End-Functional Polystyrenes via Quasiliving Atom Transfer Radical

Chapter 23

Downloaded by STANFORD UNIV GREEN LIBR on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch023

End-Functional Polystyrenes via Quasiliving Atom Transfer Radical Polymerization and New Polymer Structures Therefrom 1

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Béla Iván , Tamás Fónagy , Tibor Erdey-Grúz , György Holló-Szabó , Márta Szesztay , Ulrich Schulze , and Jürgen Pionteck 1

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Department of Polymer Chemistry and Material Science, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, Pusztaszeri u . 59-67, Ρ.Ο. Box 17, Hungary Institute of Polymer Research, Dresden, Hohe Strasse 6, D-01069 Dresden, Germany

Polystyrenes with 2-chloro-2-phenylethyl chain end (PSt-Cl) prepared by quasiliving atom transfer radical polymerization (ATRP) are useful intermediates for the synthesis of a variety of well-defined Macromolecular architectures. The preparation of star-shaped polystyrenes by quasiliving A T R P of divinylbenzene initiated by PSt-Cl and hyperbranched polystyrene by self-grafting of PSt-Cl mediated with T i C l is carried out in the course of our studies. Allyl-terminated macromonomers were also prepared by reacting PSt-Cl with allyltrimethylsilane in the presence of TiCl . This PSt-allyl was copolymerized with propylene by metallocene catalysts yielding novel poly(propylene-g-styrene) (PP-g-PSt) graft copolymers. 4

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© 2003 American Chemical Society

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction There have been significant developments in the field of quasiliving polymerizations (1) during the last decade. A s a consequence of this evolution the possibilities for the synthesis of well-defined Macromolecules have considerably broadened. Quasiliving radical polymerization processes, such as A T R P (2,3), SFRP (4) and R A F T (J), have led to a variety of polymers with predetennined molecular weight, relatively low polydispersity and high chain end functionality unavailable by other techniques. Quasiliving atom transfer radical polymerization (ATRP) is a transition metal catalyzed process yielding polymers with halogen at chain ends. These terminal halogen atoms are very useful as intermediates in the synthesis of complex Macromolecular architectures. For example, the reactive endgroup can initiate the A T R P o f a second monomer leading to block copolymers (6,7), or it can be transformed by nucleophilic substitution, elimination or radical chemistry to other advantageous groups. Several functional groups, such as allyl (8), hydroxyl (9) and amino (70) groups, were synthesized by these ways resulting in potential macromonomers or macroinitiators both in step- and chain-growth polymerizations. This study deals with our recent results on the synthesis andAppl.icationso f endfimctional polystyrenes obtained via quasiliving A T R P .

Experimental Section Polystyrenes with 2-chloro-2-phenylethyl chain end (PSt-Cl) were synthesized by quasiliving A T R P of styrene with (l-chloroethyl)benzene initiator and copper(I) chloride catalyst complexed with 2,2'-bipyridine (bpy) in bulk at 130 °C. The molar ratio of initiator/CuCLtopy was 1:1:2.5. The initiator/styrene ratio was calculated from the desired molecular weight of the polymer. The resulting polymers were purified by chromatography and precipitation. PSt-Cls with average molecular weight of 1,800 was used as macroinitiator in A T R P of divinylbenzene ( D V B ) for the synthesis of star polymers. The polymerizations were carried out in xylene with CuCl/3bpy catalyst system, the ratio of C u C l to PSt-Cl was 1:1. The resulting polymers were purified by chromatography and precipitation. The unreacted linear polystyrene was removed by passing the polymer in carbon tetrachloride solution through a chromatography column filled with neutral aluminum oxide followed by elution of adsorbed PSt-Cl with tetrahydrofuran.



