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Journal of Non-Crystalline Solids 353 (2007) 3969–3975

Self-assembling nanoscaled drug delivery systems composedof amphiphilic poly-N-vinylpyrrolidones

Andrey N. Kuskov a, Michail I. Shtilman a, Anastasia V. Goryachaya a,Ravshan I. Tashmuhamedov a, Alexander A. Yaroslavov b, Vladimir P. Torchilin c,

Aristidis M. Tsatsakis d, Apostolos K. Rizos e,*

a Laboratory of Polymeric Biomaterials, Department of Polymers, D.I. Mendeleyev University of Chemical Technology,

9 Miusskaya Square, Moscow 125047, Russiab Department of Polymers, M.V. Lomonosov Moscow State University, Moscow 119899, Russia

c Department of Pharmaceutical Sciences, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USAd Laboratory of Toxicology, Department of Medicine, University of Crete, 71409 Heraklion, Crete, Greece

e University of Crete, Department of Chemistry and Foundation for Research and Technology-Hellas (FORTH), P.O. Box 2208,

Heraklion 71003, Crete, Greece

Abstract

Certain amphiphilic water-soluble polymers including amphiphilic derivatives of poly-N-vinylpyrrolidone (PVP) were found to be anefficient base for drug delivery systems. In this study, one-end functionalized PVP of different molecular weight was hydrophobicallymodified with long-chain fatty n-alkyl or di(n-alkyl) amines through a coupling reaction using N,N 0-dicyclohexyl carbodiimide(DCC). Amphiphilic PVPs have different physico-chemical properties according to the structure of hydrophilic/hydrophobic fragments.Since synthesized polymers have the amphiphilic characteristics in aqueous solution, polymeric nanoparticles of amphiphilic PVPs wereprepared in aqueous media. Using the analysis of fluorescence excitation spectra and dynamic light-scattering it was shown that the abil-ity to form nanoparticles of 30–700 nm size for polymers with primary aliphatic end groups increases with decreasing the polymer molec-ular weight and with increasing n-alkyl group length. Polymers containing di(n-alkyl) end groups form larger aggregates at higherconcentrations. Transmission electron micrograph readings showed the spherical morphologies of the polymeric nanoparticles. In allcases the size of aggregates formed in physiological solution is lower than those obtained in distilled water.� 2007 Published by Elsevier B.V.

PACS: 87.15.He; 87.64.Cc

Keywords: Biomaterails; Biopolymers; Nanoparticles, colloids and quantum structures; Nanoparticles; Polymers and organics

1. Introduction

Nano-dimensional carriers for biologically active andpharmaceutical substances (liposomes, nano-aggregates)currently represent a fast growing area of biomedical

0022-3093/$ - see front matter � 2007 Published by Elsevier B.V.

doi:10.1016/j.jnoncrysol.2007.02.061

* Corresponding author. Tel.: +30 28 10545048; fax: +30 28 10545001.E-mail address: rizos@chemistry.uoc.gr (A.K. Rizos).

research. [1,2]. A significant role in increasing nano-carrierefficiency is displayed by water-soluble amphiphilic poly-mers, which are used for the creation of nano-aggregatesand surface modification of liposomes. One of the mostpopular and successful methods to obtain long-circulatingbiologically stable liposomes is their coating with poly(eth-ylene glycol), or PEG [3–7]. Despite the well-developedchemistry of PEG coupling to pharmaceuticals, the searchfor alternative polymers for drug delivery systems is quiteactive to get even better control over the properties of

3970 A.N. Kuskov et al. / Journal of Non-Crystalline Solids 353 (2007) 3969–3975

modified drugs and drug carriers. The suggested theoreticalmodel [8,9] and available experimental data permitted us toformulate some general requirements for polymers, suit-able for creation of drug carriers. These polymers shouldbe soluble, hydrophilic and have a highly flexible mainchain. Polymer biocompatibility has to be added to theseproperties, because polymeric nano-particles and poly-mer-liposomes are intended for medical use.

