16

Influence of Aluminium Alkyl Compounds on the High-Pressure Polymerization of Ethylene with TernaryMetallocene-Based Catalysts. Investigation of ChainTransfer to the Aluminium

Christian Götz,* Alexander Rau, Gerhard Luft

Department of Chemical Engineering, Darmstadt University of Technology,Petersenstraße 20, D-64287 Darmstadt, GermanyFax: +49 6151 16 4214; E-mail: goetz_christian@gmx.de

Keywords: aluminium alkyl; high pressure; metallocene catalysts; molecular weight distribution; polyethylene (PE);

IntroductionTernary catalyst systems based on a metallocene dichlor-ide L2MCl2 (M = Ti, Zr, Hf; L = cyclopentadienyl-basedbridged or non-bridged ligands), a large excess of an alu-minium alkyl compound (AlR3) and one to two equiva-lents of a cation-forming agent such as B(C6F5)3,[Ph3C][B(C6F5)4] or [PhNHMe2][B(C6F5)4], are relativelyinsensitive to impurities. This is because the excess alu-minium alkyl compound reacts with impurities, thus pro-tecting the active sites of the catalyst.[1–7] Ternary catalystsystems are therefore easy to use and are the catalysts ofchoice in many commercial polymerization processes.[8, 9]

Solutions of preactivated ternary catalyst systems can bedirectly introduced into existing high pressure plants.

This means that the advantages of the metallocene cata-lysts can be combined with high pressure reaction condi-tions.[10, 11] The influence of the excess aluminium alkylcompound on polymerization of olefins and polymerproperties below 1 MPa has been investigated[12–14] butlittle is known about the effects under high pressure andhigh temperature conditions.

Here we report on the influence of the aluminium alkylcompounds TIBA and TEA in a high-pressure reactor onthe metallocene-catalyzed high-pressure polymerizationof ethylene at 150 MPa and 1808C, using Cp2ZrCl2 andPh2C(CpFlu)ZrCl2 as catalyst precursors. The latter werepreactivated with TiBA and DMAP outside the reactor.

Full Paper: The influence of aluminium alkyl compoundson metallocene-catalyzed high pressure polymerizationsof ethylene has been investigated at 150 MPa and 1808Cin a continuously operated autoclave. The catalysts werebased on the metallocenes bis(cyclopentadienyl)zirco-nium dichloride (Cp2ZrCl2) and diphenylmethylene(cyclopentadienylfluorenyl)zirconium dichloride (Ph2C-(CpFlu)ZrCl2), which were preactivated outside the reac-tor with triisobutylaluminium (TiBA) and N,N-dimethyl-anilinium tetrakis(pentafluorophenyl)borate (DMAP,[PhNHMe2][B(C6F5)4]). The concentrations of triisobuty-laluminium (TiBA) and triethylaluminium (TEA) in thereactor were varied over a wide range, using a separatedosing for these two aluminium alkyl compounds. Produc-tivity and polymer properties strongly depended on thetype and the concentration of the aluminium alkyl com-pound used. Highest productivities and molecular weightswere obtained with low concentrations of TiBA in thereactor. Up to a concentration of 30 molppm Al in the

reactor, unimodal polymers were formed with M—

w/M—

n

between 2 and 3. With higher aluminium concentrationsthe products formed contained small amounts of waxes,due to oligomerization catalyzed by the aluminium alkylcompounds. The molecular weight distributions (MWDs)of these products could be described as a superimpositionof two Schulz-Zimm distributions. All MWDs were ana-lyzed with regard to the amount of waxes produced byethylene oligomerization and with regard to the influenceof chain transfer reactions to the aluminium. The rate con-stants of chain transfer to aluminium, in relation to therate constants of insertion of ethylene, were estimated.

