atactic polymerization of propylene catalyzed by mono(η5-cyclopentadienyl)titanium tribenzyloxide...

Atactic Polymerization of Propylene Catalyzed byMono(h5-cyclopentadienyl)titanium TribenzyloxideCombined with Methylaluminoxane

QING WU, ZHONG YE, QINGHAI GAO, SHANGAN LIN

Institute of Polymer Science, Zhongshan University, Guangzhou 510275, People’s Republic of China

Received 18 September 1997; accepted 2 February 1998

ABSTRACT: Propylene has been polymerized with mono(h5-cyclopentadienyl)titaniumtribenzyloxide activated with methylaluminoxane (MAO). It was found that the contentof residual trimethylaluminium (TMA) in MAO has a determinative effect on thepolymerization. An excess of TMA in the catalyst system reduces the Ti species toinactive lower valent states. The catalyst system gives medium molecular-weight atac-tic polypropylene (Mv Å 2–7 1 104) with narrow molecular weight distribution (Mw /Mn Å 1.4–1.8). The polymer has a stereoirregular structure described by Bernoullianstatistics. Statistical analysis of the regiotriad distribution of the polypropylene chainsindicates a regioblock microstructure. q 1998 John Wiley & Sons, Inc. J Polym Sci A: PolymChem 36: 2051–2057, 1998Keywords: propylene polymerization; metallocene catalyst; methylaluminoxane;atactic polypropylene; regioselectivity

INTRODUCTION allocenes are nonstereospecific and produce, ingeneral, atactic and low molecular weight PP.5

High molecular weight atactic PP, which is a elas-In the past decade, a new class of homogeneoustomeric material, was also synthesized by C2vZiegler–Natta catalysts composed of Group IVBsymmetric Me2Si(Flu)2ZrCl2.6,7

metallocene and methylaluminoxane (MAO) orPP, consisting of alternating blocks of stereoir-metallocene cations has been developed for the

regular-amorphous and stereoregular-crystalliz-stereospecific polymerizations of a-olefins.1,2 Dif-able segments, is a rubbery material with theferent stereoregulated arrangements of polypro-novel properties of a thermoplastic elastomer.pylene (PP) can be produced, owing to the steric

and electronic diversity of the cyclopentadienyl Elastomeric PP with the stereoblock structure canligands of the metallocenes. The keys to directly be synthesized by alumina-supported Group IVBcontrolling the catalytic activity, stereospecificity, tetraalkyl8 and bis(arene)9 catalysts. Chien etand molecular weight of PP are the transition al.10 used rac-ethylidene(CpMe4Ind)TiCl2 andmetal (Ti, Zr, and Hf) and the type of h5-ligand. rac-ethylidene(CpMe4Ind)TiMe2 as catalyst pre-Isotactic PP can be obtained by stereorigid chiral cursors for the synthesis of elastomeric PP. Theansa-metallocene catalysts with C2 symmetry, block structure of the elastomeric PP was ex-such as rac-Et(Ind)2ZrCl3

2 , while those with bilat- plained that the catalytic species exists in twoeral symmetry, such as iPr(CpFlu)ZrCl4

2 , pro- isomeric states that undergo occasional intercon-duce syndiotactic PP. Achiral and symmetric met- version during the chain propagation. One of the

isomers produces atactic-amorphous blocks andanother produces isotactic-crystallizable blocks

Correspondence to: Q. Wuthat form crystalline domain and act as a physical

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 36, 2051–2057 (1998)q 1998 John Wiley & Sons, Inc. CCC 0887-624X/98/122051-07 crosslink in the polymer material. Recently, Way-

2051

97169T/ 8g7a$$169t 06-22-98 12:35:43 polca W: Poly Chem

2052 WU ET AL.

mouth et al.11 designed another two-state metallo- Toluene, benzene, and hexane were distilled fromsodium under nitrogen just before use.cene, (2-PhInd)2ZrCl2, which also produces ther-

moplastic elastomeric PP by oscillating from a chi- Cyclopentadienyltitanium tribenzyloxide (CpTi-(OBz)3 ) was synthesized as described pre-ral rac-like state to an achiral meso-like state by

rotation of the indenyl ligands about the metal– viously.17

MAOs were prepared as follows: 200 mL ofligand bond axis.Studies on catalysts applying in propylene po- TMA solution (3.1M, in toluene) was added drop-

wise into a flask with appropriate amount oflymerization have so far focused mainly on themetallocenes with binary cyclopentadienyl li- ground Al2(SO4)3r18H2O in toluene at 07C. The

mixture was gradually heated to 407C and stirredgands (identical or not identical) . On the otherhand, half-titanocenes have been reported to be for 24 h, and was then filtered. The filtrate was

concentrated under reduced pressure to a whiteefficient catalyst precursors for the syndiospecificpolymerization of styrene2e as well as catalyze po- solid. Residual TMA content in the MAO was de-

termined by pyridine titration.19 The MAOs pre-lymerizations of substituted styrene,2e conjugateddiene,12 ethylene2d,13 and their copolymeriza- pared with initial [H2O]/[TMA] molar ratios of