333 Hyperbranched polymers were obtained by self-grafting of PSt-Cl. PSt-Cl with M W of 2000 was dissolved in dichloromethane or in mixture of dichloromethane and hexane (40/60) in concentration of 0.1 g/ml, then 8 eq. titanium tetrachloride was added under argon atmosphere. A t predetermined reaction times some solution was withdrawn. The samples were quenched with 2 m l methanol saturated with ammonia, then T H F was added, left for 24 hours at ambient temperature and filtered. Finally the polystyrene samples solved in T H F were precipitated into 10-fold amount of methanol, let to Sci.tle, filtered and dried in vacuum. The chain end of PSt-Cl was transformed to allyl terminus by reacting it with allyltrimethylsilane (ATMS) in the presence of T i C l in 0.1 mol/dm dichloromethane solution under argon atmosphere at 0 °C. The resulting allyl-terminated polystyrene (PSt-allyl) was copolymerized with propylene in the presence of metallocenes. Polymerizations were carried out in 200 m l toluene in a 1 liter glass autoclave (Buchi) equipped with a stirrer, manometer, thermocouple, heating and cooling units. The polymerization temperature was varied between 30 °C and 70 °C, and the total pressure was Sci. in the range of 1 bar to 3 bar. The catalysts were either Me Si(2-Me-4,5Benzind) ZrCl (MBI), Et[Ind] ZrCl (Etlnd) or Me Si[(t-Bu-N](Me) Cp]TiCl (TiN). For the copolymerizations, the A l / Z r molar ratio was 4,000 and catalyst concentration was 8x10"* mol/1. After consumption of 0.5 mol propylene the polymerization was terminated by injecting a small amount of ethanol. The reaction solution of the copolymer was precipitated into a mixture of ethanol, water and some hydrochloric acid. The resulting copolymer was filtered, washed with water and ethanol, and dried overnight in a vacuum oven at 70 °C. The unreacted polystyrene was removed by precipitation from hot xylene into cold acetone, followed by drying in vacuum oven at 70 °C. Characterization. *H N M R and C N M R spectra were recorded on a Bruker D R X 500 and a Varian 400 M H z spectrometers. Polystyrenes and polypropylene containing samples were measured at room temperature in CDC1 and at 120 °C in C D C1 , respectively. Molecular weight distributions of polystyrenes were determined by gel permeation chromatography (GPC) with a Waters/Millipore liquid chromatograph equipped with a Waters 510 pump, Ultrastyragel columns of pore sizes 1x10 , l x l O , l x l O and 500 L , a Viscotek parallel differential refractometer/viscometer and a laser light scattering (miniDawn®, Wyatt Technology Co.) detectors. Tetrahydrofuran was used as the mobile phase with a flow rate of 1.5 ml/min. Calibration was made with narrow M W D polystyrene standards. Molecular weights of PP-g-PSts were determined using high temperature size exclusion chromatography (Polymer Laboratories 210 GPC) operated at 135 °C. The G P C apparatus had a columnSci.with four columns (PL 3

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2

2

2

2

2

4

2

, 3

3

2

s

2

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334 gel 20 μπι Mixed-A). The mobile phase was 1,2,4-trichlorobenzene with a flow rate of 1.0 ml/min. The calibration was carried out by using PP with known molecular weight and distribution.


Results and Discussion Quasiliving A T R P of styrene results in polymers with predetermined molecular weight, relatively low polydispersity and 2-chloro-2-phenylethyl chain end. The presence of this terminal functional group makes such polystyrene (PSt-Cl) suitable for further transformations in order to obtain welldefined Macromolecular architectures. Numerous potential transformations of the 2-chloro-2-phenylethyl terminal group are offered by common and special chemical reactions. In the course of our recent investigations we have utilized PSt-Cl to initiate A T R P of divinylbenzene ( D V B ) for the preparation o f star polymers, to carry out self-grafting by Friedel-Crafts alkylations for obtaining hyperbranched polystyrenes, and to perform carbocationic chain end

(a)

c

\__

p

CuCI/ 01

"^^SC y

3bpy

J

^

ό TiCI

ATMS

TiCI

Ο

4

Q r 4

hyperbranched

r

star macromonomer

metallocene

0

on

graft

Scheme 1. Preparation of polystyrenes with various architectures from chlorine-terminated polystyrene obtained by quasiliving A T R P of styrene.



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derivatization with A T M S followed by metallocene catalyzed copolymerization of the resulting PSt macromonomer with propylene. These reactions and the resulting new polymers are shown in Scheme 1. PSt-Cl was used as macroinitiator for A T R P of D V B resulting in coupling of linear polystyrene segments by polymerization of the bifunctional monomer into a central polyDVB core (Scheme la). Figure 1 shows the molecular weight distribution ( M W D ) of the resulting polymers obtained at different polymerization times. A s it is exhibited in this Figure the relative concentration of the starting PSt-Cl gradually decreases while the amount of higher molecular weight fraction increases with increasing reaction time. However, the presence of the starting linear polymer is clearly indicated after 8 hours reaction time. Similar findings were reported by X i a et al. (11) by using also pentamethyldiethylenetriamine ( P M D E T A ) as ligand in catalyst of Cu-complex and several bifunctional monomers.

logM Figure 1. Molecular weight distributions o f polystyrenes obtained by A T R P o f D V B initiated by PSt-Cl in xylene. M (PSt-Cl)=1800, [PSt-Cl]=[CuCl]= 0.15 mol/dm , [bpy]= 0.45 mol/dm , [DVB]=1.11 mol/dm n

3

3

3

In order to be able to characterize the higher molecular weight fraction the rest of the unreacted PSt-Cl had to be separated. Several solvents were tested as eluent for preparative chromatography of this polymer mixture. It was found that the most suitable process is passing the polymer in carbon tetrachloride solution through a chromatography column filled with neutral aluminum oxide followed by elution with tetrahydrofuran. A s it is shown in Figure 2 the high molecular weight fraction passed through the column in the carbon tetrachloride solution, while the unreacted PSt-Cl was adsorbed to the neutral A 1 0 filler and could be eluted by tetrahydrofuran. 2

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log M Figure 2. Molecular weight distribution of polystyrene samples before and after separation of the crude polymer obtained by A T R P of D V B initiated by PSt-Cl. !

