolved in acetone,neutralized with one drop of acetic acid and purified by precipitation in ethanol.Polymeric material 5 was separated and dried under vacuum. 100% conversion(m = 1.85 g). Tg = −4◦C. SEC (light scattering, THF): Mw = 246 kg/mol,Mn = 230 kg/mol, pI = 1.06. 1H-NMR (200 MHz, CD3COCD3) δ = 0.80 (s,3H, CH3 hexyl), 1.20 (m, 12H, 2CH3 main chain and (CH2)3 CH3), 1.50 (m,2H, (CH2) (CH2)3 CH3), 4.00 (m, 2H, O CH2 hexyl), 5.00 (s, 2H, O CH2

benzyl), 5.20 (s, 1H, CH main chain), 7.10 ( m, 5H, C6H5). 13C-NMR (75 MHz,CD3COCD3): δ = 14 (CH3 hexyl), 22 (2CH3), 25, 28, 31 (4CH2), 45 (C mainchain), 66 (2O CH2), 76 (CH main chain), 129, 136 (C6H5), 168, 173 (2C O).

2.3.2. Poly(benzyl (R,S)-3,3-dimethylmalate-co-hexyl (R,S)-3,3-dimethylmalate)(PDMMLABz30-co-He70) (6). The preparation of statistical co-polymer 6 wasrealized according to the procedure previously described for statistical co-polymer 5with a mixture of (R,S)-4-benzyloxycarbonyl-3,3-dimethyl-2-oxetanone (1, 659 mg,2.18 mmol) and (R,S)-4-hexyloxycarbonyl-3,3-dimethyl-2-oxetanone (2, 1.49 g,6.57 mmol) in the presence of tetraethylammonium benzoate (8.75 µmol). 100%conversion (m = 1.98 g). Tg = 7◦C. SEC (light scattering, THF): Mw =280 kg/mol, Mn = 230 kg/mol, pI = 1.20. 1H- and 13C-NMR spectra gave thesame results as polymer 5 but with different integration values.

2.4. Synthesis of block co-polymers

2.4.1. Poly(benzyl (R,S)-3,3-dimethylmalate-b-hexyl (R,S)-3,3-dimethymalate)(PDMMLABz5-b-He95) (7). Polymer 7 was obtained by anionic polymerizationof lactone 1 (107 mg, 0.45 mmol) in the presence of tetraethylammonium ben-zoate (4.5 µmol, 89 µl) as initiator, in a solution of anhydrous THF (2 ml) at roomtemperature for 1 day. A sample (200 µl) of the mixture was analyzed by IR spec-troscopy to control the total conversion of the lactone. Lactone 2 (930 mg, 4 mmol)in a solution of anhydrous THF (2 ml) was added to the precedent solution andthe polymerization was continued for one additional week. A sample (200 µl) ofthe mixture containing block co-polymer 7 was also analyzed by IR spectroscopy.After complete conversion, the polymerization was stopped by neutralization withone drop of acetic acid. The solution was poured dropwise in ethanol to precipi-tate the co-polymer 7. (m = 950 mg). Tg = −8◦C. SEC (light scattering, THF):Mw = 217 kg/mol, Mn = 164 kg/mol, pI = 1.30. The 1H- and 13C-NMR spectraof block co-polymer 7 are identical to the spectra previously obtained for statisticalco-polymer 5.

2.4.2. Poly(benzyl (R,S)-3,3-dimethylmalate-b-hexyl (R,S)-3,3-dimethymalate)(PDMMLABz30-b-He70) (8). Block co-polymer 8 was prepared according to theprocedure previously described for block co-polymer 7, but with lactone 1 (209 mg,

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0.9 mmol) in 2 ml of anhydrous THF in the presence of tetraethylammonium ben-zoate (2.97 µmol, 37 µl) for 5 days and lactone 2 (482 mg, 2.1 mmol) in 2 ml ofanhydrous THF for an additional 5 days (m = 635 mg). Tg = −3◦C and 54◦C.SEC (light scattering, THF): Mw = 170 kg/mol, Mn = 160 kg/mol, pI = 1.06.The 1H- and 13C-NMR spectra of block co-polymer 8 are identical to the spectrapreviously obtained for statistical co-polymer 5.