Synthetic polymers of vinyl series, such as poly-N-vinyl-pyrrolidone (PVP) may serve as the most evident examplesof other potential polymers for creation of new drug deliverysystems [10,11]. Similar to PEG, PVP has a long history ofpharmaceutical applications and demonstrates a highdegree of biocompatibility [12,13]. Amphiphilic derivativesof PVP have been described containing phospholipid resi-dues and long-chain acyls as hydrophobic groups and serv-ing as efficient steric protectors for liposomes [10,11].Modification of liposomes by amphiphilic PVP leads to anincrease of their circulation time in the blood of experimen-tal mice and prevents fast capture of liposomes by the reti-culo-endothelial system (RES) [10]. Interaction ofamphiphilic PVP aggregates with blood components andseveral proteins was studied in [14,15]. It should be alsomentioned that amphiphilic PVPs possess greater biocom-patibility that PEG end-capped amphiphiles. Moreover,the semi-telechelic PVP amphiphiles with only one endhydrophobic group are expected to be very perspective basisfor drug carriers and have very few analogues (only one end-capped PEG polymers or mono-alkyl ethers of PEG).

However, the choice of both the type of hydrophobicgroup and the Mw of the polymer were quite arbitraryand based mostly on general considerations. In this studywe made an attempt to investigate peculiarities of theamphiphilic N-vinylpyrrolidone polymer behavior in aque-ous environment medium, depending on their molecularweight and structure of the end hydrophobic groups.

2. Experimental

2.1. Materials

N-Vinylpyrrolidone (VP), azobisisobutyronitrile(AIBN), mercaptoacetic acid (MAA), N,N’-dicyclohexyl-carbodiimide (CCD), octylamine, dodecylamine, hexade-cylamine, octadecylamine, dihexylamine, dioctylamine,dioctadecylamine, pyrene and substrate for electron micros-copy – 0.2% polyvinylformal, were obtained from Sigma–Aldrich (St. Louis, MO). All solvents and components ofbuffer solutions were analytical grade preparations.

2.2. Polymer synthesis

Similar to our previous experiments [11,16], amphiphilicpoly-N-vinylpyrrolidones were synthesized in two steps.Firstly, PVP with the terminal carboxylic group was pre-pared, and then hydrophobic groups were attached to thereactive terminus of the PVP molecule.

2.3. PVP with single terminus carboxylic group

The polymerization of VP was carried out under dryargon atmosphere in dioxane solution, in the presence ofinitiator, azobisisobutyronitrile, for 1 h at 70 �C. To pre-pare a carboxy-terminated PVP (carboxy-PVP) with differ-ent Mw, variable quantities of mercaptoacetic acid wereused for chain transfer. The polymers formed were precip-itated into diethyl ether, purified by repeated washing withether and dried in vacuum until constant weight. The yieldof different batches was between 75% and 90%. MW valueswere determined by either potentiometric titration or steamosmometry using a «Knauer» osmometer (Germany). Thepolydispersity of the semitelechelic polymers was deter-mined by high-performance liquid chromatography(GFHPLC; TSK Gel G4000PwxL; Toso Co., Ltd., Tokyo,Japan). For all polymer samples with different molecularweight Mw/Mn was in the range of 1.08 –1.14.

2.4. Amphiphilic PVP with terminal aliphatic group of

different structure

In order to attach n-alkyl or di(n-alkyl) groups to car-boxy-PVP with the formation of amphiphilic PVP, the solu-tion of carboxy-PVP in isopropanol was supplemented withan excess of N,N-dicyclohexylcarbodiimide in an equal vol-ume of the same solvent. The mixture was stirred at 0 �C for1 h, and then excess of a corresponding primary or second-ary amine dissolved in isopropanol was added. The mixturewas incubated for 2 h at 60 �C. The precipitated sediment(N,N’-dicyclohexylurea (CU), amine and CCD residues)was filtered and the resulting polymer was precipitated intodiethyl ether, washed with ether and dried in vacuum.