In the H N M R spectrum of the high molecular weight polymer two broadened signals appeared in the range of 5-6 ppm. This finding clearly indicates the presence of unreacted double bonds of D V B units in the polymer, and in accordance with the increased molecular weight it confirms the efficient A T R P of D V B by the PSt-Cl macroinitiator. The shape of these Macromolecules is expected to be star-like with polyDVB core and linear polystyrene arms. This was proved by the Mark-Houwink α coefficient (shape factor) calculated from G P C chromatograms. This value was found to be 0.11 indicating high branching degrees of the resulting star polymers. PSt-Cl can be regarded as multivalent reagent in Friedel-Crafts alkylation since the 2-chloro-2-phenylethyl chain end can be easily transformed to a carbocation by a Lewis-acid while the pendant aromatic rings are excellent substrates of electrophilic substitution. Thus, after treatment of PSt-Cl in dichloromethane solution with titanium tetrachloride (TiCl ) self-grafting of polystyrene by interalkylation was expected (Scheme lb). The *H N M R spectrum of the resulting polymers (Figure 3) indicate that the chlorine end of PSt-Cl was completely transformed since the signal at 4.3-4.6 ppm, assigned to the methine proton next to the chlorine atom, disappeared. Simultaneously a new signal appeared at 3.3-3.5 ppm, which can be assigned to methine protons neighbouring with two aromatic rings, indicating the successful alkylation. Figure 4 shows the results of G P C analysis. The average molecular weight and the polydispersity increased upon treatment with T i C l indicating that the intermolecular alkylation, i.e. self-grafting, was dominant over intramolecular substitution, i.e. cyclization. In order to confirm the branched structure of the resulting polymers the α parameters of the Mark-Houwink equation were also 4

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Ο'

l/l

5' m/^tmm0 ^f^


30' 60* 120'

4.5

ppm

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4.0

Figure 3. *H N M R spectra o f polystyrene samples obtained by selfgrafting of PSt-Cl. M (PSt-Cl)=2000, [PSt-Cl]=0.05 mol/dm , [TiCl ]=0.4 mol/dm , solvent: dichloromethane and hexane (40/60), T= -78 °C 3

n

4

3

% 0.8 £ α OJ

ϋ JSC

0.4

J • Ο

0.2

1

0.0

τ

20

40

60

80

100 120

Reaction time [min] Figure 4. Average molecular weight (square), polydispersity (star) and Mark-Houwink shape factor (delta) of PSt-Cl treated by T i C l as a function of time. M (PSt-Cl)=2000, [PSt-Cl]=0.05 mol/dm , [TiCl ]=0.4 moVdm , solvent: dichloromethane and hexane (40/60), T= -78 °C 4

3

n

3

4


338 calculated from the G P C chromatograms. It was found that the initial value of 0.72 decreased to 0.42 within 5 minutes reaction time and kept this value to the end of the investigated period. This value also signifies the branched structure as it is exhibited in Scheme 1. The polystyryl carbocation generated by T i C l can be reacted with a variety of other reagents. In the presence of A T M S the allylation of chain ends was expected in dichloromethane solution (Scheme l c ) . Monitored by *H N M R spectroscopy the disappearance of methine protons neighboring with chlorine atom and the appearance of signals corresponding to allyl groups (at 4.8 and 5.5 ppm) were observed. The lack of signals at 3.3-3.5 ppm assigned to methine protons with two neighboring phenyl rings indicated the domination of the reaction with A T M S over the Friedel-Crafts alkylation. Thus, allyl-terminated polystyrene (PSt-allyl) without detectable side reactions was formed (5). Since the PSt-allyl can be regarded as a substituted propylene, the copolymerization of this macromonomer with olefins by metallocene catalysis was expected to result in certain new graft copolymers. Thus a systematic investigation was carried out by us on the influence of synthesis conditions on the structure of poly(propylene-g-styrene) (PP-g-PSt) graft copolymer obtained by the copolymerization of propylene with PSt-allyl. The separation of the unreacted PSt-allyl was carried out by precipitation from hot xylene into cold acetone. A representative N M R spectrum of a purified copolymer sample is shown in Figure 5. This shows the presence of signals in the aromatic region corresponding to the PSt side chains in PP-g-PSt graft copolymer indicating that PSt-allyl incorporated into the polypropylene backbone. The molecular weights of PP-g-PSts were determined by GPC. The PSt content and the average number of side chains per molecule were calculated from the integral ratios of peaks in the Ή N M R spectra and molecular weights of PSt-allyl and the resulting PP-gPSts. These results are summarized in Table 1.


