Solvent Effects in the Polymerization of PropyleneCatalyzed by Octahedral Complexes

VICTORIA VOLKIS, ELENA SMOLENSKY, ANATOLI LISOVSKII, MORIS S. EISEN

Department of Chemistry and Institute of Catalysis Science and Technology,Technion–Israel Institute of Technology, Haifa, 32000 Israel

Received 27 February 2005; accepted 4 May 2005DOI: 10.1002/pola.20915Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The effect of solvents (toluene, dichloromethane, and hexane) was studiedon the polymerization of propylene with the octahedral complexes bis(trimethylsilyl)benzamidinate titanium dichloride(a), bis(acetylacetonate) titanium dichloride(b),and bis(diethylamino) titanium di-2-(diphenylphosphanylamino)pyridine as catalyticprecursors and methylalumoxane as the cocatalyst. For comparison, the polymeriza-tion was also performed in plain liquid propylene without the addition of any solvent.The obtained polymers were fractionated by refluxing hexane. The activity of thecomplexes and the molecular weights and tacticities of the whole polymers and theirdifferent fractions were the studied parameters. VVC 2005 Wiley Periodicals, Inc. J Polym

Sci Part A: Polym Chem 43: 4505–4516, 2005

Keywords: MAO; polymerization of propylene; solvent effect; titanium complexes

INTRODUCTION

During the last 2 decades, much academic andindustrial interest has been focused on the poly-merization of a-olefins promoted by organome-tallic complexes. The understanding of the rela-tionships between the properties of the obtainedpolymers and the catalyst structure, activity,and selectivity has been the momentum of theresearch.1 For metallocenes of groups 3 and 4,which are activated by either methylalumoxane(MAO) or perfluoro aromatic boron cocatalysts, ahigh level of sophistication has been obtainedwith respect to the stereoregularities, polydis-persities of the polymers, and control over thepolymeric architecture.2 In general, the struc-ture of the ancillary ligation affects electroni-cally and sterically the properties of the cata-lysts, influencing their activity and stereoselec-

tivity in the polymerization. Thus, complexeswith C2 or C1 symmetry are expected to produceisotactic polyolefins, whereas complexes withC2v symmetry are known to induce the forma-tion of atactic polymers.3 Although the micro-structure of the polypropylene obtained in thepresence of a well-defined single-site catalysthas been correlated to the symmetry of theactive organometallic complex, some complexeshave been found not to follow such expectationsand have produced distinctive polymeric materi-als, providing new, spectacular insights into themechanism for the polymerization of propylene.4

In the polymerization of a-olefins catalyzed byorganometallic compounds, the activity of thecomplexes and the molecular weight and tacticityof the polymers are the most relevant parametersfor describing the performance of the catalyticsystem. The activity of the complexes is charac-terized by the total amount of the polymerobtained per amount of the active catalyst speciesas a function of time. The molecular weight of thepolymers is determined by the relative rates of

Correspondence to: M. S. Eisen (E-mail: chmoris@tx.technion.ac.il)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 4505–4516 (2005)VVC 2005 Wiley Periodicals, Inc.

4505

the propagation and termination processes,whereas the tacticity of the polymers is influencedby the stereochemistry of the olefin insertion andthe relative position of the growing polymericchain to the monomer substituent.5 The tacticityof the polymer is also affected by any misinser-tions (2,1- and 3,1-insertions) and/or epimeriza-tion of the last inserted monomeric unit.6

Besides the listing parameters, one of the fac-tors strongly influencing the polymerization pro-cess is the nature of the solvents used to preparethe catalytic mixture. It has been shown that thenature of the solvent, particularly the solventpolarity, remarkably affects the activity of themetallocene systems and the properties of the poly-mers.7 For example, a linear increase in the rateof the syndiotactic propylene polymerization withthe i-Pr(CpFluo)ZrCl2/MAO catalytic system wasobserved with an increase in the content of di-chloromethane in a mixture with toluene.7(a)

However, in dichloromethane, the syndiospecific-ity of the catalyst was much lower than thatobserved in toluene. This result was assumed tobe caused by an isomerization of the free zircono-cene species, separated by the polar solvent, viamigration of the growing polymer chain beforethe next monomer insertion.7(a) A second examplerelates to the homopolymerization of 1-hexene inpolar solvents (CH2Cl2 and o-dichlorobenzene)with the aforementioned catalytic system. Theuse of the polar solvents allowed a drastic reduc-tion of the cocatalyst MAO amount to achieve amaximum activity, but the stereoregularity of thepolymer was much lower than those obtained inthe less polar solvent toluene.7(b) The effect ofsolvents with increasing donor character andsteric hindrance on the copolymerization of pro-pylene with higher a-olefins promoted by theEt(Ind)2ZrCl2/MAO system was also studied.7(c–f)

The activity of the catalyst for the homopolymeri-zation and copolymerization increased with theenhancement of the dielectric constant (e) of thesolvents in the order of o-dichlorobenzene >dichloromethane > chlorobenzene > toluene >heptane, without changes in the stereoregularityof the copolymers.7(c) The lowest 1-hexene contentin the copolymers observed in toluene wasexplained in terms of a competition between thenucleophilic medium and the monomer coordina-tion to the active site.7(e–f) In general, severalplausible schemes have been suggested for thesolvation of active species by nonpolar and non-coordinating (heptane), polar (CH2Cl2 and o-C6H5Cl), and coordinating (C6H5CH3) solvents,

regarding the copolymerization behavior of thecatalyst.7(f)