2.5. Preparation of amphiphilic PVP polymeric

nanoparticles and CAC estimation

In order to prepare polymeric aggregates, appropriateamounts of a corresponding PVP derivative was sonicatedin distilled water or in physiological solution (0.15 M NaCl,pH � 7.4) at 10 W for 15 min using a Sonic Desmembrator60 (Fisher Scientific, USA); the cycle was repeated threetimes with 1 min intervals [17]. To estimate the criticalaggregation concentration (CAC) values for different PVPderivatives, the method used was based on the solubilizationof water-insoluble fluorescent dye, pyrene, in nano-aggre-gates. For this purpose, aliquots of 10 ll of pyrene solutionin acetone (10 mg/ml) per test tube were dried under vac-uum. The tubes were supplemented with 2 ml of serial dilu-tions (10�4–10�10 M) of various PVP samples, and shakenovernight at room temperature. The samples were filteredthrough 0.2 lm filter to remove the non-solubilized pyrene,and the fluorescence intensity of solubilized pyrene wasmeasured using a Hitachi 650–10 S Spectrophotometer(Hitachi Instruments Inc., Japan). The emission wavelengthwas 390 nm for excitation spectra. The excitation and emis-sion bandwidths were 1.5 and 1.5 nm, respectively.

Table 1Carboxyl-terminated semitelehelic N-vinylpyrrolidone polymers

Polymer structure Polymer code M�n M��n �N

PVP–S–CH2–COOH 1PVP 1500 1450 12–142PVP 2000 1980 17–193PVP 2600 2640 22–244PVP 3300 3270 29–30

CH2 CH S CH2 COOH

CN

O

H + HxNRy

CH2 CH S CH2 C

CN

O

HO

NHx-1Ryn n

+ CCD

- CU

CH3(CH2)7NH2

CH3(CH2)15NH2

CH3(CH2)11NH2

[CH3(CH2)7]2NH

[CH3(CH2)5]2NH

CH3(CH2)17NH2 [CH3(CH2)17]2NHWhere HxNRy =

A.N. Kuskov et al. / Journal of Non-Crystalline Solids 353 (2007) 3969–3975 3971

2.6. Photon correlation spectroscopy (PCS)

The average size of polymeric nano-aggregates obtainedin water and in physiological solution and their size distri-bution were determined by dynamic light-scattering [18–20]. The analysis was performed at a wavelength of532 nm, at several scattering angles and at a temperatureof 25 �C.

2.7. Transmission electron microscope (TEM)

The shape of amphiphilic PVP aggregates was deter-mined by transmission electron microscopy (TEM) usingapparatus JEOL JEM-1000 (Germany) at a voltage of20 kV. For sample preparation, a drop of polymer solutionwas placed on substrate from 0.2% solution of polyvinyl-formal applied on copper mesh.

2.8. Synthesis of functional PVP derivatives and modification

of terminal reactive groups

The VP amphiphilic polymers used in this study for cre-ation of new drug nano-carriers were obtained by a two-stage process. Firstly, a semi-telehelic PVP with one endfunctional group was synthesized by free-radical polymeri-zation of VP in dioxane in the presence of initiator – azob-isisobutyronitrile, and mercaptoacetic acid, as chain-transferring agent. In the synthesis of functional VPpolymers, the ability of mercaptans to transfer the chainduring the radical attack provides an opportunity to intro-duce a functional group onto a single terminus of apolymeric chain. In this particular case, the mercaptan per-forms the chain transfer due to the formation of free radi-cals produced by elimination of a hydrogen atom from aSH-group. The above polymerization technique canproduce polymers with a single functional end group corre-sponding to the structure of the mercaptan radical [21].

5PVP 4000 4050 35–37

M�n – average molecular weight, determined by potentiometric titration.M��n – average molecular weight, determined by steam osmometry.N – number-average degree of polymerization.

