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Effect of molecular architecture on physicalproperties of tree-shaped and star-shaped poly(methylmethacrylate)-based copolymersLorella Izzo a c & Giuliana Gorrasi b ca Department of Chemistry and Biology , University of Salerno , via Giovanni Paolo II, 132I-84084 , Fisciano , (SA) , Italyb Department of Industrial Engineering , University of Salerno , via Giovanni Paolo II, 132I-84084 , Fisciano , (SA) , Italyc NANO_MATES, Research Centre for NANOMAterials and nanoTEchnology University ofSalerno- via Ponte don Melillo , I-84084 , Fisciano , (SA) , ItalyAccepted author version posted online: 25 Sep 2013.

To cite this article: Journal of Macromolecular Science, Part B (2013): Effect of molecular architecture on physical propertiesof tree-shaped and star-shaped poly(methyl methacrylate)-based copolymers, Journal of Macromolecular Science, Part B:Physics, DOI: 10.1080/00222348.2013.845058

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Received 3 February 2013

Accepted 3 July 2013

Effect of molecular architecture on physical properties of tree-shaped and star-shaped

poly(methyl methacrylate)-based copolymers

Lorella Izzo1,3

and Giuliana Gorrasi2,3*

1 Department of Chemistry and Biology - University of Salerno - via Giovanni Paolo II, 132 I-

84084, Fisciano (SA)-Italy

2 Department of Industrial Engineering - University of Salerno - via Giovanni Paolo II, 132 I-

84084, Fisciano (SA)-Italy

3 NANO_MATES, Research Centre for NANOMAterials and nanoTEchnology University of

Salerno- via Ponte don Melillo, I-84084, Fisciano (SA)-Italy

*e-mail: ggorrasi@unisa.it

Running Title: PEG-PMMA architectures: effect on physical properties

Abstract

The synthesis of star-like A(B)n copolymers based on the hydrophilic poly(ethylene glycol)

monomethyl ether (m-PEG, block A) and the hydrophobic poly(methyl methacrylate) (PMMA,

blocks B) is reported. We obtained copolymers made of one m-PEG chain and 2 or 4 PMMA

blocks using a combined “arm first”- “core first” approach. Such structures were called tree-

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shaped copolymers where the m-PEG was considered as the trunk and PMMA arms as the

branches. Star-like copolymers (B)nA-A(B)n built by two tree-shaped fragments with a

poly(propylene oxide) (PPO) as the central junction, were also synthesized according to a

previously reported procedure. The latter were called star-shaped structures and the synthesis

was performed to obtain architectures different from the tree-shaped one but characterized by a

similar length of the PMMA arms. Microstructural analysis was carried out through 1H-NMR

and GPC, and the thermal and transport properties (sorption and diffusion) to liquid water were

investigated and correlated to the molecular architecture of the two classes of copolymers.

Keywords: star-polymers; radical polymerization; poly(methyl methacrylate); PEG.

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Introduction

In recent years, there have occurred major developments in the science and technology of

polymeric materials with controlled architectures. In this direction, different types of branched

polymers have been designed.[1-3]

Depending on the functionality, the number and relative

arrangement of the branching points within the macromolecule, polymers having a branched

architecture can be further classified as star polymers, brushes/grafted polymers, randomly

branched polymers, hyperbranched polymers, dendrimers and gels.[4,5]

An ideal star polymer contains n-functional branching points at the central core and n emanating

arms. Molecular brushes and/or densely grafted copolymers contain many functional branching

points distributed along a linear back-bone and, consequently, hundreds of side chains.[6,7]

The search for synthetic routes that lead to well-defined, multifunctional polymeric architectures

aims to attain control of the structure-property relationships so that the materials can be tailored

for specific applications.[8-12]

In this framework, star-like polymers are particularly interesting

because they can potentially satisfy the increasing requests for such tailored materials with the

structure, the chemical composition and the spatial distribution of arms easily controlled. In fact,

a rational selection of functional initiators, monomers and/or cross-linkers for the polymerization

and copolymerization allows incorporation of a variety of functionalities into the structure and

the preparation of materials with predetermined properties such as degradability,

biocompatibility and environmental sensitivity. Star-polymers with a cross-linked core are

formed either when the monomer is polymerized prior to the addition of a cross-linker (“arm

first” approach), or if polymerization occurs on the functionality of the cross-linker (“core first”

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approach). Each approach results in the formation of a star, but the former is more suitable for

obtaining polymers with different specific arms.