In this communication, we report for the firsttime a comparison study of the effect of smallamounts of solvents (toluene, dichloromethane,and hexane), differing in polarity and coordinat-ing ability, on the high-pressure homopolymeri-zation of propylene catalyzed by bis(trimethyl-silyl)benzamidinate titanium dichloride (com-plex A), bis(acetylacetonate) titanium dichloride(complex B), and bis(diethylamino) titanium di-2-(diphenylphosphanylamino)pyridine (complexC) as catalytic precursors activated by MAO.The effect of the mixture of the solvents on theactivity of the complexes and stereoregularity ofthe polymers has also been investigated. Inaddition and for comparison, the polymerizationin neat liquid propylene was performed withoutthe addition of the supplementary solvents. Theobtained polymers were fractionated via reflux-ing in hexane. The activity of the complexes andthe properties of the whole polymers and theirdifferent fractions were the studied parameters.

EXPERIMENTAL

Materials and Instruments

All manipulations were carried out with theexclusion of oxygen and moisture with flamedSchlenk-type glassware and a high-vacuum (10�5

Torr) technique. For the storage of air-sensitivematerials, a nitrogen-filled Vacuum Atmospheresglovebox with a medium-capacity recirculator (1–2 ppm O2) was used. The gases (argon, nitrogen,and propylene) were purified by passage througha MnO oxygen removal column and a Davison4-A activated molecular sieve column. Analyti-cally pure solvents were distilled under argonfrom a Na/K alloy (diethyl ether and hexane),Na (toluene), K (tetrahydrofuran), or P2O5 (di-chloromethane). The MAO cocatalyst (Witco) wasprepared from a 30% suspension in toluene byvacuum evaporation of the solvent at 25 8C/10�5

Torr. Complexes A and B, the 2-(diphenylphos-phanylamino)pyridine ligand, and the homoleptictetrakis(diethylamino)titanium complex wereprepared according to literature procedures.8–11

NMR spectra were recorded on Bruker AM-200,Bruker AM-400, Bruker Avance 300, and BrukerAvance 500 spectrometers. Chemical shifts for 1Hand 13C NMR were referenced to internal solventresonances and reported with respect to SiMe4.The polypropylene NMR experiments were con-

4506 VOLKIS ET AL.

ducted in deuterated tetrachloroethane at 85 8C.The tacticities of the resultant polypropylenewere determined according to the pentad analysismethod.12 The molecular weights of the polymerswere determined by the GPC method with aWaters-Alliance 2000 instrument with 1,2,4-tri-chlorobenzene as a mobile phase at 150 8C.

Synthesis of Bis(diethylamino) Titanium Di-2-(diphenylphosphanylamino)pyridine (Complex C)

Ti(NEt2)4 (0.23 g, 6.85 � 10�3 mol)11 was addedvia a syringe under an argon flow to a solutioncontaining 0.34 g (1.22 � 10�3 mol) of the ligand2-(diphenylphosphanylamino)pyridine10 in 10 mLof dry toluene. After the mixture was stirred at 258C overnight, the solvent and diethylamine wereremoved by vacuum. The residue was washedwith hexane through a fritted glass to remove theunreacted ligand, and the orange powder of theobtained product was recrystallized at �45 8Cfrom a toluene solution; this yielded 0.45 g (88%)of orange crystals of complexC.

ELEM. ANAL. Calcd. for C42H48P2N6Ti: C,67.58%; H, 6.48%; N, 11.25%. Found: C, 68.35%;H, 6.32%; N, 10.58%. 1H NMR (200 MHz, tol-uene-d8, d): 0.86 (t, 12H, 3JHH ¼ 7 Hz, ��CH3),3.79 (br, 2H, ��CHH��CH3), 4.78 (br, 2H,��CHH��CH3), 5.95 [dd, 2H, 3JHH ¼ 4.0 Hz,3JHH ¼ 6 Hz, H(b)], 6.13 [dd, 2H, 3JHH ¼ 8.5Hz, H(d)], 6.54 [dd, 2H,3JHH ¼ 8.5 Hz, 3JHH ¼ 6Hz, H(c)], 7.22 (br, 6H, m,p-aromatic rings), 7.56(br, 2H, o-aromatic ring), 7.81 (br, 2H, o-aromaticring), 7.97 [d, 2H, 3JHH ¼ 4 Hz, H(a)]. 13C NMR(100 MHz, toluene-d8, d): 13.0 (��CH2CH3), 45.7(��CH2CH3), 111.9 (CH, Cb), 112.2 (CH,Cd), 128(CH, m,p-aromatic rings), 131.8 (CH, C1), 132.0(CH, C3), 137.5 (CH, Cc), 143.4 (CH, Ca), 168(C, Ce). 31P NMR (81 MHz, toluene-d8, d): 35.12(s, PPh2).

