C60 as core of well defined hetero-star polymers

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Synthetic Metals 103 (1999) 2348-2349

C60as core of well defined hetero-star polymers. Y. EderlC,R. Nuffer and C. Mathis* Institlrt Charles Sadron (CN~-ULP) 6, rue Boussingault 67083 Strasbowg Cedex, Fsanre

Abstract

Addition of “living” polymers such as polystyryllithium onto C60 allows the synthesis of well defmed star-shaped macromoleculeswith a C60 core bearing six PS arms of about same length and an equal number of negative charges located on the fullerene. If these latter are used to initiate the anionic polymerization of methymetlnacrylate,a hetero-star consisting of a C60core Eth 6 polystyrene and 2 poly(methylmethacrylate) arms is obtained. A second way to prepare hetero-stars is to react the “living” stars with functional polymers bearing at one end a reactive halide. We were able to graft 2 additional PS chains of exactly the same or different length asthe 6 branches of the starting 6-branch star, producing respectivelya 8 branched star or asymmetric-star. Keywords : Fullerenes

and derivatives, Star and hetero-star

shaped polymers.

By bringing together the advantages of anionic polymerization and the possibility to control multiple addition onto the double bonds of C60,it becomes possible to open a new way toward the synthesis of well controlled and elaborated macromolecular architectures. In these latter, the fullerene plays the role of a small, well defined core or knot of controlled functionality. The low polydispersity and easy tailoring of the molar masses of the polymer chains prepared via anionic polymerization guaranties the model character of the produced polymer structures. In recent publications [ 1,2] we have shown that, in non polar solvents like toluene, a maximum of 6 polystyryllitium (PS-Li) adds to double bonds on the C60.This upper limitation has been used to produce star shaped macromolecules with exactly 6 PS chains of almost equal length attached to the fullerene core. The fact that the length of the grafts can be easily controlled was demonstrated by the synthesisof a set of hexa-adducts where the molar massofthe branch was varied between 1 000 and 400 000. The functionality of 6 has been experimentally demonstrated by the determination by light scattering of the absolute molar mass of the stars. The very low apparent polydispersity of the stars (< 1.1), as low as that of the branches, is an experimental proof of the narrow dispersity in functionality.

of vinyl monomers. We have demonstrated that the reactivity of the carbamons borne by the C60 core of a “living” grafted fullerene inc!ases with the number of the negative charges (i.e. number of branches) E3].This effect results from the~well known increaseof the localization of the negative charges on a conjugated molecule if the number of carbanions increases.Six carbanions have to be located on the same fullerene so that one of them becomesable to initiate the anionic polymerization of non polar monomer likestyrene and isoprene. The length of the single chain growing out from the core can be controlled and its polymolecularjty stays low. If the molar mass of this additional F’S or PI graf’r is not to far from that of the 6 PS branches of the starting sk, an asymmetric or a hetero-star is formed. When the out growing chain becomes longer, it can be seen as the trunk of a palm-tree like architecture. Such structuresare reported in 141. Using a more reactive monomer iike methylmethac~ldtt (M), two of the 6 carbanions of 3 “living” hexa-adduct initiate the anionic polymerization of MMA [3], So, a hetero-star where 6 PS branches of almost same molar mass and 2 PiwMA chains are connected to the same Cd0 molecule. In this architecture, illustrated below, the respective length of the 6 PS and the 2 PMMA grafts can be independently varied. 1. Initiation

It has to be stressed out that, each time a PS-Li adds to a double bond on the fullerene, a carbanion is created on the c60 core. These reactive species can be used for further reactions, opening the route for the design of more sophisticated architectures. 0379-6779/99/S - seefront matter0 1999 PII: SO379-6779(98)002951

Elsevier

Science

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rightsreserved.

Y. Ederle’ et al. I Synthetic

Metals

103 (1999)

Figure 1 : SEC characterization of a (PS)&6o(PMMA)2

2348-2349

Figure 2 : SEC characterization of (PS)&

ildetector

RI ddectw

-----

PS parent

-

(ps)sc606@36

----

Afleradditicm

2349

Mp=74000 J

-

W)6C60

-----

(PS)&jO(PS)2

b”ip=SSco of h&l4 A

32

36 Elution volume

40 (ml)

44

The characterization of a typical (PS)&(PMMA)2 heterostar by size exclusion chromatography (SEC) is given in figure 1. The SEC diagram of the products obtained after initiation of the MMA shows that a main compound with a higher molar mass including COO,as demonstrated by a UV-Vis detector set at 320 run where only the fullerene is detected, is formed. This latter correspondsto the hetero-star (PS)&O(PMMA)Z. The broadening observed for this peak indicates a larger polydispersity of the PMMA chains. The small shoulder on the right side of this main peak, corresponding to a compound with a mass about half of (PS)&(PMMA)2, can be attributed to a hetero-star bearing only one PMMA chain. 2. Reaction with w-functionalized polymer chains. Another route to hetero-stars is to react the carbanions of the “living” hexa-adducts with polymers bearing at one end a reactive halide. To check the validity of this method, we made the following experiments. First, the same “living” PS has been used to produce both the “living” hexa-adduct and the functionalized polymer. This latter has been obtained by reacting the PS-Li with a large excess of dibromoparaxylene (DBPX) leading to a bromide terminated PS-CHZ-Ph-CH& which is known to be very reactive toward carbanions. Despite the large excess of DBPX, the coupling reaction could not be completely avoided. Even if this polymer having twice the molar mass of PS-Li shows up in the SEC traces, it has to be stressed out that this polymer is unreactive. The functional polymer has been reacted with the “living” 6 branched star in a stoichiometry of 6 PS-CHI-PhCHzBr per hexa-adduct. The SEC trace of the obtained product is shown in tigure 2 along with that of the hexa-adduct.

The peak corresponding to the star is clearly shirted toward higher molar masses indicating that an addition of functional polymer has taken place. The very narrow shape of the new peak demonstrates that all the stars have reacted and that the new product [(PS)(6+&0] has a very low dispersity in functionality. The number of additional chains can be determined from the absolute molar massesof [(PS)(6T$6~], (PS)&o and PS-CH2-PhCH2Br (Table 1). The increase of molar mass of the star demonstrates that 2 more chains are added giving rise to a welldefmed stars, where 8 PS arms of equal length are connected to the same fullerene core, This same procedure has been used to graft 2 PS chains of Mw = 80 000 onto a 6-branched PS star of Mw~s= 36 000. In this case the two additional arms of the heterostar are much longer as the 6 branches of the parent star (Mwbmch = 6 000). All the chains grafted onto the C60core being polystyrene, this architecture should be rather called an asymmetric-star. This same procedure is now used to graft functional polymers of various chemical nature. References : [I] Y. Ederle, C. Mathis, Macromolecules 30 (1997) 2546 [2]V. Weber, M. Duval, Y. Ederle, C. Mathis, Carbon in press [3] Y. Ederle, C. Mathis, Macromolecules 30 (1997) 4262 [4] C. Mathis, Y. Ederle, R. Nuffer, same issue

Table 1 : Molar massesevolution upon reacting a “living” hexa-adduct with PS-CH2-Ph-CHlBr PSBr

(ps)&60

X

(Ps)(6+x)c60

Mw

Mn

I

Mw

Mn

I

14000

13600

1.03

55700

50000

1.11

80000

74000

1.08

27000

25000

1.08

Mw

Mn

I

Mwr,s

81000

74300

70000

1.06

110000

2.1

36000

175000

170000

1.03

190000

1.9

MWLS