In this paper we report the synthesis and characterization of star-like copolymers based on

poly(ethylene glycol) monomethyl ether (m-PEG) and poly (methyl methacrylate) (PMMA), the

former representing a hydrophilic component of the macromolecule, the latter a hydrophobic one.

In particular, we obtained copolymers made of one m-PEG chain and 2 or 4 PMMA blocks using

a combined “arm first”- “core first” approach. Considering the overall shape of the

macromolecules obtained, we can refer to them as “tree shaped” copolymers[13]

where m-PEG is

the trunk and the PMMA arms are the branches. The distribution, the number of arms and the

relative amount of the two components can influence several physical properties both in bulk and

in solution, so thin films were obtained by casting from chloroform and the structural and

thermal properties and the transport properties (like sorption and diffusion) of liquid water were

analysed and correlated to the architecture. For comparison purposes a linear di-block copolymer

m-PEG-b-PMMA was also synthesized and characterized. Recently, we reported the synthesis of

star-like copolymers made of 2 and 4 PMMA blocks radiating from each terminal chain of a

linear tri-block poly(ethylene oxide)-poly(propylene oxide) (PEObPPObPEO).[14]

These

copolymers can be thought of as been built by two tree-shaped fragments tethered by a PPO

junction. We carried out the synthesis and characterization of such star-like structures, prepared

according to the reported procedure, with the length of the PMMA arms comparable to the ones

of the tree-shaped structures in order to have a more complete and insightful comparison

between their structure and physical properties relationship.

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Scheme 1.

Experimental

Materials

Poly(ethylene glycol) monomethyl ether (m-PEG) ( nM   = 5000 Da, w nM / M = 1.04),

polyethyleneoxide-polypropyleneoxide triblock (PEObPPObPEO) ( nM = 14600 Da, w nM / M =

1.13, PEO = 84.2 wt%), benzaldehyde dimethyl acetal, 2,2-bis(hydroxymethyl)propionic acid, p-

toluenesulfonic acid monohydrate (TsOH), acetone, N,N’-dicyclohexylcarbodiimide (DCC), 4-

(dimethylamino)pyridine (DMAP), methanol, Pd/C 10%, 2-bromoisobutyryl bromide (BMPB),

triethylamine (TEA), diethyl ether, ethanol, CuBr, 2,2’-bipyridine (bpy), chloroform and Al2O3

were purchased from Sigma-Aldrich (Germany) and used without further purification.

All manipulations involving air-sensitive compounds were carried out under nitrogen atmosphere

using Schlenk or drybox techniques. Toluene was dried over sodium and distilled before use.

CH2Cl2 and methylmethacrylate (MMA) were dried over CaH2 and then distilled, the latter under

a reduced pressure of nitrogen.

Synthesis

Br-PEObPPObPEO-Br, Br2-PEObPPObPEO-Br2, Br4-PEObPPObPEO-Br4, m-PEG-Br, m-PEG-

Br2 and m-PEG-Br4 macroinitiators were synthesized as we have reported in the literature.[14]

(PMMA)- PEObPPObPEO-(PMMA), (PMMA)2-PEObPPObPEO-(PMMA)2 and (PMMA)4-

PEObPPObPEO-(PMMA)4 were prepared by Atom Transfer Radical Polymerization (ATRP)

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also as previously reported in the literature.[14]

m-PEG-(PMMA), m-PEG-(PMMA)2, m-PEG-

(PMMA)4 were synthesized under nitrogen atmosphere, in dry conditions. A typical procedure is

described below for the copolymer of run 3, Table 1. A magnetically stirred reactor vessel (50

mL) was charged sequentially with a solution of m- PEG-Br2 (0.1 g; 1 equiv.) in 15 mL of dry

toluene, 0.006 g of CuBr (0.04 mmol; 2 equiv.), 2·10-3

g of bpy (0.06 mmol, 3 equiv) and 5 mL

of MMA. The mixture was thermostated at 90°C and magnetically stirred. The reaction was

stopped after 15 hours with n-hexane. The copolymer was recovered, dissolved in the minimum

amount of chloroform and passed over a column with activated Al2O3 to remove the catalyst.

