Macromol. Chem. Phys. 191, 155-163 (1996) 155

Gas-phase versus slurry copolymerization of ethylene with 1-butene over MgC12-supported titanium catalysts after prepolymerization

Qing Wu, Haihua Wang, Shangan Lin

Institute of Polymer Science, Zhongshan University, Guangzhou 510275, China

(Received: January 9, 1995; revised manuscript of May 2, 1995)

SUMMARY Gas-phase copolymerizations of ethylene with 1-butene over MgC1,-supported titanium

catalysts after prepolymerization were studied. The activity of a catalyst treated by ethylene/ 1 -butene prepolymerization “prep-cat-2” for the copolymerization is not affected by the monomer composition in the range of 0- 10 mol-% of 1-butene, but it is higher than that of the homopolymerization of ethylene with a catalyst treated by ethylene prepolymerization “prep-cat-I ”. Comparative studies of the polymerizations in the gas- phase and in slurry reactors showed differences in the rate-time profiles. The higher activities in the early stage and declining kinetics for the gas-phase polymerizations can be considered to be due to the higher macroparticle diffusivity and to fast deactivation of the active centres which has been confirmed by the determination of the oxidation state of titanium in the catalyst. Data from determinations of monomer reactivity ratios indicated that the gas- phase process favours the incorporation of 1-butene into the polymer chains.

Introduction

Gas-phase polymerization is one of the most important methods for the production of polyolefins. The significant advantages of low-energy consumption, unnecessity of using solvent that must be removed from the products from slurry and solution processes, and simpler technology over the other processes make it increasingly being used in commercial production ‘ 3 ,).

Semibatch slurry reactors are most commonly used in laboratory studies to assess catalysts. However, a prediction of the activity of a catalyst for gas-phase polymeriza- tion using the results from slurry polymerization could be incorrect. The differences in catalyst behaviour between gas-phase and slurry processes could result from various causes such as differences in activation and deactivation rates of the active centres by a l k y l a l ~ m i n i u m ~ * ~ ) and differences in heat and mass transfer in the particles of the catalyst 5 , 6 ) .

In the previous paper ’), we had reported homo- and copolymerizations of ethylene with prepolymerized MgC1,-supported catalysts in a slurry reactor. It was found that the catalyst prepolymerized with ethylene/l-butene exhibited higher activities than the catalyst prepolymerized only with ethylene. The prepolymerized catalysts are also effective in gas-phase polymerization of ethylene. In this paper, gas-phase homo- and copolymerizations of ethylene with 1 -butene with prepolymerized catalysts are reported. The results are also compared with those from slurry polymerizations.

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156 Q. Wu, H. Wang, S. Lin

Experimental part

Catalyst preparation and prepolymerization

previously7). The procedures of catalyst preparation and prepolymerizations have been given

Polymerization

Gas-phase polymerizations were carried out in a 250 mL glass reactor equipped with an external water jacket for temperature control and a mechanical agitator with polytetra- fluoroethylene (PTFE) blades. The agitator was set on the bottom of the reactor, in order to suspend the catalyst-polymer particles in the reactor. No agglomerations of the particles on the blades and reactor walls were found during the courses of the polymerization. The prepolymerized catalyst containing 0.2 - 0.5 mg titanium and triisobutylaluminium (A~(~-BLI)~, mole ratio N/Ti = 100) was introduced in an N, atmosphere. After removing N, via vacuum, the copolymerization of ethylene with I-butene was initiated by introduc- tion of the monomer mixture (mole ratio of monomers M ) at definite temperature and a total pressure of 120 kPa. The gaseous monomer mixture (mole ratio M') in a reservoir was fed to replenish the consumed monomers. M' is approximately equal to the composition of the corresponding copolymer so that M in the reactor can be maintained at a certain value throughout the course of the copolymerization. The polymerizations were terminated with a 2 molar HCI in C,H,OH solution. The polymer was washed with ethanol and dried in a vacuum. The slurry polymerizations were carried out according to the method described previously 7).

Measurements

The oxidation states of titanium were determined by means of redox titrations 8, after reaction of the catalyst with A1 (i-Bu)3 for 60 min and washing with hexane. Melting points (T,) and degrees of crystallinity of the polymers were determined by means of differential scanning calorimetry (CDR-1 thermal analyzer, Shanghai Balance Factory) at a scanning rate of 10 " C h i n . Chemical compositions and monomer sequence distributions of the copolymers were obtained from 13C NMR at 120 "C using a JEOL FX-90Q spectrometer operated at 22.5 MHz. Solutions were made in o-dichlorobenzene (concentration 15 g/100 mL). Chemical shift assignments for 13C resonances were based on those proposed in the literatureg).