Table 2N–vinylpyrrolidone amphiphilic polymers

Polymer structure Polymer code Mn

PVP–S–CH2–CONHC18H37 PVP-OD 1500 1500PVP-OD 2000 2000PVP-OD 2600 2600PVP-OD 3300 3300PVP-OD 4000 4000PVP-OD 5500 5500

PVP–S–CH2–CONHC16H33 PVP-HD 4000 4000PVP–S–CH2–CONHC12H25 PVP-DD 4000 4000PVP–S–CH2–CONHC8H17 PVP-Oct 4000 4000PVP–S–CH2–CON(C18H37)2 PVP-OD2 4000 4000PVP–S–CH2–CON(C8H17)2 PVP-Oct2 4000 4000PVP–S–CH2–CON(C6H13)2 PVP-Hex2 4000 4000

Mn – average molecular weight of hydrophilic polymeric fragment.

n CH2 CH

NC O

CH2 CH S CH2 COOH

CN

O

HnHS-CH2-COOH

AIBN

The Mw of the polymers depended on the concentration ofthe corresponding chain-transferring agent and decreasedwith an increase in the amount of mercaptan added tothe system. As a result, we prepared a whole set of car-boxy-PVP with Mw values varying from 1500 to5500 Da. The concurrence of Mw values, obtained bypotentiometric titration, or alternatively, by steam osmom-etry confirms that the synthesized polymer contains onefunctional end group (Table 1).

In the second stage, the synthesis of amphiphilic PVPsinvolves modification of the end functional group of semi-telehelic VP polymers that was carried out by treating car-boxy-PVP with corresponding long-chain primary or

secondary amines (octylamine, dodecylamine, hexadecyl-amine, octadecylamine, dihexylamine, dioctylamine, dioc-tadecylamine) after preliminary activation of terminalcarboxyl groups with N,N 0-dicyclohexylcarbodiimide.

The degree of end functional group substitution wasdetermined by potentiometric titration under the same con-ditions as for semitelehelic polymer MW estimation. In allsamples carboxyl groups were not detected, which signifiestheir total substitution. The method that was developedallowed production of two series of polymeric samples(Table 2). One series included PVPs with different MWs(1500, 2000, 2600, 3300, 4000, 5500 Da) and one endhydrophobic n-octadecyl group and another series withPVP MW = 4000 Da and different structures of long-chainend hydrophobic groups. No difference in solubility ofamphiphilic PVP derivatives prepared following differentprotocols was found and similar to starting functional

3972 A.N. Kuskov et al. / Journal of Non-Crystalline Solids 353 (2007) 3969–3975

PVP samples, all amphiphilic PVP samples were water-sol-uble, and, as it was ascertained in previous spin probeexperiments [17].

3. Results

Since amphiphilic PVP derivatives contain significantlylarger hydrophilic fragments than hydrophobic ones, itwas assumed that hydrophobic fragments, minimizing theircontact with water, may ‘fold’ themselves into the globuleshape surrounded by the hydrophilic block. Due to thehydrophobic character of the end long-chain fragments,the n-alkyl or di(n-alkyl) domain should be orientedtowards the core of the polymeric nanoparticles whilehydrophilic PVP is oriented in an outward direction asan outer shell. This leads to the formation of the core-shelltype polymeric nanoparticle structure. Our task was tostudy the influence of synthesized polymer structure ontheir ability to form aggregates.

Generally, amphiphilic polymers are able to associate insolutions at concentration higher than some threshold con-centration limit (the so called critical micelle concentration).

0

200

400

600

800

1000

1200

1 10 100

1 10 100

Polymer concentration, M x 106

Flu

ore

scen

ce in

ten

sity

, I 38

5

1

2

3

4

5

6

0

200

400

600

800

1000

1200

Polymer concentration, M x 106

Flu

ore

scen

ce in

ten

sity

, I 3

85 1

2

3

4

5

6

a

b

Fig. 1. Influence of N-vinylpyrrolidone amphiphilic polymers concentra-tion on solubilized pyrene fluorescence intensity (k = 385 nm). (a) Poly-mers of different MW with end n-octadecyl group. (1 PVP-OD 1500, 2PVP-OD 2000, 3 PVP-OD 2600, 4 PVP-OD 3300, 5 PVP-OD 4000, 6 PVP-OD 5500) and (b) polymers with Mn = 4000 and with end n-alkyl and di(n-alkyl) groups of different structure (1 PVP-HD 4000, 2 PVP-DD 4000, 3PVP-Oct 4000, 4 PVP-Hex2 4000, 5 PVP-Oct2 4000, 6 PVP-OD2 4000).