The solution was then dried in vacuo and the polymer was washed with cold methanol before

drying. The polymer was characterized by NMR spectroscopy and GPC analysis. 1H NMR

(400MHz, CDCl3, 25 °C), ppm): 1.54 (m, -CHCH3-), 3.64 (s, -CH2-), 5.15 (m, - CHCH3-).

The same procedure was analogously performed for all the tree-shaped copolymers and the

corresponding linear one of Table 1: the m-PEG macroinitiators (linear m-PEG, or m-PEG- Br4

0.1 g) were reacted with CuBr and bpy (molar ratio macroiniziator/CuBr/bpy 1/1/2 and 1/4/8

respectively) in 15 mL of dry toluene at 90 °C, with stirring, for 8 and 15 hours respectively. The

reactions were then stopped with n-hexane. The copolymers were recovered, dissolved in the

minimum amount of chloroform and passed over a column with activated Al2O3 to remove the

catalyst. The solutions were then dried in vacuo and the polymers were washed with cold

methanol before drying. The polymers were characterized by NMR spectroscopy and GPC

analysis.

Preparation of films by casting

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Thin films were prepared by dissolving 200 mg of polymer in 50 mL of CHCl3 at 25°C. The

solution was cast in a Teflon petri dish (diameter 6 cm) and the solvent evaporated at room

temperature. The film was removed from the petri dish and stored in a vacuum oven at 30°C for

three days. For comparison purpose, we used PMMA purchased from Sigma-Aldrich (Germany)

( wM = 350 KDa).

Characterization

1H NMR spectra were recorded on a Bruker Advance 400 MHz (Germany) spectrometer at 25°C

with a delay time D1 = 1 s. The samples were prepared by introducing 20 mg of copolymer in

0.5 mL of CDCl3 into a tube (0.5 mm outer diameter). TMS was used as an internal reference.

GPC chromatograms were recorded on a system equipped with a Waters 1525 (USA) binary

pump, a Waters 2414 RI detector and four styragel columns (range 103–10

6 Å). The

measurements were carried out at 25°C, using THF as eluent (1.0 mL/min) and polystyrene

standards as references.

Thermogravimetric analysis (TGA) was carried out with a Mettler TC-10 thermobalance (Italy).

Specimens (about 10 mg) were heated from 25 to 600 °C at 10°C/min heating rate under air and

nitrogen flow. The weight loss was recorded as a function of temperature.

Differential scanning calorimetry (DSC) measurements were carried out using a thermal analyzer

Mettler DSC 822/400 (Italy) under N2 atmosphere at a heating rate of 20° C/min between 25 and

200°C.

Water sorption was detected on different strips of the sample by immersion in distilled water at

25°C. The weight uptake was followed as a function of time, weighing the sample after

removing the excess water from the surface. Equilibrium sorption was assumed when no further

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weight changes were observed. The data were averaged on three samples.

Results and discussion

The synthesis of the hetero arms tree-shaped copolymers was carried out using a combined “arm

first”- “core first” approach in order to provide a better control over the final structure.

Preformed m-PEG chains with nM = 5000 Da and w nM / M = 1.04 were used to obtain m-PEG-

[OH]2 and m-PEG- [OH]4 by reaction with benzylidene anhydride as reported in the literature.[15]

The reaction of the modified m-PEGs with BMPB yielded the hydrophilic block with the

multifunctional core at one end of the chain. The following ATRP of MMA in the presence of

CuBr/bpy started on each of the n functionalities available on the m-PEG so that the tree-shaped

copolymers were characterized by two or four PMMA arms per m-PEG chain. In the following,

they are indicated as AB2 and AB4 structures, where A is the m-PEG and B the PMMA blocks.