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11111111 I^Ii11111111

11111111 1^1 11111111^111111111^111111 111^111111111^111111111^111111II i^i 111 \ π ι i^j ι ι 11111 tij

ppm Figure 5. *H N M R spectrum of PP-g-PSt (copolymerization at 1.5 bar propylene pressure with PSt-allyl with M W of 2000, M B I catalyst at 50 °C).


339 Table 1. Effect of temperature (7), propylene pressure (p), molecular weight of PSt-allyl and type of catalyst on molecular weight, PSt content of PP-g-PSt, and average number of side chains per Macromolecule in metallocene catalyzed copolymerization of propylene with PSt-allyl. PSt content Averag number of M (PP-g-PSt) side chains (w/w%) g/mol n


T(°C) 30 50 70 Ρ (bar) 1 1.5 2 M„(PSt-allyl) 2,000 11,000 18,000 Catalyst Etlnd MBI TiN

p=1.5bar, cata lyst: M B I , M„(PSt-allyl)=l 8,000 0.37 2.4 285,000 6.7 0.33 89,400 0.20 10.8 33,200 T=50 °C, catalyst: M B I , M„(PSt-allyl)=l 8,000 0.33 7.4 78,900 0.33 6.7 89,400 0.22 4.9 183,300 T=50 °C, p=1.5 bar, catalyst: M B I 8.7 54,000 7.0 80,300 6.7 89,400

2.1 0.56 0.33

T=50 °C, p=1.5 bar, M„(PSt-allyl)=18,000 10,600 89,400 194,900

6.2 6.7 7.4

0.04 0.33 0.80

The PSt content of the resulting graft copolymers significantly increases with increasing polymerization temperature. This tendency can be explained by the more favorable concentration ratio of PSt-allyl to propylene. The change in the apparent reactivity ratio with temperature may also contribute to the increased PSt-allyl incorporation. Similar to homopolymerization of propylene, the M of the resulting graft copolymers decreases with increasing polymerization temperature. The PSt incorporation increases and the M W of copolymers decreases with decreasing propylene pressure. These changes are caused by the decreased concentration of propylene in the solutions. Some decrease in the PSt content and moderate increase in M W can be observed with increasing M W of PSt-allyl. This finding can be explained by the decreased apparent reactivity and decreased molar concentration (the molal concentration was kept constant) of longer chains while the increased M W is caused by the decreased PSt-allyl incorporation. A s expected, the catalyst has considerable influence on the M W of copolymers. These catalysts provide the same tendency as in the case of propylene homopolymerizations. The average number of side chains per Macromolecule was generally found to be lower than 1 with the n


340 exception of PSt-allyl with M W of 2000 indicating the possibility of successful synthesis of PP-g-PSt graft copolymers which might be efficient blending agents for polypropylene and polystyrene (12).


Conclusions Polystyrenes with 2-chloro-2-phenylethyl chain end (PSt-Cl) prepared by quasiliving atom transfer radical polymerization are useful intermediates for the synthesis of a variety of well-defined Macromolecular architectures. In this paper, we presented the preparation of star-shaped polystyrenes by quasiliving A T R P of divinylbenzene initiated by PSt-Cl and hyperbranched polystyrene by self-grafting o f PSt-Cl mediated by T i C l . Allyl-terminated macromonomers were also synthesized by reacting of PSt-Cl with allyltrimethylsilane in the presence of T i C l . This PSt-allyl was successfully copolymerized with propylene by metallocene catalyst yielding a variety of PP-g-PSt graft copolymers. 4

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Acknowledgements. The authors are grateful to D. Voit and E . Tyroler for G P C and H . Komber for N M R measurements. The authors acknowledge loaning the miniDawn® laser light scattering detector by Wyatt Technology Co. Financial support by the Hungarian Scientific Research Fund ( O T K A T29711, T25933, T33107) and Sàchsisches Ministerium fur Wissenschaft and Kunst is also acknowledged.

References

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8. Iván, B.; Fónagy, T. ACS Symp. Ser. 2000, 768, 372 9. Coessens, V . ; Matyjaszewski, K. Macromol. Rapid Commun. 1999, 20, 127 10. Matyjaszewski, K.; Nakagawa, Y.; Gaynor, S. G . Macromol. Rapid Commun. 1997, 18, 1057 11. Xia, J.; Zhang, X.; Matyjaszewski, K. Macromolecules 1999, 32, 4482 12. Schulze, U.; Fonagy, T.; Komber, H.; Pompe, G . ; Pionteck, J.; Ivan, B . to be published


End-Functional Polystyrenes via Quasiliving Atom Transfer Radical

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