Macromol. Mater. Eng. 2002, 287, No. 1 i WILEY-VCH Verlag GmbH, 69469 Weinheim 2002 1438-7492/2002/0101–0016$17.50+.50/0

Macromol. Mater. Eng. 2002, 287, 16–22

Influence of Aluminium Alkyl Compounds on the High-Pressure Polymerization ... 17

Experimental Part

Materials

All experiments with air-sensitive materials were carried outunder an atmosphere of argon, using standard Schlenk tech-niques. Hexane and toluene were dried over a Na-K alloy,then distilled and stored under argon. Ethylene (Linde,99.8%) was purified by passage through columns containingmolecular sieve (4 �) and a copper catalyst (BASF). TEA,TiBA and Cp2ZrCl2 (Aldrich) were all used as purchased.Ph2C(CpFlu)ZrCl2 and [PhNHMe2][B(C6F5)4] (BASF AG)were used as purchased. All solids were stored under argon.

Analytical Methods

Molecular weight distributions (MWDs) were determined bymeans of high temperature gel permeation chromatography(GPC). A Waters 150C plus was equipped with two Styragelcolumns (Polymer Standard Services) as stationary phaseand a differential refractometer (RI 150) as detector. The set-up was calibrated with polyethylene standards of knownmolecular weight and narrow polydispersity (Polymer Stan-dard Services).

Preparation of Ternary Catalyst Systems

The ternary catalyst systems were prepared according to anoptimized standard procedure.[15, 16] 1.5 ml (5.9 mmol) TiBAwas added to 16.3 mg (29.3 lmol) Ph2C(CpFlu)ZrCl2 dis-solved in 12 ml toluene (Al/Zr = 200), and stirred for 30 minat 208C. 1.8 ml of this mixture was added to a solution of4.1 mg (5.1 lmol) [PhNHMe2][B(C6F5)4] dissolved in28.2 ml toluene (B/Zr = 1.3). The catalyst solution was stir-red for 1.5 h before being metered into the reactor.

Polymerization of Ethylene

Ethylene was polymerized in a continuously operated,100 ml volume, high-pressure autoclave. The set-up used hasbeen described elsewhere.[10, 16] For the experimentsdescribed here the pressure was set at 150 l 1 MPa, the tem-perature at 180 l 38C and the residence time at 240 l 3 s. 90mol-% ethylene and 10 mol-% hexane were continuously fedinto the reactor.

Four series of experiments were carried out. Differentamounts of TiBA and TEA were added to the hexane feed,to vary the concentration of the aluminium alkyl compoundin the autoclave. Cp2ZrCl2/TiBA/DMAP and Ph2C(CpFlu)-ZrCl2/TiBA/DMAP were used as catalyst systems. Theywere preactivated outside the reactor with a Zr/Al/B ratio of1/200/1.3. The catalyst solutions were separately meteredinto the reactor.

Concentrations are given in ‘molppm’, where ‘molppm’stands for the mol fraction of a compound (xi = ni/nges) inparts per million.

Results and Discussion

Influence of [Al] in the Reactor on Productivity

The catalyst system Cp2ZrCl2/TiBA/DMAP showed thehighest productivity at a concentration of 8 molppmTiBA in the reactor, which was adjusted without addi-tional aluminium alkyl compound in the feed (Figure 1).When the concentration of aluminium in the reactor wasincreased to 40 molppm, by addition of TiBA to the feed,the productivity decreased from 1000 to 700 kg PE/g Zr.When TEA was used, the productivity dropped to 400 kgPE/g Zr. A further increase in the concentration of thealuminium alkyl compound resulted in almost no changein the productivity.

Much higher productivities were obtained with the cat-alyst system Ph2C(CpFlu)ZrCl2/TiBA/DMAP (Figure 2).With 3 mol TIBA in the reactor this catalyst system pro-duces about 2500 kg PE/g Zr. Increasing the TiBA con-

Figure 1. Influence of [Al] in the reactor on productivity ofCp2ZrCl2/TiBA/DMAP (Zr/Al/B = 1:200:1.3) in the high-pres-sure polymerization of ethylene.

Figure 2. Influence of [Al] in the reactor on productivity ofPh2C(CpFlu)ZrCl2/TiBA/DMAP (Zr/Al/B = 1:200 :1.3) in thehigh-pressure polymerization of ethylene.

