Starpolymers by soft-hard-soft block copolymers

Die Angewandte Makrornolekulare Chernie 245 (1997) 183-191 (NK 4291)

‘Department of Applied Chemistry, 2Department of Solid State Physics, Kossuth Lajos University, H-4010 Debrecen,

Hungary

Starpolymers by soft-hard-soft block copolymers

S. KCki’, C. Bogacs’, L. Bog&csl, L. Dar6czi2, M. Zsuga’*

(Received 5 July 1996)

SUMMARY A novel synthesis of starpolymers from a soft-hard-soft triblock has been reported.

The triblock was obtained by adding isobutene (IB) “in situ” to the living styrene (S) polymerization mixture initiated by the p-dicumy1chloride/TiCl4/pyridine system. Microphase separation was observed by transmission electron microscopy and differential scanning calorimetry. Treatment of the PIB-PS-PIB triblock with CC14 in the presence of AlCl, led to the starpolymer and the number of arms was estimated by size-exclusion chromatography to be 20.

ZUSAMMENFASSUNG: Es wird uber eine neuartige Synthese fur Sternpolymere aus einem Weich-hart-

weich-Triblockcopolymeren berichtet. Das Dreiblock-Copolymere wurde durch in- situ- Addition von Isobuten (IB) zu einer mit Dicumylchlorid/TiC14/Pyridin initiier- ten lebenden Polymerisation von Styrol (S) erhalten. Untersuchungen mittels Trans- missionselektronenmikroskopie und Differentialkalorimetrie zeigen eine Mikropha- senseparation des PIB-PS-PIB-Copolymeren. Durch Umsetzung des Triblock-Copo- lymeren mit CC14 in Gegenwart von AlCl, wurde das Stempolymere erhalten, des- sen Anzahl der Arme mittels GroBenausschluBchromatographie zu 20 bestimmt wurde.

Introduction

Star-shaped polymers, containing at least three linear polymer chains (“arms”) connected to a central “core” region, represent a special class of branched polymers.

A potential application of starpolymers in oil industry is because they de- crease the viscosity of lubricating oils in a wide temperature range and increase the viscosity index’. Star-shaped polymers can be prepared by living ionic polymerization in different ways including the reaction of the living

* Correspondence author.

0 1997, Hiithig & Wepf Verlag, Zug CCC OOO3-3146/97/$07.00 183

S. KCki, C. BogBcs, L. BogAcs, L. Darbczi, M. Zsuga

polymer with a small amount of crosslinking agents'-6. Although the synthesis of related compounds was originally elaborated for anionic polymerization"*. Kanaoka et al. have shown that the star-shaped polymers of vinyl ethers are also available by cationic polymerization using linking agents, i. e. divinyl compounds24. The first four-arm star poly(isobutene) (PIB) was synthesized by Huang et al.9, and great efforts have been devoted to obtain multi-arm star PIB-s5. For the variation of arms the first methods have been proposed6 most recently. In these procedures an excess of divinylbenzene as linking agent was introduced to the living PIB chains.

Our synthetic strategy was to prepare a soft-hard-soft block copolymer, where the hard segment is suitable for crosslinking to develop the core of a star polymer. Such hard segments are supposed to be linked to each other physically and reversibly. The present paper reports the synthesis of a soft- hard-soft PIB-polystyrene (PS)-PIB block copolymer with the aim of inves- tigating the possibilities of crosslinking of a hard segment in both physical (i. e. phase separation) and chemical ways to produce star-shaped structures.

Experimental

Materials

1,4-Bis(2-chloro-2-propyl)benzene (dicumylchloride, pDDC) was prepared from 1,4-bis(2-hydroxy-2-propyI)benzene in CH2C12 as described earlier". Pyridine (Rea- nal) was distilled from potassium hydroxide. Methylcyclohexane (Aldrich) was purified by distillation. TiCI, (Merck) was distilled under reduced pressure and under nitrogen atmosphere. CHzC12 (Aldrich) was distilled from P,Olo. The stabilizer was removed from styrene (Aldrich) by extraction with a sodium hydroxide solution. After neutralization, the solvent was dried over CaH, and distilled under reduced pressure. Isobutene (TIFO, Hungary), CC14 (Reanal) and methanol (Reanal) were used as received.