Propylene Polymerization

The polymerization experiments were carriedout in a 100-mL, stainless steel reactor equipped

with a magnetic stirrer and a thermocouple.The reactor was charged inside a glovebox with10 mg of the complex and the correspondingamount of MAO to keep the Al/Ti molar ratio at800. After the introduction of the solvent (e.g.,6 mL of the solvent) via a syringe under anargon flow, the reactor was frozen in liquidnitrogen and pumped down. Liquid propylene(40 mL) was then vacuum transferred into a fro-zen reactor, the valve was closed, and the tem-perature was quickly (5 min) raised to 25 8C.During this heating time, no stirring was per-formed. At the desired temperature, the stirrerwas operated, and the reaction was stirred for4 h at a pressure of 10.2 atm. The reaction mix-ture was then quenched by the exhaustion ofthe unreacted propylene, followed by the intro-duction of 40 mL of a mixture of acetyl acetoneand water. The polymer was filtered, washedwith methanol and acetone, and dried in vacuo.The polymerizations were repeated to obtain atleast three good reproducible results (<5%).

The possibility of polypropylene formation dur-ing the fast warming of the reaction mixture fromthe temperature of liquid nitrogen to room tem-perature without stirring was found to be negli-gible. When the polymerization reaction wasquenched immediately after room temperature(þ25 8C) was reached, the amount of the polypro-pylene formed was less than 0.3% of the totalamount of the polymer obtained at room tempera-ture with normal stirring. In addition, the activ-ities of the studied complexes were not highenough to induce heating of the reaction mixtureand, consequently, a change in pressure.

Fractionation of Polypropylene

The polymer samples were fractionated byextraction with refluxing hexane. The polymers(�1 g) were packed into a cellulose thimbleplaced in a Soxhlet extractor. The latter wasattached to a round-bottom flask (250 mL) filledwith hexane (100 mL). After extraction for 24 h,the solvent was evaporated. Thus, for everypolypropylene sample, two fractions [hexane-soluble (HS) and hexane-insoluble (HI)] wereobtained.

RESULTS AND DISCUSSION

We start the presentation of the results withthe synthesis of the complexes. The complexesTiCl2[PhC(NTMS)2]2 (A) and TiCl2(CH3COCH-

POLYMERIZATION OF PROPYLENE 4507

COCH3)2 (B) were prepared by the reaction in tol-uene of TiCl4 with 2 equiv of the bis(trimethyl-silyl)benzamidinate lithium � tetramethylethylene-diamine (TMEDA) salt (eq 1)8 or the bis(acetylaceto-nate) lithium salt (eq 2), respectively.9 The complexTi(Ph2PNpy)(NEt2)2 (C) was prepared by the reac-

tion of 2 equiv of the neutral ligand Ph2PNHpy10

and 1 equiv of the homoleptic tetrakis(diethylami-no)titanium11 (eq 3).13 These complexes activated byMAO produced active catalytic systems for the poly-merization of propylene, yielding under high mono-mer pressure an elastomeric polypropylene:

To shed some light on the conceptual questionregarding the effects of different solvents on thepolymerization of propylene promoted by octahe-dral complexes, we present here the results ob-tained with three complexes (A, B, and C) dif-fering in their ancillary ligation. The effects of

the solvents on the polypropylene yields, the cat-alytic activity of the complexes A, B, and C, andthe molecular weights and tacticities of the poly-mers are presented in Table 1. The calculatedrates of the propylene insertions for each of thecomplexes in the specific solvent, the rate for

4508 VOLKIS ET AL.

chain termination, and the number of mono-meric units in each polymer are also presentedin Table 1.

We begin the discussion of the polymerizationresults with a comparison of complexes A andB. As can be seen in Table 1 and Figure 1, forthese precatalysts, the polymerization rate inthe presence of dichloromethane (e ¼ 8.93) islower than that in toluene (e ¼ 2.38). At thesame time, in both solvents, complexes A and Bare considerably less active in comparison withhexane (e ¼ 1.90) and liquid propylene (e ¼ 1.90).These results are opposite to those in the liter-ature data, according to which the polymerizationactivity of zirconocenes in a polar solvent(CH2Cl2) is higher than that in toluene, whereasthe yields of the polymers in a nonpolar medium(heptane) are considerably lower in comparisonwith dichloromethane and toluene.7(a–f)

The different characters of the solvent effectson the polymerization of propylene, observed inour experiments and described in the literature,are due to the unique structures of the studiedactive titanium octahedral complexes, in com-parison with those of the metallocenes, and thepolymerization conditions.3,8,9 As for the reasonsfor the inferior polymerization rates observed indichloromethane and toluene, in comparison

with hexane and neat propylene, let us examinelikely interactions of the former solvents withthe active cationic centers of the titaniumdichloride benzamidinate (A) and acetylaceto-nate (B) complexes.

One of the known mechanisms for the inter-action of polar solvents with the organometalliccomplexes activated by MAO has been proposedto be the separation of the ion couples betweenthe cationic complex and the cocatalyst counter-ion forming a coordinative unsaturated cationic

Table 1. Effects of Different Solvents on the Polymerization of Propylene with Complexes A, B, and C Activatedby MAOa

Complexi

A B C

dcm tol hex prop dcm tol hex prop dcm tol hex prop

PPb 1.75 2.31 12.4 9.75 1.64 5.08 11.1 10.6 1.32 1.67 1.74 1.69Ri � 10�21c 6:27 8:28 44:96 34:95 5:88 18:21 39:79 38:00 4:73 5:98 6:24 0:65