18 C. Götz, A. Rau, G. Luft

centration to 10 molppm resulted in a sharp increase inproductivity. Productivity increased to 10000 kg PE/g Zr.A further increase in the TiBA concentration led to adecrease in productivity. With 420 molppm TiBA,2000 kg PE/g Zr was obtained. With additional TEA inthe reactor, productivity decreased from 2500 to 850 kgPE/g Zr.

Results obtained for both catalyst systems showed thatthe high-pressure polymerization system used hererequired a TiBA concentration of about 10 molppm in thereactor for highest productivities. Below this concentra-tion a reduced productivity was found with the catalystbased on Ph2C(CpFlu)ZrCl2, whereas with the catalystsystem based on Cp2ZrCl2 no polymerization wasobserved, if the TiBA concentration was adjusted below8 molppm. An explanation could be that the aluminiumalkyl compounds are able to act as scavengers and protectthe active centers.[2, 3, 7] When the concentration of the alu-minium alkyl compounds present in the reactor was lessthan 10 molppm there were not enough thereof to reactwith impurities, hence the active sites were consumedand productivity decreased. When the concentrations ofaluminium alkyl compounds in the reactor where higher,the productivity decreased. This could possibly be due toreactions between the active metallocene cations and theexcess aluminium alkyl compound. The most likelyexplanation is the reversible complexation of active siteswith the aluminium alkyl compound (Scheme 1).

With low concentrations of aluminium alkyl com-pounds the equilibrium shifts to the side of the freecation, resulting in high concentrations of active sites,leading to high productivities. With high concentrationsof aluminium alkyl compound the number of heterodi-nuclear complexes increases, resulting in a decrease ofproductivity. The active sites of heterodinuclear com-plexes are blocked for olefin coordination and insertion.Bochmann et al. proved that [Cp92ZrMe]+ forms heterodi-nuclear complexes of the type [Cp92Zr(l-Me)2AlMe2]+ inthe presence of TMA. The equilibrium reaction competeswith the olefin insertion.[17] Bochmann also characterizedheterodinuclear hafnium-based metallocene cations of thetype [Cp2Hf(l-Et)2AlEt2]+ by means of NMR spectro-scopy.[18] His work showed that TEA complexes betterwith the active sites than TIBA, probably due to increasedsteric hindrance. This explains the lower productivitieswith TEA than with TIBA in the reactor. Naga et. alfound polymerization results similar to ours. They alsoobtained higher productivities with ternary metallocene

catalysts if they used TiBA instead of TEA as the alumi-nium alkyl compound in the catalyst system.[12, 13]

Detailed investigations into the complexation betweenTIBA and the metallocene cation were carried out by us.Ternary catalyst systems were analyzed by NMR spectro-scopy. These results and a description of a bridged spe-cies between the active complex and TiBA in highly con-centrated catalyst solutions are presented in another pub-lication.[15]

Influence of [Al] in the Reactor on Polymer Properties

The molecular weights of the polymers were stronglyinfluenced by the concentration of aluminium in the reac-tor. As shown in Figure 3, the polyethylene producedwith the catalyst based on Cp2ZrCl2 had the highest M

—n

(52000 g/mol) with 15 mol/ppm TIBA in the reactor. Thepolymer obtained with the catalyst based onPh2C(CpFlu)ZrCl2 had the highest M

—n (42000 g/mol) with

a TiBA concentration of 3 to 10 molppm in the reactor

Scheme 1. Reversible deactivation of active metallocenecations.

Figure 3. Influence of [TiBA] in the reactor on M—

w/M—

n andmolecular weight of PE samples obtained with Cp2ZrCl2/TiBA/DMAP (Zr/Al/B = 1:200 :1.3) in the high-pressure polymeriza-tion of ethylene.

Figure 4. Influence of [TEA] in the reactor on M—

w/M—

n andmolecular weight of PE samples obtained with Cp2ZrCl2/TiBA/DMAP (Zr/Al/B = 1:200 :1.3) in the high-pressure polymeriza-tion of ethylene.