In this case, structures of different morphology can beobserved in solution [22]. For all samples of synthesizedPVP amphiphilic polymers the aggregates obtained are ofcomplex structure and their size exceed the size of simplemicelles corresponding to polymers of given MWs. That iswhy in this study we use the term ‘critical aggregation con-centration’ (CAC) to describe such a concentration limit.

A method of measuring a fluorescent probe (poorly-sol-uble in water pyrene) was adapted for CAC estimation(Fig. 1). After introduction of polymeric aggregates intothe system, an amount of fluorescent probe passed intoaqueous medium can be measured after separation of theundissolved part of the probe [17]. Fluorescence emissionspectra showed the increased intensity along with the con-centration of amphiphilic PVPs. From the plot of I337/I334

versus logC of the excitation spectra, the CAC values weretaken from the intersection of the tangent to the curve atthe inflection with the horizontal tangent through thepoints at low concentrations. The estimated CAC valuesof various amphiphilic PVP samples are shown in Table 3.

As one can see (Fig. 1 and Table 3) CAC values are in themicro-molar range. This means that the obtained amphi-philic polymers form aggregates at relatively low concentra-

Table 3Critical aggregation concentration and average aggregate size values forN-vinylpyrrolidone amphiphilic polymers

Polymercode

Critical aggregationconcentration, lM

Particle average diameter, nm

Cp � CAC Cp = 10 mg/ml

Watera Waterb Physiologicalsolutionb

PVP-OD1500

4.0 28 520 280

PVP-OD2000

5.3 26 560 294

PVP-OD2600

5.8 30 560 298

PVP-OD3300

6.5 30 580 306

PVP-OD4000

7.3 32 580 300

PVP-OD5500

9.4 35 620 340

PVP-HD4000

15.4 32 640 360

PVP-DD4000

18.5 30 680 370

PVP-Oct4000

22.1 32 690 380

PVP-Hex2

4000

12.1 36 710 400

PVP-Oct2

4000

14.0 40 800 440

PVP-OD2

4000

25.2 90 1800 1000

Cp – amphiphilic polymer concentration.a The average error is ±3 nm.b The average error is ±12 nm.

A.N. Kuskov et al. / Journal of Non-Crystalline Solids 353 (2007) 3969–3975 3973

tions. All tested PVP derivatives, formed micelle-like aggre-gates in water solutions with the average size between 30 and90 nm and narrow size distribution as follows from the par-ticle size measurement. The ability of water-soluble amphi-philic polymers for self-organization is deeply connectedwith interaction of their hydrophobic fragments. Becausesuch interaction depends not only on the relative share ofhydrophobic fragments in the macromolecule but also ontheir accessibility, it was found interesting to ascertain,how these factors affected the synthesized series of polymers.The CAC values for various particles depend on both thetype of the hydrophobic residue and the molecular size ofthe hydprophilic PVP block. In general, the increase in thelength of the PVP block increases the CAC value. The big-gest difference in CAC values was found for amphiphilicPVP samples with intermediate size of PVP blocks between1500 and 6000 Da. Table 3 shows data on the CAC measure-ment by the pyrene method for different PVP amphiphiles. Itis clearly seen (Table 3) that the CAC value for PVP-OD4000 is lower than the CAC value for PVP-Oct 4000 witha similar MW of both polymers, i.e. PVP-OD 4000 producesmore stable aggregates. One may interpret these results interms of the relative intensity of hydrophobic interactionsbetween n-alkyls of different lengths forming the micellecore and keeping the micelle together and the energy ofthe free motion of hydrophilic PVP chains forming themicelle shell in water and destabilizing the micelle. In theshort n-alkyl chains, as the carbon number decreases, thehydrophilicity of the whole polymer increases. On the con-trary, when PVP chains are short, all the n-alkyls formaggregates at low CAC values. When PVP chains are long,the energy of the free motion of the PVP block in water ishigher than for short blocks, CAC value increases. The dif-ference between the lengths of the hydrophobic residues alsoplays a certain role. This role is, however, most pronouncedat intermediate lengths of PVP blocks, when stearyl pro-vides stronger hydrophobic interactions in the micelle corethen all other n-alkyls. The obtained data for amphiphilicPVPs behavior in aqueous media is in full agreement withthe data obtained for PEG end-capped amphiphilic poly-mers [23,24]. But in comparison with the PEG polymersend-capped on only one end, amphiphilic PVPs possess10–25 times lower CAC values and therefore form stableaggregates much easier.