1.0, 1.3, and 1.8 contain the residual TMA 28.0,tion.14 Very little work on propylene polymeriza-tion by monocyclopentadienyl catalysts, however, 22.9, and 15.4%, respectively, and are referred to

MAO1, MAO2, and MAO3. MAO2 was redissolvedhas been reported in open literature. Soga et al.15

utilized heterogeneous catalysts, SiO2-supported in toluene and then the solvent was vaporized at607C in vacuo, to give MAO2 *.(or MgCl2-supported) XTiCl3 (X Å Cp, MeCp, Ind,

and Me7Ind), to yield atactic PP, and isotactic PPwhen a Lewis base is added. Polymerization Procedure

Monocyclopentadienyltitanium (IV) complexes Polymerizations were carried out in a 100-mLare not very stable in the presence of cocatalyst, glass flask equipped with a magnetic stirrer.and tend to be reduced to lower oxidation states, Twenty milliliters of toluene saturated with thefor example, Ti3/ . The Ti3/ species are active for monomer and desired amount of MAO and CpTi-syndiotactic polymerization of styrene, but not for (OBz)3 were introduced in this order. The gaseousa-olefin polymerization,16 which could be only ini- monomer was fed on demand. The total pressuretiated by Ti4/ species. Hence, the key to success was maintained at 130 kPa throughout the coursein propylene polymerization with half-titanocene of the polymerization. The polymerizations werecatalysts lies in protection of the metallocene spe- stopped after 1 h and the polymers were precipi-cies from reduction. tated by addition of acidified alcohol. The re-

Recently, we have performed styrene polymer- sulting polymers were washed with alcohol andization with mono(h5-cyclopentadienyl)titanium dried in vacuo to constant weight.tribenzyloxide, (CpTi (OBz)3 ) , combined withMAO as a cocatalyst,17 and found that the cata- Characterizationlytic activity depends upon the content of the re-

Molecular weight of the polymer was obtainedsidual trimethylaluminium (TMA) in MAO. Thisfrom intrinsic viscosities20 measured at 1357C inis probably attributable to the residual TMA,tetrahydronaphthalene. The molecular weightwhich reduces the Ti species under the polymer-distributions were measured on a Waters-208 LC/ization conditions.18

GPC with chloroform as solvent at 407C. Monodis-In this article, we report a new result fromperse polystyrene was used as a standard. 13C-aspecific polymerization of propylene promotedNMR spectra of the polymers in o-dichloroben-by CpTi(OBz)3–MAO through controlling thezene were recorded on a Bruker-AM400 at 1207C.amount of the residual TMA in MAO. StructuralDSC scans were obtained on a Perkin–Elmerproperties of the obtained products are illustratedDSC-7c at a scanning rate of 107C/min from040–herein.2007C.

EXPERIMENTAL RESULTS AND DISCUSSION

Materials Propylene Polymerization

MAO is the ubiquitous cocatalyst used in metallo-Trimethylaluminum (TMA) was commerciallyavailable and used without further purification. cene catalyzed olefin polymerization. It always


ATACTIC POLYPROPYLENE 2053

Table I. Oxidation State Distributions of Titanium in Propylene Polymerizations Using CpTi(OBz)3 withVarious MAOsa

MAO Preparation Oxidation Statesb,mol % Activity

TMA in MAO, External kg PP/ Tg,Run No. H2O/Al mol % TMA, mM Ti4/ Ti3/ Ti2/ (mol Tirh) Mw

c 1 1003 7C

1 MAO1 1.0 28.0 0 12.1 83.3 4.7 trace2 MAO2 1.3 22.9 0 15.4 80.9 3.7 2.203 MAO3 1.8 15.4 0 75.3 23.6 1.1 87.7 53.2 08.14 MAO3 1.8 15.4 16.7 29.4 67.4 3.2 trace5 MAO2* d 17.4 0 73.7 25.7 0.6 58.9 44.3 09.2

a Polymerization conditions: CpTi(OBz)3, 1.04 mM; MAO, 170 mM; 407C, 1 h.b Ti oxidation states were measured by redox titration.21

c Molecular weight from intrinsic viscosity measurement.20-

d MAO2* was obtained by redissolving MAO2 in toluene and then vaporizing the solvent at 607C in vacuo.

contains some amount of unreacted TMA that activity for propylene polymerization (run 3), de-activates the catalyst.cannot be removed easily by vacuum distillation.