Ri � 10�6d 4:31 3:00 22:90 16:97 1:39 1:71 4:40 6:02 0:69 0:87 0:67 0:80

ne 124 198 440 442 331 2244 1281 1231 744 830 694 1154

A � 10�4f 2.82 3.72 19.9 15.7 1.30 4.03 8.97 8.42 2.46 3.11 3.25 3.15Mw � 10�3 23 40 50 52 64 264 183 150 122 129 137 160MWDg 4.4 4.8 2.7 2.8 4.6 2.8 3.4 2.9 3.9 3.7 4.7 3.3mmmmh 12.3 10.2 9.2 8.1 24.5 20.5 16.0 12.2 9.7 9.3 9.0 8.9

a Amount of the complex ¼ 10 mg; Al/Ti molar ratio ¼ 800; volume of the solvent ¼ 6 mL; volume of liquid propylene ¼40 mL; temperature ¼ 25 8C; pressure ¼ 10 atm; reaction time ¼ 4 h.

b Yield of polypropylene (g).c Rate of monomer insertion (molecules/h).d Rate of chain termination (mol of polymer/h).e Number of monomeric units in the polymer.f Activity (g of PP � mol of Ti�1 � h�1).g Molecular weight distribution (Mw/Mn).h mmmm pentad content (%).i dcm ¼ CH2Cl2; tol ¼ toluene; hex ¼ hexane; prop ¼ neat liquid propylene.

Figure 1. Effect of the solvent on the activity ofcomplexes A, B, and C in the polymerization of propy-lene (DCM ¼ dichloromethane).

POLYMERIZATION OF PROPYLENE 4509

complex, which does not strongly interact withthe solvent (Scheme 1).7(f),8(a,b) This effect makesthe separated cationic species more accessiblefor the incoming monomer and, in principle,must result in the augmentation of the polymer-ization rate as a result of the better charge sep-aration caused by the solvents with higherpolarity. However, as noted, both complexes Aand B were unexpectedly found to be moreactive in the less polar solvents (Table 1 andFig. 1).

Hence, another possible effect responsible forthe polymerization activity of the complexesmay be the coordination of CH2Cl2 and toluene,due to their donor abilities, to the free orbital ofthe metal obtained in the formation of the cati-onic complex by the transfer of the methidegroup (CH3

�) to MAO (Scheme 2).7(f),8(a,b) Suchcoordination would stabilize the active cationiccenter, thus decreasing its accessibility and rap-idity for the insertion of the monomer molecules.In comparison with toluene, this effect must be

larger for the more polar dichloromethane, asfollows from the data of Table 1.

It can be deduced that the summarized effectsof the solvents are determined by both kinds ofinteractions (Schemes 1 and 2). Apparently,under the studied conditions, precatalysts A andB are more sensitive to the stabilization actionof the polar solvents (dichloromethane and tol-uene) than to the charge separation effect, thatresulting in a decrease in the polymerizationactivity of the complexes. This effect foundfor the polar solvents may be corroborated bythe fact that all the studied complexes werefound to be inactive when the polymerizationswere performed with the more polar solventtetrahydrofuran, having a higher coordinationcapability.

The polymerization of propylene in dichloro-methane, toluene, or hexane was performedwith a large excess of liquid monomer withrespect to the amounts of the solvents. There-fore, we can assume the coexistence of solvent-

Scheme 1. Plausible separation of the cationic center and the MAO counterion bythe polar solvent (M ¼ metal; L–L ¼ ancillary ligation; S ¼ solvent).

Scheme 2. Plausible scenario for the coordination of polar solvents (dichlorome-thane and toluene) to the cationic center at octahedral complexes (M ¼ metal; L–L ¼ancillary ligation).

4510 VOLKIS ET AL.

separated and propylene-separated ion pairs, inaccordance with the literature data.1(a),2(c),14

Noticeably, in the polymerization of propy-lene, larger rates were observed when the reac-tion was carried out in neat monomer (Table 1and Fig. 1). Clearly, in this case, liquid propy-lene also acts as a solvent, ensuring the forma-tion of the active species without other externalinteractions hindering the insertion of the in-coming monomer molecules. The high polypropy-lene yields in hexane, similar to those observedin liquid propylene (Table 1 and Fig. 1), are indi-cative of the lack of interactions of the formersolvent with the cationic complexes.

A similar but less pronounced effect of thesolvents on the polymerization of propylene wasalso observed for complex C (Table 1 and Fig. 1).The small effect of the solvents is the result of aplausible existence of complex C in two equili-brium forms with both a closed pseudo C2 sym-metry (I and I0) and an opened pseudo C2v

configurations (II), in addition to the internalcoordination of the same ancillary ligations(Scheme 3).13

The variation of the molecular weight of thepolymers as a function of the nature of the sol-vent is presented in Table 1 and Figure 2. Forcomplexes A and C, the molecular weight of thepolymers slightly decreased with increasing sol-vent polarity. For complex B, the lowest molecu-lar weight was obtained when the reaction wascarried out in CH2Cl2, whereas in the less polarsolvents, polymers with much higher molecularweights were obtained.

To rationalize these results, we must take intoaccount the relationships of the rates for chainpropagation and termination for each complex.