Results and discussion

Gas-phase polymerization

Ethylenell -butene copolymerizations were performed over the prepolymerized catalysts with 3 - 10 mol-Vo of I-butene in the monomer feed (the I-butene content in the reactor was much higher) for the preparation of linear low-density polyethylene (LLDPE). The results are shown in 'Ittb. 1. The catalytic activities for the copolymeri- zations essentially d o not change with monomer composition (runs 1-4). Melting points (T,) and degrees of crystallinity of the products decrease remarkably with in- creasing content of 1 -butene, indicating that the copolymerizations were effective and LLDPE's were obtained. Keeping the content of I-butene at 5 mol-Vo, the polymeriza-

Gas-phase versus slurry copolymerization of ethylene with 1-butene . . . 151

Tab. 1. merized catalysts a)

Results of gas-phase copolymerizations of ethylene with 1-butene with prepoly-

Run Prepolymerized 1-Butene content') Tp d, Activitye) T, f, Degree of - - oc crystallinityg)

in Vo "C catalyst b, in mol-Vo

1 2 3 4 5 6 I 8 9

10 11 12

prep-cat-2 (1) prep-cat-2 (2) prep-cat-2 (3) prep-cat-2 (4) prep-cat-2 (2) prep-cat-2 (2) prep-cat-2 (2) prep-cat-2 (2) prep-cat-2 (2) prep-cat-1 prep-cat-1 prep-cat-1

3 5 I

10 5 5 0 0 0 0 0 0

I0 22,4 I0 23,9 I0 23,6 I0 24,l 50 16,4 90 26,l 50 14,3 I0 22,2 90 25,2 50 5,7 I 0 15,6 90 21,3

130 62 126 49 123 41 122 35 121 50 126 48 131 I1 139 19 138 80 139 19 138 80 138 80

a) Polymerization conditions: mass of Ti = 0.5 mg; mole ratio AI/Ti = 100; total pressure = 121 kPa.

b, Catalyst prep-cat-1 was prepared in the absence of 1-butene; prep-cat-2 contains 1-butene units: 3 mol-Vo (prep-cat-2 (l)), 5 mol-Vo (prep-cat-2 (2)), 7 mol-Vo (prep-cat-2 (3)), 10 mol-Vo (prep-cat-2 (4)).

c, In monomer feed. d, Polymerization temperature. e, In units kg polymer per g Ti per h. f, Melting temperature measured by means of differential scanning calorimetry (DSC). g) Calculated from heat of fusion AHf.

tion activities increase with raising the polymerization temperature from 50 "C to 90 "C (runs 2, 5 , 6) and reach 26.7 kg polymer per Ti per h at 90°C. This result of an increasing polymerization rate constant k, is in accordance with an Arrhenius dependence.

Homopolymerizations of ethylene with prepolymerized catalyst containing 5 mol-Vo of I-butene units (prep-cat-2 (2), runs 7-9) gave almost the same activities as those of the copolymerizations at the same temperature. But the products yielded high melting temperatures T, (137 "C- 139 "C) and high degrees of crystallinity (17% - 8O%), i. e. the value of high-density polyethylene (HDPE).

However, lower activities were obtained in ethylene homopolymerizations with catalyst prep-cat-1 prepared without 1-butene (runs 10- 12) than in both ethylene homopolymerizations and copolymerizations with prep-cat-2. Such differences can be attributed to the formation of a porous structure of prep-~at-2~), in favour of monomer diffusion through catalyst particles, whilst in the homopolymerizations with prep-cat-I resistance to monomer diffusion may exist to some extent depending on polymerization temperature.

158 Q. Wu, H. Wang, S. Lin

Time dependences of the polymerization rates are shown in Fig. 1. Gas-phase polymerization rates show time dependences of the decay-type. The highest rates appear at the beginning of the polymerizations and then they decay with time, except for the polymerization with prep-cat-1 at low temperature.