The residual TMA can be a factor influencing the The effects of the reaction condition on propyl-ene polymerization carried out with the CpTi-catalytic activity of metallocenes and molecular

weight of polyolefins. Four MAOs with various (OBz)3/MAO3 system were studied. As showed inTable II, the catalyst system exhibits relativelyamounts of residual TMA were tested as cocata-

lyst in the propylene polymerization with catalyst high activities in the temperature range of 40–507C. The molecular weight of the polymer in-precursor CpTi(OBz)3. The activity of the cata-

lyst is very sensitive to the TMA content in MAO, creases slightly with decreasing polymerizationtemperature. Kinetic profiles of the polymeriza-as showed in Table I. Using MAOs with residual

TMA content ranged from 22.9 to 28.0 mol % tions at various temperatures are the decay type(see Fig. 1). The optimum molar ratio of MAO to(MAO1 and MAO2), the catalyst shows consider-

ably poor activities for propylene polymerization the titanocene for the polymerization is approxi-mately 150, much lower compared to the zircono-(runs 1 and 2). High catalytic activity is achieved

only with MAO3, which containing 15.4 mol % of cene catalyst systems.2a Raising the Al/Ti ratiocauses an obvious decrease of the catalytic ac-TMA (run 3). Reducing the amount of residual

TMA from 22.9 mol % (MAO2) to 17.4 mol % tivity.To give a better insight into the effect of the(MAO2 * ) , by redissolving MAO2 in toluene and

then vaporizing the solvent in vacuo at 607C, TMA in MAO on the polymerization, oxidationstate distribution of the Ti species was measuredcauses a dramatic increase in catalytic activity

from 2.2 kg PP per mol Ti per h (run 2) to 58.9 by redox titration21 under similar conditions inthe polymerizations. The values of titanium oxi-kg PP per mol Ti per h (run 5). On the contrary,

the addition of external TMA into CpTi(OBz)3/ dation states from the reaction of the titanocenewith MAO are given in Table I. Large fractions ofMAO3 system (run 4), which give originally high

Table II. Propylene Polymerizations with CpTi(OBz)3/MAO3 Catalyst

Run [Ti], mM [MAO3], M Al/Ti, mol/mol Tp, 7C Activity, kg/(mol Tirh) Mw, 11004

6 1.04 0.150 150 60 88.5 2.327 1.04 0.150 150 50 100.2 4.058 1.04 0.150 150 40 97.0 5.359 1.04 0.150 150 20 70.7 7.61

10 1.04 0.085 85 50 50.3 6.4411 1.04 0.252 240 50 31.2 3.8212 0.52 0.252 480 50 22.1 1.82


2054 WU ET AL.

Figure 2. DSC curves of the polypropylenes preparedFigure 1. Profiles of propylene polymerizations with with CpTi(OBz)3 activated with MAO3 (a); MAO2 *CpTi(OBz)3–MAO3 at 207C (1); 407C (2); 607C (3). (b).

ples do not exhibit a melting endotherm. Figuretitanium are in the reduced oxidation states2 shows that the glass transition lies at 010–087C.([Ti3/ ] Å 80.9–83.3% and [Ti2/ ] Å 3.7–4.7%) 13C-NMR analysis confirms that the titanocene/when the titanocene reacts with MAO1 or MAO2MAO system produces, at temperature range ofhaving high TMA content, whereas the reactions20–607C, atactic polypropylene. Figure 3 showswith MAO3 and MAO2 * containing less TMA givethe methyl region of sample 8. The concentrationsmostly Ti4/ species ([Ti4/ ] Å 73.7–75.3%). Withof stereo-diad and -triad sequences of the polypro-the addition of a small amount of external TMApylenes prepared at 20, 40, and 607C, respectively,into the catalyst system with MAO3 originallyare listed in Table III. The triad distributions ofcontaining less residual TMA, [Ti3/ ] increases re-these polypropylenes are not so different frommarkably from 23.6 to 67.4% and [Ti2/ ] increaseseach other. It can be observed that the polymersfrom 1.1 to 3.2%, and the catalytic activity dropsare stereoirregular with rr ú mm . A satisfactiondown correspondingly.of Bernoullian statistics, by mr / (2mm / mr )The results indicate that the catalyst activity /mr / (2rr /mr ) Å 1, indicates that the insertioncan be correlated with titanium valence states.of propylene with the CpTi(OBz)3/MAO catalystTitanium in the catalyst system with MAO con-system follows a chain end control model23 fa-taining less residual TMA can remain mostly invoring slightly racemic enchainment.a tetravalent state, and promotes propylene poly-

merization efficiently. On the other hand, a highRegioselectivityTMA content in MAO or addition of external TMA

favors the reduction of titanium to the lower va- The present polymers are regioirregular and con-lent states that do not catalyze the polymerization tain a nonnegligible proportion of propylene unitsof propylene. Therefore, we propose that the ac-tive species for the propylene polymerization inthe titanocene–MAO system are in tetravalentstate, and the Ti species in the reduced valencesare inactive for propylene polymerization, but ac-tive for styrene syndiotactic polymerization.17,21,22