Instruments

A Waters size-exclusion chromatograph (SEC) equipped with Waters differential refractometer, UV-detector and five Ultrastyragel columns (7.8 x 300 mm) was used for the determination of molecular weights and molecular weight distributions. The system was calibrated with polystyrene standards, and tetrahydrofuran was used as the eluent.

184


A TG-Mettler DSC 30 was applied for the determination of glass transition tem- peratures, T,. The microphase separation was visualized by a Jeol 2000 FX2 transmission electron microscope.

The relative viscosity values were determined with an Ostwald-type instrument. The ‘H NMR spectra were recorded on a Bruker WP 200 SY instrument.

Synthesis of the PIB-PS-PIB block copolymer

The block copolymerization was carried out in a 1000 mL three-necked flask in a Dry-Box, at -8O”C, under dry nitrogen atmosphere by stirring with a mechanical stirrer. Styrene (15 mL, 0.13 mol), pyridine (0.12 mL, 1.5 mmol) and pDCC (0.277 g, 1.2 mmol) were dissolved in a mixture of methylcyclohexane/CH2C12 (60/ 40, v/v). The volume of the solution was 600 mL and the polymerization was started by adding 24 mmol (2.64 mL) TiC14 to the solution. After 15 min, a sample was taken out from the mixture and “in situ” isobutene (30 mL, 0.3855 mol) was added to the living polystyrene (PS) reaction mixture. The polymerization was quenched with 30 mL of precooled methanol after 120 min, and the resulting polymer was purified by four successive precipitations from its hexane solution with methanol (yield: 34 g; 96%).

Synthesis of the star-shaped polymer

A representative experiment for the crosslinking of the PIB-PS-PIB block copolymer was carried out as follows: 1 g (58.8 pmol) triblock was dissolved in 20 mL CC14 (an appropriate solvent for both segments) and refluxed in the presence of AlC1, (0.4 g) for 6 h. The reaction was quenched by destroying A1CI3 with methanol (10 mL) and the polymer formed was precipitated. The inorganic compounds were removed by washing the polymer with a dilute HCI solution, and after neutralization it was dried in vacuum at room temperature (yield: 0.065 g insoluble polymer, 0.85 g soluble polymer).

Results and discussion

For our purposes the PIB-PS-PIB triblock was selected because its central block can be crosslinked chemically with a Friedel-Crafts type reaction, i. e. with CCl, in the presence of AlC1,. Prior to the synthesis of the PIB-PS-PIB block copolymer, styrene polymerization initiated by the pDDC/TiCl,/pyridine system was investigated and the results are shown in Fig. 1.

185

S. KCki, C. BogBcs, L. Bog&, L. Darbczi, M. Zsuga

4000 - 3000 - 2000 -

1000 7

0

Fig. 1. M, vs. weight of polymer, W,, and ln([M]d[M])-time plots for living polystyrene polymerization initiated by the pDCC/TiCI4/pyridine system; [pDDC] = 2 x m o m ; [S] = 0.21 m o m ; [Py] = 4 x

m o m ; [TiC14] = 4 x mom; T = -80°C; V = 200 mL).

The M, versus weight of the polymer formed (W,) and ln([M]d[M])-t plots are rectilinear, indicating the living nature of styrene polymerization.

With increasing time both the conversion and the M, increase regularly, al- lowing to design the molar mass of the central block of the block copolymer.

In order to obtain the block copolymer, isobutene was introduced "in situ" to the living polymerization mixture of styrene. For estimation of the length of the PS segment, a sample was taken and the M, of the central block was determined by SEC. The SEC traces of the triblock obtained and those of the starting PS are shown in Fig. 2.

The observed significant shift of the peak indicates that chain extension takes place, and comparison of the RI and UV traces shows that the chromo- phoric central PS block is distributed evenly. Independently, the M, of the triblock-copolymer was also determined by 'H NMR to check the validity of PS calibration of SEC for the PIB-PS-PIB triblock. The M, and MWD values of PS and PIB-PS-PIB are summarized in Tab. 1.

The good agreement in M, values (within the experimental error) indicates that SEC calibrated with linear PS standards is an appropriate tool for the determination of the M, of the PIB-PS-PIB triblock-copolymer.

186


0.8

0.7

h 0.6

90.5 z

0.4

0.3

CI -

0.2 4 I I I I I

0 10 20 30 40 50 60 V,(mL)

Fig. 2. SEC traces of the PS formed (15 min) and of the PIB-PS-PIB triblock.