2.4.3. Poly(benzyl (R,S)-3,3-dimethylmalate-b-hexyl (R,S)-3,3-dimethymalate)(PDMMLABz50-b-He50) (9). Polymer 9 was obtained by anionic polymerizationof lactone 2 (456 mg, 2 mmol) in the presence of tetraethylammonium benzoate(0.02 equivalent, 80 µl) as initiator, in a solution of anhydrous THF (2 ml) at roomtemperature for 3 h. A sample (200 µl) of the mixture was analyzed by IR spec-troscopy to control the total conversion of the lactone. Lactone 1 (361 mg, 1.54mmol) in a solution of anhydrous THF (2 ml) was added to the precedent solu-tion and the polymerization pursued for 4 additional days. A sample (200 µl) ofthe mixture containing block co-polymer 9 was also analyzed by IR spectroscopy.After complete conversion, the polymerization was stopped by neutralization withone drop of acetic acid. The solution was poured dropwise in ethanol to precipitateco-polymer 9 (m = 538 mg). Tg = 0◦C and 12◦C. SEC (light scattering, THF):Mw = 26 kg/mol, Mn = 24 kg/mol, pI = 1.11. The 1H- and 13C-NMR spectra ofblock co-polymer 9 are identical to the spectra previously obtained for the statisticalco-polymer 5.

3. RESULTS AND DISCUSSION

The different co-polymers obtained in this study were synthesized by living anionicring-opening polymerization of two different lactones: (R,S)-4-benzyloxycarbonyl-3,3-dimethyl-2-oxetanone (1) and (R,S)-4-hexyloxycarbonyl-3,3-dimethyl-2-oxetan-one (2). Their structures are shown in Fig. 2.

These α,α,β-trisubsituted β-lactones were prepared in a few steps from com-mercial racemic diethyloxalpropionate according to the procedure described pre-viously [12, 13]. First, the two main homo-polymers 3 (PDMMLABz) and 4(PDMMLAHe) were synthesized from the corresponding lactones 1 and 2, respec-tively. The anionic polymerization of monomer was carried out at 37◦C in bulk inthe presence of tetraethylammonium benzoate as initiator (10−3 mol/mol monomer)

Figure 2. Structure of the two main lactones.

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Scheme 2. Synthesis of homo-polymers 3 and 4 and statistical co-polymers 5 and 6.

Table 1.Characteristics of the synthesized homo-polymers and statistical co-polymers

Polymer Mn,Th (kg/mol) Mn (kg/mol)a Mw (kg/mol)a Mw/Mna Tg (◦C)b

3 234 320 340 1.05 +454 228 177 200 1.10 −95 229 230 246 1.06 −46 230 230 280 1.20 +7

Mn,Th = theoretical Mn.a SEC in THF, light scattering.b Determined by DSC.

(Scheme 2). The conversion ratio of the polymerization, the total after a few days,was confirmed by IR spectroscopy with the disappearance of the band at 1850 cm−1

corresponding to the cyclic carbonyl of lactones.These two homo-polymers 3 and 4 were characterized by 1H- and 13C-NMR,

size-exclusion chromatography (SEC) and differential scanning calorimetry (DSC).The results are summarized in Table 1. In both cases, SEC measurements showedhigh experimental molecular weight and polydispersity indexes close to unity. Theliving anionic process can be explained by the presence of two methyl groups in theα-position of α,α,β-trisubsituted β-lactones. The absence of hydrogen atom in theα-position avoids transfer by H-abstraction and therefore side reactions [1]. On theother hand, DSC data gave two different Tg values. The benzylic homo-polymerPDMMLABz 3 has a Tg at 45◦C. This value can be explained by the stiffnessof aromatic group in the lateral chain of homo-polymer 3. In the case of hexylichomo-polymer PDMMLAHe 4, the low Tg at −9◦C is due to the high flexibility ofthe aliphatic chain of the hexyl ester group.