The mechanism for the termination of thepolymerization corresponding to each of the

complexes has been previously studied. We havefound that b-methyl elimination, b-H elimina-tion, and aluminum-transfer mechanisms werethe major operative termination pathways forcomplexes A, B, and C, respectively.8,9,13 Thedata in Figure 2 allow us to assume that forcomplex A the effect of the solvent is primarilyrelated to the rate of termination rather than tothe rate of propagation. From the rate of propa-gation, we should expect higher molecularweights for the less polar solvents (Fig. 1). How-ever, because the obtained molecular weightsare similar for all solvents, the rate of termina-tion must follow the trend hexane > propylene> toluene � dichloromethane to compensate thepropagation rate yielding the tendency observedin Figure 2. This assumption is corroborated bythe calculated rate of chain termination pre-sented in Table 1. For complex B, a similar

Scheme 3. Dynamic behavior of cationic complex C in the presence of MAO.

Figure 2. Dependence of the molecular weight ofthe polymers obtained with complexes A, B, and C onthe nature of the solvent (DCM ¼ dichloromethane).

POLYMERIZATION OF PROPYLENE 4511

trend for the rate of termination also must beoperative, though much stronger (for hexaneand propylene), besides the more polar CH2Cl2,which induces a low number of insertions. Forcomplex C, similar catalytic activities andweight-average molecular weight (Mw) of thepolymers were observed in all solvents; thisindicated comparable rates for monomer inser-tion and chain terminations, as corroborated bythe calculated rates presented in Table 1.

Scheme 4 shows the possible interactions ofthe cationic center with the monomer, polymerchain, and solvents. By comparing plausiblerates of solvation of the active complexes withthe rate of termination and taking into accountthat the rate of solvation affects both the ratesof propagation and termination, we can explainthe dependences shown in Figure 2. For complexA, the rate of rehybridization of the methylgroup (k1 ¼ rate of chain termination) must befaster than the rate of solvation (k2) of the cati-onic center (k1 > k2). This effect increases the b-CH3-elimination rate and, as a result, decreasesthe molecular weight of the polymers. The neteffect of the different solvents can be estimatedrelatively by the amount of the monomeric unitsinserted into the polymer chain (Table 1). Forcomplex B, the interaction of the b-hydrogen(centrosymmetric orbital interaction) with themetal (k3) must be much weaker than the inter-action of the strong polar solvent (k3 < k2), thusimpeding fast monomer insertions. If this is theonly operative mechanism, it will lead to a de-

crease in the molecular weight by increasing thesolvent polarity. In addition, the rate of termina-tion will be increased by the reduction of thepolarity of the solvent. Therefore, the higher Mw

obtained in toluene, in comparison with thatobtained in hexane or plain propylene, is the neteffect obtained from the competition betweenthe insertion of propylene (high concentration ofthe monomer-intermolecular process) and chaintermination (low concentration of the catalyst-intramolecular process), allowing a large amountof monomer insertions but a small amount ofchain eliminations. The same effect is alsoobserved for CH2Cl2, although this solventimpedes much more strongly the monomerinsertion, producing low activity as well as lowmolecular weights. The b-hydrogen interactionshave been proved to be extremely important,inducing the first step for the epimerization ofthe last inserted monomer in the polymerizationof olefins by metallocenes, but not causing amajor reduction in the molecular weight of thepolymers.15 The use of hexane as a solvent withcomplex B unexpectedly promotes the formationof a polymer with a slightly higher molecularweight in comparison with plain propylene, andthis indicates that the b-H elimination in theformer is much slower than that in the plainmonomer. This result is related to the lack ofstabilization of the cationic center by the hexanesolvent, which allows a higher chain of propaga-tion (stabilization via the monomer) in compari-son with the chain of termination (see Table 1).

Scheme 4. Main-chain termination interactions in the polymerization of propylenefor octahedral cationic complexes (k1 ¼ rate of b-methyl elimination; k2 ¼ rate ofsolvation; k3 ¼ rate of b-hydrogen elimination; k4 ¼ rate of aluminum transfer; S¼ solvent; L–L ¼ ancillary ligation).

4512 VOLKIS ET AL.

For complex C, the rate of the interaction of thepolar solvent with the metal (k2) must be compa-rable to or slightly larger than the rate for alu-minum transfer (k4; k2 ˜ k4). For complex C,the molecular weight of the polymer decreasedwith an increasing amount of MAO (beyond Al/Ti ¼ 800) when the reaction was carried out inplain propylene, hexane, or toluene. Evidently,the rate of termination is a combined effect ofthe MAO concentration and its competitiontoward the stabilization of the cationic center bythe solvents.16 Such an effect was not observedfor complexes A and B.

For complexes A and C, the effect of the sol-vents on the tacticity of the polymers seems to benegligible, whereas for precatalyst B, the mmmmpentad contents in the polymers increased asthe polarity of the solvents increased (Fig. 3).The last result suggests that during the reactionin the presence of CH2Cl2, which is in closeinteraction with the empty d-orbital of theactive cationic complex, the stereoregular dis-crimination of the two olefinic enantiofaces islarger, allowing an easier insertion of the pre-ferred face leading to higher isotacticities, ascorroborated by the lowest activity of the systemB/MAO observed in dichloromethane (Table 1and Fig. 1).17 The isospecificity of complex A is,in general, lower than that for B because of thehigh amount of misinsertions.12(d) An independ-

ence of the mmmm content in the polymers,obtained with the system C/MAO, of the polarityof the solvents presumably may be a result ofthe dynamic behavior of this complex and inter-nal ligand coordination.13