Influence of Aluminium Alkyl Compounds on the High-Pressure Polymerization ... 19

(Figure 5). Up to an [Al] of about 30 molppm the poly-mers showed narrow molecular weight distributions(MWDs), with M

—w/M

—n between 2 and 3.

With a aluminium alkyl compound concentration ofmore than 400 molppm in the reactor the molecularweights of the polymers were below 5000 g/mol and thepolydispersity was between 20 and 40 (Figure 3 to 6).Increasing the TiBA or TEA concentrations led to anincrease in the formation of waxes in the reactor, due toethylene oligomerization at aluminium centers. TheMWDs therefore became bimodal, with increasing alumi-nium concentration. Concurrently, the number of chaintransfer reactions from the zirconium to the aluminiumincreased. Both reactions resulted in a decrease in M

—n and

the formation of waxes caused an increase in polydisper-sity.

The M—

n’s of the polymers obtained with TEA wereslightly lower and the polydispersities were significantlyhigher than the molecular weights and polydispersities ofthe polymers obtained with TiBA (Figure 3 to 6). Naga et

al. found similar results in polypropylene polymerizationscatalyzed by ternary metallocene catalysts using TEAand TIBA as cocatalysts.[12]

Analysis of MWDs

The MWDs were fitted using the Schulz-Zimm distribu-tion (Equation (1)).[19]

wðNpÞ ¼X

n

Cn NPZn N eÿPyn ð1Þ

withP = degree of polymerizationw(NP) = distribution functionZn = index of dispersion

Cn = constant independent of Pyn = M

—n/(Z +1)

n = number of Schulz-Zimm distributions

The Schulz-Zimm distribution is very suitable fordescribing the bimodal (n = 2) MWDs of polymers con-taining waxes. The MWDs can be described as a superim-position of two Schulz-Zimm distributions. Figure 7shows the measured and the calculated MWDs of a poly-mer obtained with a TEA concentration of 422 mol in thereactor. By adjusting all parameters of the two Schulz-Zimm distributions by iteration, using the Levenberg-Marquardt algorithm, the experimental data could befitted. By eliminating one correlating distribution term ata time (e.g. the wax peak is described by n = 1 and thepolymer peak by n = 2) it is possible to separately calcu-late the low- and the high-molecular mass fractions of theMWD (Figure 8).

The average results of the calculations are summarizedin Table 1 and 2. The tabulated results showed that evenwhen there was a high aluminium concentration in the

Figure 5. Influence of [TiBA] in the reactor on M—

w/M—

n andmolecular weight of PE samples obtained with Ph2C(CpFlu)-ZrCl2/TiBA/DMAP (Zr/Al/B = 1:200 :1.3) in the high-pressurepolymerization of ethylene.

Figure 6. Influence of [TEA] in the reactor on M—

w/M—

n andmolecular weight of PE samples obtained with Ph2C(CpFlu)-ZrCl2/TiBA/DMAP (Zr/Al/B = 1:200:1.3) in the high-pressurepolymerization of ethylene.

Figure 7. MWD of a PE sample obtained with Cp2ZrCl2/TiBA/DMAP (Zr/Al/B = 1:200 :1.3) with [Al] = 420 molppm in thereactor and same MWD fitted by a superimposition of twoSchulz-Zimm distributions.

20 C. Götz, A. Rau, G. Luft

reactor the fraction of waxes in the polymer (the lowmolecular mass part) was very low (a0.5 wt.-%). Theamount of oligomers obtained with TEA was twice ashigh as with TiBA. This proved that TEA has a strongerinclination towards oligomerization of ethylene thanTiBA.

In all cases, the M—

n of the high molecular mass fractionof the MWDs decreased with increasing aluminium con-centration. The type of the metallocene used had a stronginfluence. The molecular weights obtained with Cp2ZrCl2

were generally lower than the molecular weights obtainedwith Ph2C(CpFlu)ZrCl2. The M

—w/M

—n ratio increased sig-

nificantly in the case of Cp2ZrCl2 and slightly in the caseof Ph2C(CpFlu)ZrCl2. The broadening of the molecularweight distributions and the decreasing molecularweights with increasing concentrations of aluminiumalkyl compound could be attributed to the increasingnumber of chain transfer reactions to the aluminium. Theresults showed that with Cp2ZrCl2, more chain transferreactions occurred than with Ph2C(CpFlu)ZrCl2. Furthermore chain transfer reactions occurred with TEA thanwith TiBA.