Tab. 1. M, and MWD of PS and PIB-PS-PIB in batch polymerization (*by SEC; ** by 'H NMR).

PS PIB-PS-PIB

6700* 17 OOO* 15000**

1.54 1.37 -

The TEM investigations of a film of the triblock (Fig. 3 ) shows a microphase separation and the two T,'s of the triblock support this finding (Fig. 4).

According to these results, in solid state of the triblock the hard segments form microdomains, and the PIB chains remain unlinked at one end, resulting in star-shape microstructures (Scheme 1).

Such a microstructure may remain unchanged in solution in solvents which dissolve the PIB segment well but do not dissolve the PS segment (e.g. in hexane). The existence of a star structure in hexane, a solvent which is a model for aliphatic lubricating oils, permits the application of the PIB- PS-PIB block copolymers as viscosity index improvers without further chemical treatment. To prove this behavior of the PIB-PS-PIB triblock, its intrinsic viscosity was determined in toluene (a good solvent for both PIB and PS) and in hexane (a good solvent for PIB, but bad solvent for PS). By extrapolating the inherent viscosity to zero concentration, intrinsic viscosity values of 18.3 and 26.2 mWg were obtained in hexane and in toluene, re-

187

S. KCki, C. Bogics, L. BogBcs, L. Darbczi, M. Zsuga

Fig. 3. TEM micrograph of the PIB-PS-PIB triblock stained by OsO, (the dark dots indicate the PS segments).

Fig. 4. DSC trace of the PIB-PS-PIB triblock copolymer; T, of PIB blocks at -59.7 "C, T, of PS block at 60.3 "C.

Scheme 1. Model of the starpolymer formed from the PIB-PS-PIB block copolymer by microphase separation. The circle is the model of glassy domain, acting as solvent and temperature sensitive crosslink, holding the core of the star in place (- PS segments, \/\/\ PIB segments).

188

Starpolymers by so@-hard-soft block copolymers

spectively. The lower intrinsic viscosity of the triblock in hexane may be attributed to the formation of star-shape structures. As expected, a lower hy- drodynamic volume can be observed in the case of a star-polymer than for a linear polymer".

According to one of our invents'* the aromatic rings are capable of react- ing with CC14 in the presence of AlCl,. On the basis of this finding the chemical crosslinking of the hard segments of the triblock was achieved with AlCl, in CC14 solution, i.e. in a good solvent for both segments (Scheme 2).

Scheme 2. Model of chemically crosslinked PIB-PS-PIB triblock; the circle is the model of the core of the starpolymer, - PS segments, - PIB segments, chemically bonded crosslinking points.

The SEC traces of the soluble fraction of the treated and non-treated tri- blocks are shown in Fig. 5.

After chemical modification, a significant shift can be recognized, indicating the increase in molar mass, and thus providing a direct proof for chemical crosslinking of the PS segments. The peak of the crosslinked star is fairly narrow, indicating a controlled micelle aggregation. Since beyond M, = 5000 g/mol the SEC calibration based on linear standards is not ac- ~eptable '~, the M, of the starpolymer can only be roughly estimated. The number average molar mass of the starpolymer is 176000 g/mol by SEC, from which we estimated the minimum number of arms to be about 20.

189

S. K&, C. Bog@ L. Bogics, L. Darbczi, M. Zsuga

0.44 0.42

A 0.4 50.38 u’0.36

0.34 0.32 0.3

v)

0 10 20 30 40 50 60 Ve(mL)

Fig. 5. Representative SEC trace of a mixture of the soluble polymer obtained by chemical crosslinking of the PIB-PS-PIB triblock and of the non-crosslinked PIB-PS-PIB triblock.

Conclusions

The living polymerization of styrene with “in situ” blocking by isobutene is a suitable method for the synthesis of well-defined PIB-PS-PIB block copolymers. By crosslinking of the hard-segments of the triblock with CCl, in the presence of AlC1, starpolymers can be obtained. Variation of the lengths of the PIB and PS chains in the triblock, as well as the time of the crosslinking reaction, allows the synthesis of starpolymers with tangling arms of different length.

This work was financially .supported by the National Research Foundation (OTKA, Grant No. T 007456 and T 019508).

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Starpolymers by soft-hard-soft block copolymers