The most important question concerns theability of small amounts of the polar solvents (inthe presence of an excess of liquid propylene) toinfluence drastically the polymerization activityof the complexes and properties of the polymers(vide infra). This remarkable fact can be illus-trated by the results obtained when the poly-merization reaction was performed with the cat-alytic system B/MAO in binary mixtures ofCH2Cl2 with toluene or with hexane (Table 2),followed by fractionation of the polymers in therefluxing hexane. The polypropylene obtained indichloromethane was more soluble in hot hexanethan the polymer formed in toluene (entries 1and 2). In addition, for the polymer obtained intoluene, the isotacticity of the HS fraction washigher than that for the polymer obtained indichloromethane, whereas the opposite effectwas observed for the isotacticity of the HI frac-tion. Increasing the amount of dichloromethane,while the total volume (solvent and liquid propy-lene) was kept constant, did not practicallyinfluence the yield of the polypropylene, thepolymerization rate, and the molecular weightand isotacticity of the whole polymer (before frac-tionation) and both HS and HI fractions (entries 2and 3). However, when the reaction was carriedout in a mixture of CH2Cl2 and toluene, theactivity of the catalytic system and the proper-ties of the polymers were mainly affected by thepresence of the polar solvent (CH2Cl2), as ob-served for both soluble and insoluble fractions.Remarkably, this effect is independent of theconcentration of the polar solvent in the reactionmixture (cf. entries 5 and 6). These results shedlight on the large ability of the polar solvent,even in small amounts in a mixture of solvents,to control the activity of the organometallic com-plex in the polymerization of propylene as wellas the properties of the obtained polypropylene.As already mentioned, the highest polypropy-lene yield was reached in hexane (entry 4).Nevertheless, the data obtained in mixtures ofhexane and dichloromethane (entry 7) are alsocomparable to those obtained in dichlorome-thane (entry 2). Hence, the polymerization activ-ity of the complexes is mostly operative by theinteraction of minute amounts of the polar sol-vent (dichloromethane) with the active cationic

Figure 3. Dependence of the tacticity of the poly-mers as a function of the solvent polarity in the poly-merization of propylene catalyzed by complexes A, B,and C (DCM ¼ dichloromethane).

POLYMERIZATION OF PROPYLENE 4513

center, via their electron-donor/coordinatingabilities, almost independently of the presenceof additional nonpolar solvent in the reactionmixture.

The results of the propylene polymerizationwith catalytic system B/MAO, carried out in amixture of solvents (Table 2), corroborate theaforementioned effect of the solvents on themolecular weight of the polymers. When equalamounts of CH2Cl2 and toluene (3 mL of each)were used, Mw of the polymer (66,000) was notdistinct from that of the polymer obtained inCH2Cl2 alone (Mw ¼ 64,000; entries 2 and 5). Inaddition, when the polymerization was carriedout in a mixture of CH2Cl2 and hexane (3 mLof each), a similar result was obtained (Mw

¼ 68,000), indicating again the large effectcaused by the more polar solvent.

The separation of the polypropylene into twofractions, soluble and insoluble in hot hexane,correlates with the wide polydispersity of thepolymers obtained. Taking into account thatthese fractions are produced, at least, by thetwo different active sites, we can propose thatthe initial active complexes undergo a specifictransformation under the influence of solvents.Because the separated fractions of the polymersstrongly differ in isotacticity (Table 2), it is veryplausible that the active sites responsible fortheir formation also differ by their stereospeci-ficity, depending on the kinds of interactionswith the solvent. We can propose the activeintermediates 1 and 2 responsible for the forma-tion of the heterotactic and isotactic polymericfractions, respectively. Active site 1 has anempty d-orbital (in addition to the cationic orbi-tal) that can be coordinated by the nonpolar sol-vent, propylene, at the expense of an a-agosticinteraction from the polymer chain. This kind ofinteraction will not avoid any rotation of thepolymer, and therefore, the insertion of themonomer molecules will not be maintained transbetween the methyl group of the monomer andthe growing polymer chain, a requirement forstereoregular insertions.5

Table

2.

Data

onthePolymerization

ofPropylenePerform

edbytheCatalyticSystem

B/M

AO

inDifferentSolven

tsandTheirMixturesa

Entry

mLb

PP

Yield

(g)

A�

10�4

c

WholePolymer

HSFraction

HIFraction

tolb

dcm

chex

dM

w�

10�3

mmmm

(%)d

%M

w�

10�3

mmmm

(%)

%M

w�

10�3

mmmm

(%)

16

00

5.08

4.03

264

21

81

210

17

19

621

60

20

60

1.64

1.30

64

24

94

56

86

197

77

30

12

01.80

1.43

62

23

90

54

710

183

78

40

06

11.1

8.8

183

16

91

128

12

9610

69

53

30

1.97

1.56

66

1192

57

98

136

78

65.5

0.5

01.63

1.29

54

993

51

67

153

66

70

33

2.28

1.81

68

14

90

49

10

10

336

75

aAmou

ntof

B¼

10mg;Al/Tiratio¼

800;volumeof

liquid

propylene¼

40mL;temperature

¼258C

;pressure

¼10atm

;time¼

4h.

btol¼

toluen

e;dcm

¼dichloromethane;

hex

¼hex

ane.

cActivity(g

ofPP�

mol

ofTi�

1�

h�1).

dPen

tadcontent.