The findings were similar for polymer coated liposomes,where it was shown that the extent of protective activity fordifferent polymers towards liposomes in vivo depends onthe length of hydrophobic anchor and polymer molecularweight [10,11]. When modified with the same long hydro-phobic anchor, various soluble and flexible polymers (suchas PEG, PVP, polyacryl amide) of similar MW and concen-trations, all provide efficient steric protection for lipo-somes. The protective activity of polymers with shorterhydrophobic moiety or with higher MW was, however,much lower. From the thermodynamic point of view, itwas assumed that the relatively short hydrophobic groupis unable to keep a polymer molecule on the liposome sur-

face. The energy of the polymeric chain motion in the solu-tion might be higher than the energy of a shorthydrophobic group interaction with phospholipid sur-roundings within the liposomal membrane. The longeranchor provides much firmer polymer binding with lipo-some (higher energy of interaction with hydrophobic mem-brane core due to the larger number of membrane-embedded CH2– groups), and thus much better liposomesteric protection. On the other hand, even the length of a‘long’ anchor is insufficient to firmly fix a much larger poly-mer on the liposome surface because of the higher energyof polymer chain motion in water compared with that forshorter polymers [25].

As for the polymers with di(n-alkyl) groups, the largerbranched groups increase the hydrophobicity of the wholepolymer molecule. It can be expected that the CAC valuesincrease by the hydrophobic/hydrophilic balance to formcore-shell structure polymeric nanoparticles. Due to theirgreater hydrophobic share, especially for the polymer withdistearyl anchor, this polymer showed poor solubility inwater and high value of CAC (Table 3 and Fig. 1(b)). Itcan be also explained by the steric hindrances for fragmentsinteraction. For the other two polymer samples withshorter di(n-hexyl) and di(n-octyl) end groups the CAC val-ues are lower, even when they possess less hydrophobicfragment share in the polymer.

4. Discussion

As the polymeric carriers on the basis of PVP amphiphilicderivatives are planed to be used for drug solubilization andinjection administration it was found reasonable to studythe polymeric particles sizes and shape in the wide rangeof polymers concentrations. The size of polymeric nano-particles obtained in aqueous environment was studiedusing dynamic light scattering. At concentrations near theCAC values all the PVP derivatives, form micelle-like aggre-gates in water solutions with the average size between 30 and90 nm and narrow size distribution (Table 3). The differencebetween the average sizes of the particles for polymers withdifferent structure corresponds well to the difference in themolecular size of aggregate-forming individual amphiphilicPVP unimers. In polymers with concentrations 100–1000fold higher than CAC an increase in polymeric aggregatessize is observed. The presented particle size distribution(see Fig. 2) for amphiphilic PVP with MW 3300 Da and n-octadecyl hydrophobic group at concentration 10 mg/mlshows that the produced dispersion contains particles from60 nm up to 450 nm size. Thus, at the concentrations higherthan CAC values the small micelle-like particles form biggeraggregates of complex structure. The size of such aggregatescan be controlled and decreased during the dispersion for-mation by mechanical or ultrasonic processing, but in anycase the sizes of the particles will be greater than at CACand apparently are the most thermodynamically advanta-geous for each amphiphilic PVP derivative and for eachpolymer concentration.

0

0.02

0.04

0.06

0.08

0.1

0.12

0 40 80 120 160 200 240 280 320 360 400 440 480 520Particle diameter. nm

Sh

are

Fig. 2. Size distribution of PVP-OD 3300 particles in physiologicalsolution (C = 10 g/ml).