Estimation of Rate Constants of Chain Transfer to theAluminium

The degree of polymerization P can be formallydescribed by the ratio of the rate constants of polymeriza-tion to termination (Equation (2)).[20]

P ¼R

kP½C��½M�adt

½C�� þRðktr;b þ ktrA½Al�b þ ktrM½M�cÞ½C��dt

ð2Þ

Transformation of Equation (2) leads to Equation (3):

1P¼ ktr;b þ ktrM½M�c

kP½M�aþ ktrA½Al�b

kp½M�að3Þ

withP = degree of polymerizationkP = rate constant of polymerizationktr, b = rate constant of b-hydrogen transfer to the metalktrM = rate constant of b-hydrogen transfer to the monomer

Table 1. Average values of the high- and low molecular mass fractions of polymer samples obtained with Cp2ZrCl2/TiBA/DMAP(Zr/Al/B = 1:200 :1.3) using regressive fits of a superimposition of two Schulz-Zimm distributions.

½Al� in the reactormolppm

High molecular mass fraction Low molecular mass fractionwt.-% Mn

g=molM—

w/M—

n wt.-% Mn

g=molM—

w/M—

n

TiBA 39 99.96 24300 3.1 0.04 1400 1.2121 99.93 17400 4.0 0.07 640 1.2419 99.71 15700 4.5 0.29 490 1.3

TEA 36 99.86 21100 3.0 0.14 1800 1.3122 99.87 20000 3.7 0.13 670 1.3422 99.59 13900 6.4 0.41 550 1.3

Table 2. Average values of the high- and low molecular mass fractions of polymer samples obtained with Ph2C(CpFlu)ZrCl2/TiBA/DMAP (Zr/Al/B = 1:200:1.3) using regressive fits of a superimposition of two Schulz-Zimm distributions.

½Al� in the reactormolppm

High molecular mass fraction Low molecular mass fractionwt.-% Mn

g=molM—

w/M—

n wt.-% Mn

g=molM—

w/M—

n

TiBA 11 100 39800 2.0 – – –102 99.88 36200 2.3 0.12 750 1.3406 99.79 35300 2.2 0.21 620 1.3

TEA 20 100 27000 2.6 – – –110 99.82 25100 3.7 0.13 790 1.4410 99.60 19600 3.5 0.40 510 1.3

Figure 8. Calculated wax and polymer peaks of a MWD of aPE sample obtained with Cp2ZrCl2/TiBA/DMAP (Zr/Al/B =1:200:1.3) with [Al] = 420 molppm in the reactor using twoSchulz-Zimm distributions.

Influence of Aluminium Alkyl Compounds on the High-Pressure Polymerization ... 21

ktrA = rate constant of chain transfer to aluminium[M] = concentration of monomer[Al] = concentration of aluminium[C*] = concentration of active centersa, b, c = reaction orders

In earlier work we proved that the reaction order withrespect to the concentration of ethylene (a) is one forhigh-pressure polymerizations.[11] In all experiments theconcentration of ethylene ([M]) was kept constant. Withthe assumption that the reaction order with respect to theconcentration of aluminium (b) is also one,[12] Equation(3) can be simplified (Equation (4)).

1P¼ C þ ktrA½A�

kp½Ethylene� with C ¼ ktr;b þ ktrM½M�c

kp½M�að4Þ

In order to estimate the rate constant of chain transferto the aluminium, only the high molecular mass fractionsof the MWDs were considered. The low molecular masswaxes were not included in the calculation. It wasassumed that the M

—n of the high molecular mass fraction

only decreased by chain transfer reactions to the alumi-nium. From a linear fit of the reciprocal degree of poly-merization (1/P) of the high molecular mass fraction ver-sus the concentration of aluminium in the reactor ([Al]),the ratio ktrA/kP could be estimated (Table 3 and Figure 9).