The 1H-NMR spectrum of benzylic homo-polymer 3 confirmed the presence ofbenzyl group with peaks at 5.00 and 7.10 ppm. For the hexylic homoplymer 4,peaks at 0.80, 1.50 and 4.00 ppm described its structure with the hexyl group. Inboth cases, the peak at around 1.10 ppm represents the two methyl functions on themain chain of polymers.

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Table 2.Characteristics of the synthesized block co-polymers

Polymer Mn,Th (kg/mol) Mn (kg/mol)a Mw (kg/mol)a Mw/Mna Tg (◦C)b

7 226 164 217 1.30 −88 232 160 170 1.06 −3, +549 20 24 26 1.11 0, +12

Mn,Th = theoretical Mn.a SEC in THF, light scattering.b Determined by DSC.

In parallel, two statistical co-polyesters, PDMMLABz10-co-He90 (5) andPDMMLABz30-co-He70 (6), were also synthesized by anionic polymerization ac-cording to the same protocol as described below for homo-polymers. The re-action was realized with well-defined molar proportions of two lactones 1 and 2(Scheme 2). Co-polymers 5 and 6 contain in the lateral chain benzyl/hexyl groupsin the ratios 10:90 and 30:70, respectively. Characterization by 1H-NMR confirmedthat the proportion of two units were the same as in the initial monomers feed. Theinitiator anion forms a benzoate end group which cannot be detected by 1H-NMRbecause of the high molecular weight of the polymer. The results of SEC and DSCmeasurements are also summarized in Table 1.

As homo-polymers 3 and 4, the statistical co-polymers 5 and 6 also had highexperimental molecular weights. The DSC data showed an increase of the Tg valueswith the proportion of benzyl ester group within the polymer. The lower content ofbenzyl group (10%) in co-polymer 5 had a minor stiffness effect and its Tg is −4◦C,whereas for the co-polymer 6 the presence of 30% benzyl groups involved a moreimportant stiffness and, therefore, a higher Tg (7◦C).

The second part of our work was to synthesize new block co-polymers from theseα,α,β-trisubsituted β-lactones. For the preparation of these block co-polymers,the same proportions of lactones used for statistical co-polymers were used. Onthe other hand, the protocol for anionic polymerization was realized in solutionof anhydrous THF at room temperature in the presence of tetraethylammoniumbenzoate as initiator (Scheme 3). To obtain desired diblock co-polymers, the firstlactone (lactone 1, 10 mol%) reacted with the initiator for a few days until the bandat 1850 cm−1 had disappeared, as monitored by IR spectroscopy. After completeconversion, the second lactone (lactone 2, 90% mol) was added to the solution andstirred during 10 days more to give the final block co-polyester 7. According tothe same protocol, two other di-block co-polymers 8 and 9 were synthesized fromdifferent molar proportions of lactone 1/lactone 2, 30:70 and 50:50, respectively.

The three different block co-polyesters were analyzed by NMR spectoscopy,DSC and SEC. The SEC measurements showed experimental molecular weightscorresponding to the expected ones and narrow molar mass distributions (Table 2).These results obtained with this synthetic protocol confirmed the living process ofanionic polymerization and those described previously [1]. In fact, we had realized

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Sche

me

3.S

ynth

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ofbl

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co-p

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7,8

and

9.

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New block and statistical co-polyester by anionic ROP 33

preliminary synthesis and analysis of di-block co-polyester. SEC results had shownan increase of molecular weight between the intermediate block and the final di-block polymer with a very good polydispersity index value.