4514 VOLKIS ET AL.

With active site 2, the polar solvent (tolueneor dichloromethane) is strongly attached to theempty d-orbital, impeding the free rotation ofthe growing polymer chain, thus inducing ahigher isotacticity of the polymer. These plausi-ble kinds of interactions can be corroborated bythe trend observed for the HI fractions producedby the system B/MAO. It may be seen that thehigher the polarity of the solvent, the larger theisotacticity of the polymers obtained. When mix-tures of the solvents are used, the more polarsolvent mainly induces the isospecificity (exceptthe mixture of a large amount of toluene and asmall amount of dichloromethane, when a ki-netic competition of the solvents is operative).An additional corroboration can be seen fromthe data obtained in the polymerization ofpropylene in the absence of a polar solvent(Table 1). Performing the reaction in hexane orin the neat propylene resulted in the formationof only heterotactic polymer. These results arealso confirmed by the fact that the more polarthe solvent is, the less active the catalytic sys-tems are, but the better the stereoselectivities ofthe polymerization are.

CONCLUSIONS

Solvents of various polarities and coordinationabilities (dichloromethane, toluene, and hexane)influence the high-pressure propylene polymer-ization catalyzed by the octahedral titaniumdichloride complexes A, B, and C activated withMAO. In comparison with CH2Cl2 and toluene,all the complexes are considerably more activein the nonpolar hexane and in liquid propylene(bulk polymerization). The different effects ofthe solvents were assumed to be determined bytwo effects: the separation of the ion couplesbetween the cationic complex and cocatalystcounterion and the stabilization of the metal viathe coordination of the solvents. Increasing thesolvent polarity resulted in a decrease in themolecular weight of the polymers produced bycomplexes A and B (only for CH2Cl2), whereasfor complex C, a less drastic effect was observed.We have rationalized the variation of the poly-mer molecular weight as a function of the sol-vent nature, taking into account the relation-ships of the rates for the chain propagation andtermination for each complex. For complex B,the isotacticity of the polymers increased with

an augmentation of the solvent polarity,whereas the stereoregularities of the polymersproduced by complexes A and C were nearlyindependent of the solvent nature.

The authors thank the German Israel Foundation(contract I-621-27.5/1999) and the Technion VPRFund for the Promotion of Research at the Technionfor their support of this work. V. Volkis thanks theTechnion Institute of Catalysis Science and Technol-ogy for a postdoctoral fellowship. A. Lisovskii thanksthe KAMEA program for its financial support.

REFERENCES AND NOTES

1. (a) Kaminsky, W. InAdvances in Catalysis; Gates,B. C.; Knozinger, H., Eds.; Academic:San Diego,2002; Vol. 46, p 89; (b) Kaminsky, W.; Arndt, M.Adv Polym Sci 1997, 127, 143 and referencestherein; (c) Brintzinger, H. H.; Fisher, D.; Mul-haupt, R.; Rieger, B.; Waymouth, R. M. AngewChem Int Ed Engl 1995, 34, 1143; (d) Alt, H. G.;Kopll, A. Chem Rev 2000, 100, 1205 and referen-ces therein; (e) Coates, G. W. Chem Rev 2000,100, 1223; (f) Ittel, S. D.; Johnson, L. K.; Brook-hardt, M. Chem Rev 2000, 100, 1169; (g) Chen,Y.-X.; Marks, T. J. Chem Rev 2000, 100, 1391; (h)Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F.Angew Chem Int Ed 1999, 38, 428 and referencestherein; (i) Kaminsky, W. J Polym Sci Part A:Polym Chem 2004, 42, 3911.

2. (a) Mulhaupt, R. In Polypropylene: An A–Z Refer-ence; Karger-Kocsis, J., Ed.; Kluwer Academic:Dordrecht, 1999; p 454; (b) Metz, M. V.;Schwartz, D. J.; Stem, C. L.; Nickias, P. N.;Marks, T. J. Angew Chem Int Ed 2000, 39, 1312;(c) Lanza, G.; Fragala, I. L.; Marks, T. J. J AmChem Soc 2000, 122, 12764; (d) Williams, V. C.;Piers, W. E.; Clegg, W.; Elsegood, M. R. I.; Collins,S.; Marder, T. B. J Am Chem Soc 1999, 121, 3244;(e) Temme, B.; Erker, G.; Karl, J.; Luftmann, H.;Frohlich, R.; Kotila, S. Angew Chem Int Ed Engl1995, 34, 1755.

3. (a) Averbuj, C.; Tish, E.; Eisen, M. S. J Am ChemSoc 1998, 120, 8640; (b) Even, J. A.; Jones, R. L.;Razavi, A.; Ferrara, J. D. J Am Chem Soc 1988,110, 6255; (c) Mohring, P. C.; Coville, N. J. JOrganomet Chem 1994, 479, 1; (d) Even, L. A. JAm Chem Soc 1984, 106, 6355; (e) CatalystDesign for Tailor-Made Polyolefins; Soga, K.; Ter-rano, M., Eds.; Elsevier: Tokyo, 1994.

4. (a) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi,F. Chem Rev 2000, 100, 1253 and referencestherein; (b) Rodriguez-Delgado, A.; Hannant, M.D.; Lancaster, S. J.; Bochmann, M. MacromolChem Phys 2004, 205, 334 and referencestherein.