The ratios of insertion of ethylene to chain transfer toaluminium in Table 3 show that ethylene insertion ismuch faster than chain transfer to the aluminium. Asmentioned earlier the types of metallocene catalyst andaluminium alkyl compound used in the reactor have astrong influence on the number of chain transfer reac-tions. Catalyst systems based on Ph2(CpFlu)ZrCl2 morestrongly suppress chain transfer reactions than catalystsystems based on Cp2ZrCl2. More chain transfer reactionsoccur with TEA than with TiBA.

Again, similar results were found by Naga et al. Forpropylene polymerizations with the catalyst systemPri(CpFlu)ZrCl2/AlR3/[Ph3C][B(C6F5)4] at 408C and pres-sures below 1 MPa, they found ratios of propylene inser-tion to chain transfer of 20400 using TiBA and 2000 to3000 using TEA, as cocatalyst.[12] Chien et al. investi-gated the polymerization of ethylene with the catalyst

system Cp2ZrCl2/MAO at 708C and pressures below 1MPa. They found ratios of ethylene insertion to chaintransfer to the aluminium between 14000 and 20100,depending on the concentrations of ethylene and alumi-nium.[21]

ConclusionsThe results show that the type and the concentration ofthe aluminium alkyl compound as well as the type of themetallocene catalyst used in the reactor during the high-pressure polymerization of ethylene with ternary metallo-cene-based catalyst systems had a strong influence on thepolymerization of ethylene and the resultant polymerproperties. Both ternary catalyst systems, Cp2ZrCl2/TiBA/DMAP and Ph2C(CpFlu)ZrCl2/TiBA/DMAP,showed higher productivities with TiBA than with TEAin the reactor. Below a TiBA concentration of 10 molppmin the reactor, catalyst systems based on Cp2ZrCl2 showedno productivity while catalyst systems based onPh2C(CpFlu)ZrCl2 showed reduced productivity. Thehighest productivities were obtained with TiBA concen-trations in the range of 10 molppm. Ph2C(CpFlu)ZrCl2

was then twice as productive as Cp2ZrCl2. With increas-ing concentration of the aluminium alkyl compounds, the

Table 3. Estimation of kp/ktrA for the high pressure polymerization of ethylene based on the calculated high molecular mass fractionsof the obtained polymer samples.

Cp2ZrCl2/TiBA/DMAP b) Ph2(CpFlu)ZrCl2/TiBA/DMAP b)

TEA TiBA TEA TiBA

[ethylene]ma)/(mol/l) 11.7 10.3 11.6 11.4

ktrA/(kP N [ethylene]m)/(l/mol) 1.84 N 10–6 1.37 N 10–6 1.00 N 10–6 1.88 N 10–7

kp/ktrA 45500 71400 83300 500000

a) Averaged ethylene concentration.b) Zr/Al/B = 1:200 :1.3.

Figure 9. Influence of [Al] in the reactor on 1/P with P = aver-aged calculated degree of polymerization of the high molecularmass fraction (polymer peak) of the MWDs of the PE samples.

22 C. Götz, A. Rau, G. Luft

productivities decreased. This was attributed to the for-mation of heterodinuclear complexes (dormant sites) con-sisting of active species and the aluminium alkyl com-pounds. When TEA was used, the productivities of bothcatalyst systems dropped to a constant level, even withlow concentrations of TEA in the reactor. Ph2C(CpFlu)-ZrCl2 was then twice as productive as Cp2ZrCl2.

The molecular weights obtained with TiBA wereslightly higher than the molecular weights obtained withTEA. With aluminium alkyl compound concentrations ofabout 10 mol in the reactor both catalyst systems pro-duced polymers with the highest molecular weights withnarrow MWDs. Higher amounts of aluminium alkyl com-pound led to a decrease in molecular weights and anincrease in polydispersity, due to formation of waxes andchain transfer processes to the aluminium.