1 I

- Time of polymerization in min - 0 30 60 30 60 30 60

Fig. 1. Rate of gas-phase homo- and copolymerizations at 50°C (a), 70 "C (b), and 90 "C (c). (1): Copolymerizations with 5 mol-Vo of 1-butene in the feed with catalyst prep-cat-2 (2); (2): ethylene homopolymerizations with catalyst prep-cat-2 (2); (3): ethylene homopoly- merizations with catalyst prep-cat-l . (For catalyst abbreviations see Tab. 1)

Comparison with slurry polymerization

In order to compare the activities in gas-phase polymerizations with those of slurry polymerizations, it is necessary to normalize the activities with respect to monomer concentrations which can be very different for these two processes. These activities are expressed as kilograms of polymer per gram of titanium and per mole of monomer in one litre per hour. Tab. 2 shows the results from gas-phase polymerizations and slurry polymerizations in octane. In a slurry process the copolymerizations with prep- cat-2 (2) gave higher activities than the homopolymerizations with prep-cat-I by a factor of 5,3 at 50"C, 2,9 at 70°C and 1,7 at 90"C, respectively, while for a gas-phase process the corresponding factors are only 2,9 at 50 "C, 1 3 at 70 "C and 1,l at 90 "C.

The effect of rate-enhancement of a-olefins upon the polymerization of ethylene in slurry processes over Ziegler-Natta catalysts has been frequently reported in the literature For an ethylene homopolymerization in a slurry process, a rate limita- tion can arise from the resistance of monomer diffusion through the macropores between the microparticles. This rate limitation can be quite obvious in the early stage of the polymerization and sometimes over a longer period. Addition of a small amount

Gas-phase versus slurry copolymerization of ethylene with 1-butene . . . 159

Tab. 2. Comparison between gas-phase polymerization and slurry polymerization

Medium Tp a) Activity b, F __

with l-butened) without 1-butene') "C

octane 50 205 70 869 90 1207

monomer 50 363 gas 70 562

90 664

38,7 296 73 1

126 367 580

a) Polymerization temperature. b, Activities (in units kg polymer per g Ti per mc.. L per h) were normalizec Rith respect

to monomer concentration. c, Ratio of activities of polymerization with 1 -butene to activities of polymerization

without I-butene. d, Polymerization with prep-cat-2 (2); (see Tab. 1). e, Polymerization with prep-cat-1; (see Tab. 1).

of I-butene favours the formation of a porous structure of the polymer-catalyst particles, resulting in an obvious enhancement of the activity. On the contrary, minor rate-limitation occurs in a gas-phase polymerization due to the high values of the particle diffusivity6), viz. 2 - 3 orders of magnitude higher than in a slurry process. Hence, the activities of gas-phase polymerizations are not so sensitive to the additions of 1 -butene comonomer.

As shown in Fig. 2, a higher rate of ethylene homopolymerizations in the early stages of gas-phase processes as compared with slurry processes can be clearly observed, while the rates in the beginning of gas-phase copolymerizations are very close to those of slurry copolymerizations, as shown in Fig. 3. The reason is the same as mentioned above. The lower rates in the beginning of slurry homopolymerizations cannot be due to low concentration or slow diffusion of Al(i-Bu), at the conditions used herein, because the rates in the beginning of the polymerizations did not respond to an Al/Ti mole ratio increasing from 100 to 300 with fixed Ti concentration.

Generally gas-phase polymerization rates decay rapidly with time reaching a level below those of slurry polymerization rates in the later stages of polymerization. This might result from an increasing difficulty of monomer diffusion through the polymers growing around the active sites or from a deactivation of the active centres by the alkylaluminium. An experiment with delayed addition of the monomer gave evidence that there is no diffusion resistance responsible for the decay4), in which the rate of polymerization for a delayed addition of the monomer was in accordance with that in the later stages of a conventional polymerization.

In order to verify the effect of alkylaluminium on rate versus time profiles, the oxidation states of Ti ions in the catalyst activated by Al(i-Bu), at the conditions of gas-phase and of slurry processes were determined. Tab. 3 illustrates the distributions

160 Q. Wu, H. Wang, S. Lin

c

0 ' 0 20 LO 60 80

Polymerization time in min

Fig. 2.

al +

2 1200 C 0

0 N

.- 1

.- 5 900 - 0 n

600

300

0- 0 20 LO 60 80

Polymerization time in min

Fig. 3.