Considering that the polymerization was living,[13]

the length of the PMMA arms was controlled

by the polymerization time, when in excess of monomer, or by the amount of monomer. The

linear copolymer AB was obtained by reaction of the linear m-PEG with BMPB and finally with

MMA in presence of CuBr/bpy.

The 1H-NMR spectra of all the copolymers showed the pattern of signals of both m-PEG and

PMMA (Figure 1). Besides, the GPC curves were monomodal and with narrow molecular weight

distribution ( w nM / M = 1.2-1.5) thus indicating that the copolymers were not mixtures of

copolymers and/or homopolymers.

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Figure 1

The star-shaped copolymers with two and four PMMA arms per terminal chain of the triblock

PEObPPObPEO (B2A-AB2 and B4A-AB4 respectively) and the corresponding linear one (BA-

AB) were also synthesized according to our previous report.[14]

Also in this case the length of the

PMMA arms can be controlled by the polymerization time.

It might be convenient to think about the star-shaped copolymers as two tree-shaped structures,

each of which was characterized by n PMMA branches united by a central PPO block (see

Scheme 1). The final star-shaped architectures, as a consequence, show about double the amount

of MMA units with respect to the corresponding tree-shaped copolymers. In Table 1 the

microstructural characterization of linear, “tree-“ and star-shaped copolymers is reported and one

can note that architectures having the same number of PMMA arms per each terminal chain (i.e.

BA-AB and AB; AB2 and B2A-AB2; AB4 and B4A-AB4) were characterized by a comparable

length of these blocks. The theoretical number average molecular weights of the copolymers

( nM theor) were calculated from the equation:

nM theor = n,IM + (([M]/[I]0)(MWMMA)) (1)

where n,IM is the molar mass of the linear, m-PEG-Br2 and m-PEG-Br4 macroinitiators, [M] is

the consumption of MMA, evaluated from the MMA in the feed and the polymerization yield,

[I]0 is the initial molar concentration of the macroinitiator and MWMMA is the molar mass of

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MMA.[16,17]

Table 1.

Films of linear, “tree-“ and star-shaped copolymers were obtained by casting from chloroform.

They were dried at 30°C under vacuum for three days before analysis. Figures 2 and 3 report the

TGA curves and DTG analyses, obtained in air and in nitrogen atmosphere, respectively. Table 1

reports the values of the maximum degradation temperature (Tdmax), as evaluated from DTG

analysis, where Tdmax was considered the temperature of the DTG main peak. It is worth noting

that the different molecular architectures deeply influenced the degradation of the materials; in

fact the tree-shaped structures showed a delay in the degradation with respect to the star

architectures, both in air and in nitrogen. The higher degradation temperature observed for the

tree-shaped copolymers could be explained considering both the structure and the nature of the

terminal groups. It is widely reported that chain terminal groups can strongly influence the

degradation temperature of a polymeric matrix since they represent a starting point for the

degradation mechanism. In this respect, mechanisms of thermodegradation of PMMA, either in

the presence of oxygen or in an inert athmosphere (i.e. N2 or He), have been widely studied.[18-23]

In all the structures here reported, the PMMA arms terminate with a quaternary halogenated

carbon atom –CH2(COOCH3)C(CH3)-Br (Figure 1) that is particularly prone to homolitic

cleavage under physical stimuli such as heat. Consequently, the PMMA arms are particularly

vulnerable towards the thermal degradation. The differences between the analogue “tree-“ and

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star-shaped structures reside in the number of PMMA blocks and in the corresponding number of

ester-based junctions between PMMA arms and PEG terminal chain. In fact, star-shaped

copolymers can be thought as two tree-shaped fragments united by a PPO block and so they have

twice the –CH2(COOCH3)C(CH3)-Br terminals per macromolecule and twice the ester-junctions

with respect to the corresponding tree-shaped copolymers; these can favour the faster

degradation. Such behaviour can be rationalized if considering that the architecture of the tree-

shaped copolymers induced a sort of “protection” from degradation of the m-PEG since the

radical attack propagated faster from the PMMA arms that in the case of tree-shaped copolymers,

are present on just one side of the macromolecule. Besides, it is worth noting that the Tdmax of the

PEObPPObPEO triblock was higher than the one of m-PEG both in air and in N2; nevertheless

this fact seems to have less influence on the entire degradation mechanism.