The bimodal MWDs were described using a superim-position of two Schulz-Zimm distributions. The contentof waxes in the polymer samples was determined usingthe calculated MWDs. Even with high aluminium con-centrations the fraction of waxes was below 0.5 wt.-%.With TEA the low molecular mass fraction was twice ashigh as the low molecular mass fraction obtained withTiBA. From the calculated high molecular mass fractionof the MWDs, the ratios of the rate constants of insertionof ethylene to chain transfer to the aluminium were esti-mated. The type of the aluminium alkyl compound andthe ligand system of the metallocene had a strong influ-ence on the number of chain transfer reactions. The chaintransfer reactions to the aluminium increased accordingto the following combinations of metallocenes and alumi-nium alkyl compounds: Ph2C(CpFlu)ZrCl2/TiBA a

Ph2C(CpFlu)ZrCl2/TEA a Cp2ZrCl2/TiBA a Cp2ZrCl2/TEA. Compared to chain termination by transfer to themonomer and b-H transfer to the transition metal, chaintransfer to the aluminium played a minor role in high-pressure polymerizations.

Acknowledgement: The authors wish to thank the Bundesmi-nisterium für Bildung und Forschung and BASF AG for financialsupport.

Received: June 25, 2001Revised: September 6, 2001

Accepted: September 6, 2001

[1] W. Tsai, M. D. Rausch, J. C. W. Chien, Appl. Organomet.Chem. 1993, 7, 71.

[2] J. C. W. Chien, W. Song, M. D. Rausch, Macromolecules1993, 26, 3229.

[3] J. C. W. Chien, W. M. Tsai, Makromol. Chem., Makromol.Symp. 1993, 66, 141.

[4] W. Tsai, J. C. W. Chien, J. Polym. Sci., Part A: Polym.Chem. 1994, 32, 149.

[5] J. C. W. Chien, W. Song, M. D. Rausch, J. Polym. Sci.,Part A: Polym. Chem. 1994, 32, 2387.

[6] N. Naga, K. Mizunuma, Macromol. Rapid. Commun. 1997,18, 581.

[7] C. Pellecchia; D. Pappalardo, J. A. M. van Beek, Macro-mol. Symp. 1995, 89, 335.

[8] A. Yano, S. Sone, S. Yamada, S. Hasegawa, A. Akimoto,Macromol. Chem. Phys. 1999, 200, 917.

[9] A. Yano, S. Hasegawa, S. Yamada, A. Akimoto, J. Mol.Catal. A: Chem. 1999, 148, 77.

[10] C. Götz, G. Luft, A. Rau, S. Schmitz, Chem. Eng. Tech.1998, 12, 954.

[11] A. Rau, S. Schmitz, G. Luft, Chem. Eng. Tech., in press.[12] N. Naga, K. Mizunuma, Polymer 1998, 39, 5059.[13] N. Naga, T. Shiono, T. Ikeda, J. Mol. Catal. A: Chem.

1999, 150, 155.[14] H. H. Brintzinger, S. Beck, J. Suhm, R. Mülhaupt, Macro-

mol. Rapid Commun. 1998, 19, 235.[15] A. Rau, C. Götz, G. Luft, J. Mol. Catal. A: Chem. 2002,

3504, 1.[16] G. Luft, A. Rau, A. Dyroff, C. Götz, S. Schmitz, T. Wiec-

zorek, R. Klimesch, A. Gonioukh, in: “Metalorganic Cata-lysts for Synthesis and Polymerisation”, W. Kaminsky,Ed., Berlin 1999, p. 651.

[17] M. Bochmann, S. J. Lancaster, Angew. Chem. 1994, 106,1715.

[18] M. Bochmann, S. J. Lancaster, Organomet. Chem. 1995,497, 55.

[19] R. H. Boyd, P. J. Phillips, in: “The science of polymermolecules”, R. H. Boyd, Ed., Cambridge Univ. Press 1993,p. 18–59.

[20] P. J. Flory, “Principles of Polymer Chemistry”, CornellUniversity Press, Ithaca, NY 1986.

[21] J. C. W. Chien, B. P. Wang, J. Polym. Sci., Part A: Polym.Chem. 1990, 28, 15.