Fig. 2. Comparison of rate vs. time dependences between gas-phase and slurry homopoly- merizations of ethylene with prep-cat-1. G1: gas-phase polymerization at 70 "C. G2: gas- phase polymerization at 90°C. S1: slurry polymerization at 70 "C. S2: slurry polymeriza- tion at 90°C. (Polymerization rate in units mol monomer per g Ti per mol/L per h)

Fig. 3. Comparison of rate vs. time dependences between gas-phase and slurry ethylene/l-butene copolymerizations with 5 mol-Vo 1-butene content in the feed with prep- cat-2 (2). G1: gas-phase polymerization at 70 OC. G2: gas-phase polymerization at 90 "C. S1: slurry polymerization at 70 "C. S 2 slurry polymerization at 90 "C. (Polymerization rate in units mol monomer per g Ti per mol/L per h)

Tab. 3. of gas-phase and slurry reactions a)

Oxidation states of titanium activated by triisobutylaluminium for the conditions

Medium Temp. Contents of in "C

Ti4+ Ti3+ Ti2+ in mol-Vo in mol-Vo in mol-Vo

NZ 50 03 34,O 65,2 90 0,7 26,5 72,8

octane 50 27,9 50,4 21,6 90 22,8 46,4 30,8

a) See Experimental part.

Gas-phase versus slurry copolymerization of ethylene with 1-butene . . . 161

of the oxidation state of titanium ions at different temperatures. More titanium ions were reduced to the lower oxidation state (Ti2+) in gas-phase processes than in slurry processes. Ti2+ ions are believed to be inactive in a-olefin polymerization but active in ethylene polymerization 11* 15). However, Ti2+ exhibited a much lower activity for the polymerization of ethylene than Ti3+ ions. As reported by Kashiwa et al.") the catalytic activity was reduced to 16% of that of the original catalyst when a MgC1,- supported catalyst was pretreated with triethylaluminium and 80% of the total titanium is present as Ti2+.

Monomer sequence distribution

The monomer sequence distributions of the ethylene/l-butene copolymers were measured using l3C NMR spectroscopy analysis. The spectra of ethylene/ 1-butene copolymers prepared by gas-phase and by slurry processes are shown in Fig. 4. The diad and triad distributions are determined according to Randall g).

The monomer reactivity ratios r , and r2 can be calculated by the following equations:

2@E) r, = - x . (EB)

2 x . (BB) @B)

r2 =

LO 30 20 10

Fig. 4. 13C NMR spectra of ethylene/ I-butene copolymers. (A): Prepared by gas- phase polymerization; (B): prepared by slurry polymerization

6 in ppm

162 Q. Wu, H. Wang, S. Lin

where x is the mole ratio of ethylene to I-butene in the reactor. (E), (B), (EE), (EB) and (BB) represent the mole fractions of the respective sequences of monomeric units from ethylene (E) and I-butene (B). The number-average sequence lengths of ethylene and of I-butene, f i , and ii,, respectively, are derived from Eqs. (3) and (4):

Monomer sequence distributions and calculated reactivity ratios rl and r2 for two copolymer samples separately from gas-phase and slurry polymerization are listed in Tab. 4. The data of rl and r2 indicate that the gas-phase process is more favourable to an incorporation of I-butene monomer into the polymer chain as compared with the slurry process. The products of reactivity ratios show that the polymerization products are random copolymers with somewhat of a block character.

Eib. 4. Monomer sequence distribution parameters and reactivity ratios rl and r2 of ethylenell-butene copolymers from gas-phase and slurry processes (Numerical values of sequences expressed as mole fractions)

Sequencea) parameters

Sample 1 b, Sample 2c)

0,938 0,062 0,884 0,109 0,007 0,841 0,086 0,009 0,001 0,012 0,05 1

1,12

0,091 2,08

17,2

22,9

0,931 0,069 0,870 0,121 0,008 0,820 0,100 0,009 0,002 0,012 0,056

1,13

0,104 1,90

15,4

18,2

a) E monomeric unit from ethylene. B: monomeric unit from 1-butene; iE and ii, are

b, Prepared by slurry polymerization. ') Prepared by gas-phase polymerization.

number-average sequence lengths of E and B units, resp.

Gas-phase versus slurry copolymerization of ethylene with 1-butene . . . 163

Financial support of this research from the National Natural Science Foundation of China and the Chinese Petrochemicals Corporation is gratefully acknowledged.

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