POLYMERIZATION OF PROPYLENE 4515

5. Miller, A. A.; Bercaw, J. E. Organometallics 2002,21, 934 and references therein.

6. For misinsertions, see ref. 1(c). For epimerizationmechanisms, see (a) Busico, V.; Cipullo, R. J AmChem Soc 1994, 116, 9329; (b) Busico, V.; Capor-aso, L.; Cipullo, R.; Landriani, L. J Am Chem Soc1996, 118, 2105; (c) Resconi, L. J Mol Catal A1999, 146, 167.

7. (a) Herfert, N.; Fink, G. Macromol Chem Phys1992, 193, 773; (b) Coevoet, D.; Cramail, H.; Def-fieux, A. Macromol Chem Phys 1999, 200, 1208;(c) Forlini, F.; Tritto, I.; Locatelli, P.; Sacchi, M.;Piemontesi, F. Macromol Chem Phys 2000, 210,401; (d) Forlini, F.; Fan, Z.-Q.; Tritto, I.; Locatelli,P.; Sacchi, M. C. Macromol Chem Phys 1997, 198,2397; (e) Fortini, F.; Princi, E.; Tritto, I.; Sacchi, M.C.; Piemontesi, F. Macromol Chem Phys 2002, 203,645; (f) Sacchi, M. C.; Fortini, F.; Losio, S.; Tritto,I. Macromol Chem Phys 2003, 193, 45; (g) Makio,H.; Fujita, T. Macromol Symp 2004, 213, 221.

8. (a) Volkis, V.; Shmulinson, M.; Averbuj, C.; Lisov-skii, A.; Edelmann, F. T.; Eisen, M. S. Organome-tallics 1998, 17, 3155; (b) Volkis, V.; Nelkembaum,E.; Lisovskii, A.; Hasson, G.; Semiat, R.; Kapon,M.; Botoshansky, M.; Eishen, Y.; Eisen, M. S. JAm Chem Soc 2003, 125, 2179.

9. Shmulinson, M.; Galan-Fereres, M.; Lisovskii, A.;Nelkenbaum, E.; Semiat, R.; Eisen, M. S. Organo-metallics 2000, 19, 1208.

10. (a) Xavier, K. O.; Smolensky, E.; Kapon, M.; Aucott,S. M.; Woollings, J. D.; Eisen, M. S. Eur J InorgChem 2004, 4795; (b) Aucott, S. M.; Slawin, A. M.Z.; Woollins, J. D. Organomet Chem 1999, 582, 83.

11. Bradelly, D. C.; Thomas, I. M. J Chem Soc 1959,340.

12. (a) Stehling, F. S.; Knox, J. R. Macromolecules1975, 8, 595; (b) Song, W.; Yu, Z.; Chien, J. C. W.J Organomet Chem 1996, 512, 131; (c) Busico, V.;Cipullo, R.; Caporaso, L.; Angelini, G.; Serge, A.L. J Mol Catal A 1998, 128, 53; (d) NMR of Poly-

mers; Bovey, F. A.; Miray, P. A., Eds.; Academic:San Diego, 1996.

13. In the reaction of complex C with MAO, bridgingCH3 or Al moieties were not trapped on NMR tubeexperiments; see (a) Smolensky, E. M.Sc. Thesis,Technion–Israel Institute of Technology, 2003; (b)Smolensky, E.; Kapon, M.; Woollings, D.; Eisen,M. S. Submitted for publication, 2005.

14. (a) Busico, V.; Castelli, V. V. A.; Aprea, P.; Cipullo,R.; Segre, A.; Talarico, G.; Vacatello, M. J AmChem Soc 2003, 125, 5451; (b) Beck, S.; Lieber,S.; Schaper, F.; Geyer, A.; Brintzinger, H. H. JAm Chem Soc 2001, 123, 1483; (c) Zhou, J.; Lan-caster, S. J.; Walker, D. A.; Beck, S.; Thomton-Pett, M.; Bochmann, M. J Am Chem Soc 2001,123, 223; (d) Xu, Z.; Vanka, K.; Firman, T.;Michalak, A.; Zurek, E.; Zhu, C.; Ziegler, T. Orga-nometallics 2002, 21, 2444.

15. (a) Busico, V.; Castelli, V. V. A.; Aprea, P.; Cipullo,R.; Segre, A.; Talarico, G.; Vacatello, M. J AmChem Soc 2003, 125, 5451; (b) Beck, S.; Lieber,S.; Schaper, F.; Geyer, A.; Brintzinger, H. H. JAm Chem Soc 2001, 123, 1483; (c) Zhou, J.; Lan-caster, S. J.; Walker, D. A.; Beck, S.; Thomton-Pett, M.; Bochmann, M. J Am Chem Soc 2001,123, 223; (d) Xu, Z.; Vanka, K.; Firman, T.;Michalak, A.; Zurek, E.; Zhu, C.; Ziegler, T. Orga-nometallics 2002, 21, 2444.

16. The larger the amount of MAO was, the lowerMw of the polymers was. When the reaction wasquenched after a short period, no olefin chain endgroups were observed.

17. At this stage, besides the empty d-orbitalobtained via the reaction with MAO, which isresponsible for the activity of the complexes andMw of the polymers, an additional empty d-orbi-tal is available for coordination by the monomer,solvent, or agostic interactions with the polymerchain, being responsible for the stereoregularityof the insertions.

4516 VOLKIS ET AL.