3974 A.N. Kuskov et al. / Journal of Non-Crystalline Solids 353 (2007) 3969–3975

In Fig. 3 the influence of polymer structure on averageaggregate size depending on its concentration in solutionis presented. In all cases an increase of aggregate size is

300

350

400

450

500

550

600

650

0 1 2 3 4 5C, mg/ml

Ag

gre

gat

e av

erag

e si

ze, n

m

123456

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 1 2 3 4 5

C, mg/ml

Ag

gre

gat

e av

erag

e si

ze, n

m

1234567

a

b

Fig. 3. Influence of N-vinylpyrrolidone amphiphilic polymer concentra-tion on aggregate size. (a) Polymers of different MW with end n-octadecylgroup. (1 PVP-OD 1500, 2 PVP-OD 2000, 3 PVP-OD 2600, 4 PVP-OD3300, 5 PVP-OD 4000, 6 PVP-OD 5500) and (b) polymers with Mn = 4000and with end n-alkyl and di(n-alkyl) groups of different structure (1 PVP-HD 4000, 2 PVP-DD 4000, 3 PVP-Oct 4000, 4 PVP-Hex2 4000, 5 PVP-Oct2 4000, 6 PVP-OD2 4000, 7 PVP-OD 4000).

observed with increasing polymer concentration. At thesame time, for all concentrations, a tendency for averageaggregate size increase with increasing polymer MW takesplace (Fig. 3(a)). Also the average diameter of aggregatesrises by increasing the end n-alkyl group length and withtransition to polymers with end di(n-alkyl) group(Fig. 3(b)) and it is strongly manifested for amphiphilicPVPs with di(n-octadecyl) group.

As shown in Table 3, for primary hydrophobic groupsthe particle sizes decreased when increasing the carbonnumber and decreasing molecular weight and reached aminimum value for octadecyl group and molecular weight1500 Da. For the larger branched secondary hydrophobicchains, due to the increased ratio of hydrophobic domains,the amphiphilic PVPs formed larger and loosely packedpolymeric nanoparticles. Therefore by changing the ratioof hydrophilicity versus hydrophobicity, we can producepolymeric nanoparticles with different particle sizes andself-association behavior.

Average diameters of aggregates in different media areshown in Table 3. In all cases, the average size of obtainedparticles was smaller for physiological solution that forwater. Such a decrease in particle size can be explainedby the strengthening of hydrophobic interactions in solu-tions with higher ionic force, which leads to formation ofa more compact structure. The morphology of amphiphilicpolymer aggregates is quite diverse. In general, it is sup-posed that amphiphilic polymers produce particles closeto spherical form. However, in a number of reports non-spherical micelle forms [26] have been shown. Usually, suchmicellar forms are obtained from polymers with an asym-metric structure. In our study, the shape of nano-aggre-gates was investigated by using transmission electronmicroscopy. Experimental data, obtained by TEM provideevidence of spherical particle formation with a wide rangeof size (Fig. 4).

Fig. 4. TEM picture of the polymeric nanoparticles (PVP-OD 3300,C = 5 mg/ml).

A.N. Kuskov et al. / Journal of Non-Crystalline Solids 353 (2007) 3969–3975 3975

5. Conclusions

Carboxyl-containing PVP derivatives can be successfullyprepared by free-radical polymerization of VP in the pres-ence of mercaptan that is able to perform the chain transferdue to the formation of free radical produced by elimina-tion of a hydrogen atom from a SH-group. The abovepolymerization technique can produce polymers with a sin-gle functional end group and different molecular weightscorresponding to the amount of mercaptan introduced.The hydrophobic end groups of different structures canbe introduced onto these polymers by direct reaction ofcarboxy-PVP using a corresponding amine in the presenceof N,N’-dicyclohexylcarbodiimide. Terminal hydrophobicgroups of amphiphilic PVP derivatives demonstrate a goodaccessibility for the interaction with other hydrophobicligands. As a result, in aqueous solutions, amphiphilicPVPs can spontaneously form spherical nano-aggregatesof different size and at certain concentration (CAC) values,determined by the structure of the polymer hydrophilic andhydrophobic fragments. The experimental results are con-sistent with an initial association into well defined smallspherical aggregates. The prepared nanoscaled polymericaggregates appear to be perspective carriers for drugdelivery.

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