Stereospecificity

The obtained products are amorphous solids, andsoluble in ether, hydrocarbons, and chlorinatedsolvents. Very narrow molecular weight distribu-tions of the polypropylenes, Mw /Mn Å 1.4–1.8measured by GPC, indicate a ‘‘single-site’’ cata- Figure 3. Methyl region of 13C-NMR spectrum of

sample 8.lytic polymerization. DSC analyses of these sam-



Table III. Stereospecificity of the Polypropylene Synthesized by CpTi(OBz)3/MAOCatalyst

Sample 6 Sample 8 Sample 9Sequence (607C)a (407C)a (207C)a

m 0.328 0.340 0.341r 0.672 0.660 0.659mm 0.0996 0.109 0.115mr 0.456 0.463 0.451rr 0.444 0.428 0.434

mr2mm / mr

/ mr2rr / mr

1.04 1.03 1.00

a Sample numbers are from the run numbers in Table II.

arranged in head-to-head and tail-to-tail se- where kij is the rate constant of j -insertion of apropylene monomer into the propagating chainquences. The regioirregular sequences can be ob-

served from 13C-NMR spectrum24 (Fig. 4) at 31.1 with i-oriented terminal, the subscripts 1 and 2refer to 1,2-insertion (1,2-orientation) and 2,1-in-ppm (Tbg) , 34.7–35.7 ppm (Tab , this region in-

cludes partly Sab) and 38.3 ppm (Tag) . No evidence sertion (2,1-orientation), respectively. Assuminga first-order Markov statistics and stationary-of 1,3-isomerization25 is seen in the spectra of the

present polymers. Therefore, as viewed from regi- state conditions for the binary copolymerization,27

the conditional probabilities of the propagationoselectivity, the propylene polymerization can beconsidered to be a binary copolymerization26,27 in- reactions are determined as follows:volving the following four propagation steps:

P11 Åk11

k11 / k12Å q (5)

------Ti / C3H6 rk11

- - - - - -Ti (1)

P12 Åk12

k11 / k12Å 1 0 q (6)------Ti / C3H6 r

k12

- - - - - -Ti (2)

------Ti / C3H6 rk21

- - - - - -Ti (3) P22 Åk22

k21 / k22Å p (7)

------Ti / C3H6 rk22

- - - - - -Ti (4)P21 Å

k21

k21 / k22Å 1 0 p (8)

Mol fractions of 1,2-oriented and 2,1-orientedunits, F1 and F2 , in a polypropylene sample aregiven by

F1 Å1 0 p

2 0 q 0 p(9)

F2 Å1 0 q

2 0 q 0 pÅ 1 0 F1 (10)

Using the above relations, the theoretical ex-pressions for the mol fractions of the regio-triadsequences, Tbb( / ) , Tbg( / ) , Tab(/ ) , and Tag( / ) , measured by the 13C-NMR spectra, can be derived with the conditionalFigure 4. Methine region of 13C-NMR spectrum of

sample 8. probabilities (see Table IV). Thus, we can calcu-


2056 WU ET AL.

Table IV. Regiospecificity of the Polypropylene Synthesized by CpTi(OBz)3/MAO Catalysta

Calculated Parametersc

TriadC-Type Concentration Sequence Expression q p F1 F2 r1 r2 r1r2

Tbb 0.768 / (1 0 q)p2 / q2(1 0 p)

2 0 p 0 q0.900 0.842 0.61 0.39 9.00 5.33 48.0

Tbg 0.108 / (1 0 q)(1 0 p)(p / q)

2 0 p 0 q(0.842) (0.900) (0.39) (0.61) (5.33) (9.00)

Tab 0.108b / (1 0 q)(1 0 p)(p / q)

2 0 p 0 qTag 0.016 / (1 0 q)(1 0 p)

a Sample 8 from run 8 in Table II.b The value was estimated from Tab Å Tbg, because peak at 35.4 ppm is attributable to Sab carbons besides Tab carbons.c See text.

Financial support of this research from the Nationallate the reactivity ratios, r1 and r2 , for the binaryNatural Science Foundation of China is gratefully ac-copolymerization using the following equations:knowledged.

r1 Åk11

k12Å q

1 0 q(11)

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r2 Åk22

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