Figures 2 and 3

The transport properties (sorption and diffusion) of liquid water were evaluated as well.

Generally, these properties can give useful information about the amorphous phase of the

polymeric matrix. Sorption is related to the thermodynamic interaction between the amorphous

phase and the penetrating molecules, whereas diffusion, that is the kinetic parameter of molecule

transport, is mainly dependent on the free volume and on the obstacles that the travelling

molecule can find. In addition, with these samples being biocompatible, it is important to

observe their behaviour in an interactive environment such as liquid water.[24-26] By measuring

the increase of weight with time for the samples exposed to the solvent it is possible to obtain the

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equilibrium values of sorbed solvent, Ceq. Moreover, in the case of Fickian behaviour, that is a

linear dependence of sorption on square root of time, it is possible to derive the mean diffusion

coefficient D from the linear part of the reduced sorption curve, reported as Ct/Ceq vs. square root

of time, by Equation 2.[27,28]

1/ 24t

eq

C Dt

C d

(2)

where Ct is the penetrant concentration at the time t, Ceq is the equilibrium value, and d is the

thickness of the sample (in cm). Both sides of the samples were exposed to vapour. All the

samples showed a Fickian behaviour during the sorption of liquid water, so it was possible to

evaluate the diffusion coefficient, D(cm2/s), following Equation 2. Very few differences

concerning the maximum water concentration at equilibrium were evident between the samples

having the same architecture (see Table 2). However, the thermodynamic interactions of the

samples with liquid water were different for different architectures. Interestingly, even if BnA-

ABn copolymers have double the amount of both MMA and PEO units with respect to ABn

copolymers, the star-shaped architectures and the corresponding linear copolymer BA-AB

showed the higher sorption of water. Apparently, the hydrophilic component of the copolymers

influenced this parameter more than the hydrophobic one, probably because of the structural

organization of the polymeric blocks in the bulk that make the hydrophilic core less shielded by

the hydrophobic PMMA blocks in the star-shaped structures.

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The kinetic parameter, D(cm2/s), did not show any significant difference for linear and tree-

shaped structures, ranging in the same order of magnitude with respect to PMMA (4.21x10-9

cm2/s). However, for the star-shaped sample B2A-AB2 (run 4) the diffusion coefficient was

higher by about one order of magnitude according to our previous findings. [14]

While in all cases

the diffusion appeared “PMMA dominated”, this structure, also showing the highest water

sorption, displayed an influence of the hydrophilic and more permeable core also regarding the

diffusion probably because of a smaller shielding arrangement of the PMMA chains.

Table 2.

Another important aspect related to the presence of the hydrophilic component in the copolymers

architecture is the change of the hydrodynamic volume of the macromolecules in solution. GPC

analyses are not useful for evaluating the molecular weight of star-like or hyperbranched

polymers because of the different hydrodynamic volume of polymers characterized by complex

architectures with respect to the linear ones generally used as standards.[29]

However, we can use

the Mn evaluated from GPC for comparing the hydrodynamic volume of the polymers in

solution before and after sorption of water. In this respect, it was found that after sorption of

water the star-shaped copolymers increased their hydrodynamic volume more than the tree-

shaped ones, in accordance with the data of sorption of water (Table 2). In particular, the star

copolymer with the most branched structure B4A-AB4 was the most affected by the increasing of

the hydrodynamic volume (see i.e. run 6 in Table 2 and Figure 4).

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Figure 4.