Finally, 1H-NMR spectra of co-polymers 7 and 8 showed the presence of peakscorresponding to benzyl and hexyl functions with a benzyl/hexyl molar ratio ofapproximately 10:90 and 30:70, respectively. The DSC data were very interesting.For co-polymer 7, only one Tg at −8◦C, which is identical to that of homo-polymer4, was observed. Indeed, polyester 7 contains only about 10% of benzyl estergroup and in this case no benzyl function was detected. On the other hand, thethermograms of co-polymers 8 and 9 gave two Tg values. For the di-block polyester8, Tg at −3◦C and +54◦C corresponded to hexyl block (70%) and benzyl block(30%), respectively. In the case of co-polymer 9, the two Tg values were 0◦Cand +12◦C. The Tg at 12◦C of this block co-polymer 9 may be attributed to itslower molecular weight. In parallel, a physical mixture with a 50:50 molar ratioof homo-polymers 3 and 4 was prepared and analyzed by DSC. The thermogramshowed two Tg values at −15◦C and 43◦C, corresponding to homo-polyesters 3and 4, respectively. For di-block co-polyesters, the presence of two Tg values onthe thermograms allowed to conclude that the two polymers, poly(benzyl (R,S)-3,3-dimethylmalate) and poly(hexyl (R,S)-3,3-dimethylmalate), are incompatible.

In parallel, the sequences of homopolymers 3 and 4, statistical co-polymer 6 anddi-block co-polymer 8 were conveniently characterized by 13C-NMR spectroscopyon the basis of the C O signals of lateral ester groups at 167–168 ppm. This sig-nal is particularly sensitive to structural and configurational changes in their neigh-bourhood. Figure 3 illustrates the difference between two different homo-polymersand statistical and block co-polymers derivated from (R,S)-3,3-dimethyl β-lactones.For each homopolymer, 13C-NMR spectrum displayed one triplet corresponding toisotactic, syndiotactic and heterotactic triads. The two homopolymers are sepa-rated with a low chemical shift difference. The triad of hexylic homopolymer 4(167.6 ppm) is slightly deshielded in comparison with triad of benzylic homopoly-mer 3 (167.4 ppm).

In the case of statistical co-polymer 6, the 13C-NMR spectrum displayed onlyone triplet (167.3–167.7 ppm). Indeed, in 13C-NMR spectroscopy it is verydifficult to differentiate between the two units which are distributed in the polymericmaterial in a random way. On the other hand, for di-block co-polymer 8, the 13C-NMR spectrum displayed two well-defined triplets corresponding to the two homo-polymers 3 and 4 with exactly the same chemical shifts. However, at this stage wecannot accurately attribute the isotactic, syndiotactic or heterotactic sequence peaks.To settle this question, our forthcoming aim is to synthesize the optically activeα,α,β-trisubstituted β-lactone and prepare the corresponding stereo-polymer. The13C-NMR analysis of this new stereo-polyester will give us exactly the chemicalshift of isotactic triad.

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Figure 3. 75 MHz 13C-NMR spectra of C O signals of lateral ester groups of different homopoly-esters and co-polyesters: (a) hexylic homopolymer 4; (b) benzylic homopolymer 3; (c) statisticalco-polymer 6; (d) di-block co-polymer 8.

To conclude from these 13C-NMR results, these different data allowed to confirmthe structure of statistical and di-block co-polymers obtained by anionic ring-opening polymerization of α,α,β-trisubstituted β-lactones.

4. CONCLUSIONS

The synthesis of α,α,β-trisubsituted β-lactones has opened the route to new deriv-atives in the malic acid polymer family. More important, the living process of theanionic polymerization allows the tailoring of materials such as multiblock and sta-tistical polyesters with well-controlled molecular weights and narrow polydisper-sity. After deprotection of the benzyl group, these new amphiphilic polymers couldform self-assemblies, nanosized micelle-like aggregates of various morphologies in

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aqueous solution. Poly(hexyl (R,S)-3,3-dimethylmalate) as hydrophobic segmentwill form the core of the micelle to incorporate lipophilic drugs, while the depro-tected poly(benzyl (R,S)-3,3-dimethylmalate) as hydrophilic segment will form thecorona which protect and increase the in vivo life-time of the drug carrier. Statisti-cal co-polymers with high percentage of hydrophobic units could be a good candi-date for the preparation of nanoparticles. Therefore, our future investigations willinclude a careful study on micelles and nanopoarticles from these new polymericmaterials.

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