The fact that the sorbed water influences the hydrodynamic volume so much, especially for the

star-shaped structure, can be potentially important for conformational changes and/or molecular

association of the copolymers in solution. Such an aspect plays a fundamental role in the

application of polymers in the drug delivery field and deserves a further and deeper investigation.

Conclusions

tree-shaped copolymers A(B)n and the analogous star-shaped (B)nA-A(B)n, where A is a PEG-

based chain, B is a PMMA block and n = 1, 2 or 4 were synthesized. The star-shaped copolymers

can be considered as having been built from two tree-shaped fragments united by a central PPO

junction. The sets of samples were synthesized to have a similar number of MMA units per block.

Films were obtained by casting from chloroform and microstructural, thermal and barrier

properties to liquid water were determined. Thermogravimetric analysis demonstrated that the

different molecular architectures strongly influenced the degradation of the materials. The tree-

shaped structures showed a delay in the degradation with respect to the star architectures and

commercial PMMA, both in air and in nitrogen. Sorption and diffusion to liquid water showed

no essential differences in maximum water uptake at equilibrium between the samples having the

same architecture. Sorption results were higher for star-shaped copolymers compared to the tree-

shaped structures. The diffusion parameter showed a decrease of about one order of magnitude

for the star-shaped copolymer B2A-AB2, probably because of the PMMA chains arrangement

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that shielded the PEG hydrophilic core less than in the other structures, confirming our

previously reported findings.

Gel permeation chromatograms, after liquid water sorption, showed that the molecular

architecture also influenced the hydrodynamic volume; the star-shaped copolymers increased

their hydrodynamic volume more than the tree-shaped ones.

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Table 1. Microstructural characterization, thermal and degradation properties of linear, star-

shaped and tree-shaped copolymers. a

wM , bnot detected,

cevaluated from DTG analysis

run Architecture

nM

(theor)

(KDa)

w nM / M

MMA

units/arm

PEO

units

Tg

(°C)

Tdmax(air)

c

(°C)

Tdmax(

N2)c

(°C)

1 linear

AB 46 1.2 408 112 94 350 384

2 linear

BA-AB 88 1.2 367 280 101 293 364

3 tree-shaped

AB2 51 1.3 229 112 96 287 375

4 star-shaped

B2A-AB2 115 1.4 249 280 101 243 364

5 tree-shaped

AB4 59 1.5 133 112 92 367 387

6 star-shaped

B4A-AB4 115 1.4 124 280 108 280 377

7 PMMA

(Aldrich) 350

a n.d.

b --- --- 101 293 353

8 m-PEG 5 1.04 --- 112 n.d.b 235 360

9 PEObPPObPE

O 14.6 1.13 --- 280 n.d.

b 252 376

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Table 2. Sorption, Ceq (g/100g), and diffusion, D (cm2/s) ,to liquid water of linear, tree-shaped and

star-shaped copolymers. a evaluated from GPC;

b the increased hydrodynamic volume (ihv) was

calculated by the formula: [(Mwet-Mdry)/Mdry]·100

Run Sample Ceq (g/100g) D (cm2/s)

nM (KDa) a

(dry

samples)

nM (KDa)a

(wet

samples)

ihv

(%)

1 linear

AB 2.16% 4.00x10

-9 37 44 19

2 linear

BA-AB 3.38% 5.12x10

-9 70 96 37

3 tree-shaped

AB2 2.10% 4.15x10

-9 44 68 54

4 star-shaped

B2A-AB2 3.52% 3.30x10

-8 84 142 69

5 tree-shaped

AB4 1.42% 7.00x10

-9 70 96 37

6 star-shaped

B4A-AB4 3.15% 2.30x10

-9 70 160 128

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Scheme 1. Tree-shaped and star-shaped architectures of copolymers synthesized in this work

Figure 1. 1H-NMR spectrum of a tree-shaped copolymer.

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Figure 2. TGA curves in air of star-shaped () and tree-shaped (---) copolymers

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Figure 3. TGA curves in nitrogen of star-shaped () and tree-shaped (---) copolymers

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Figure 4. GPC profile of B4A-AB4 copolymer before (A) and after (B) sorption of water.

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