effect of ethyl benzoate on the microstructure distributions of olefin homopolymers and copolymers...

Makmmol. Chem. 191,89-97 (1990) 89

Effect of ethyl benzoate on the microstructure distributions of olefin homopolymers and copolymers prepared with supported titanium catalyst

Qing Wu and Nan-Loh Yang*

City University of New York, Chemistry Department, College of Staten Island, Staten Island, NY 10301

Shangan Lin

Institute of Polymer Science, Zhongshan University, Guangzhou, P. R. China

(Date of receipt: February 1, 1989)

SUMMRY: The microstructure distributions of ethylene-propene (EP) copolymers and propene homopoly-

mers prepared with supported titanium catalyst were analyzed by 13C NMR spectroscopy. A two- site model with an isospecific site and a non-stereospecific site was used to describe the stereospecific polymerization of propene. A second two-site model with individual r,r2 = 1 was applied to the analysis of the ethylene-propene copolymerization. The addition of an external base, ethyl benzoate (EB), alters the relative concentrations of catalytic sites. Based on the comparison of the results from the homopolymers and copolymers prepared under different EB concentrations, the correlations between the stereospecificity and the activity toward the monomers were established the isospecific site produces the copolymer with lower ethylene content; the non-stereospecific site gives the copolymer with higher ethylene content.

Introduction

A series of attempts has been made to use MgCl, as the carrier for TiCl, by various methods for the stereospecific polymerization of a-olefins I). However, a catalyst obtained simply by reacting MgCl, with TiCl, and activating with alkylaluminium is poor in stereospecificity1*2) although it has a high catalytic efficiency. One or more Lewis bases are needed to enhance the stereospecificity of the catalyst. Investigations to expound the roles of Lewis base on Ziegler-Natta catalysts have been reported. The main conclusions from various hypotheses on its roles have been summarized by Chien et al. 3, and Galli et al. 4): selective poisoning of less stereospecific active sites, aug- menting the number and/or the yield of isotactic sites, complexing and removal of inhibitors from the catalyst surface, decrease of alkylaluminium reducing power and stabilization of the structure and chirality of the active center.

It is well known that polypropylene prepared with a Ziegler-Natta catalyst can be fractionated into several fractions having different tacticities 5) . The difference in the tacticities of the fractions is attributable to the presence of multiple catalytic sites having different stereospecificities. Several statistical models have been suggested to interpret the mechanism of stereospecific polymerization of propene. Among these, a well accepted model is the mixed statistical model7v8). In this model, a stereospecific propagation at an isospecific site proceeds in obedience to asymmetric Bernoullian

0 1990, Hilthig & Wepf Verlag, Base1 CCC 0025-1 16X/90/$03.00

90 Q. Qu, N.-L. Yang, S. Lin

statistics and a second propagation at a non-stereospecific site following symmetric Bernoullian statistics (for a general discussion on polymerization statistics, see ref. 9)).

Based on experimental NMR data of the diad and triad distributions of ethylene- propene (EP) copolymers, a multisite model has been suggested for describing the copolymerization systems catalyzed with heterogeneous catalysts. In a previous paper Is), we analyzed the monomer sequence distribution of EP copolymer prepared with a supported titanium catalytic system in the absence of an external base. The result showed that a multisite Bernoullian model is suitable for the description of the copolymerization system. There exist two types of catalytic sites having different activities toward monomers in the catalytic system.

Characterizations of the EP copolymers obtained over a TiCI,/MgCl,-supported catalyts with ethyl benzoate (EB) as the external base have been reported 16*1’).

However, the correlation between stereoselectivity and activity toward comonomers for each catalytic site still remains to be established clearly. The purpose of this work is to develop this correlation. For this purpose, suitable statistical models are used to analyze the experimental data of 13C NMR spectra of propene homopolymers and EP copolymers prepared with a TiCI,/MgCl,-supported catalyst in the presence of different concentrations of external EB. The effects of external EB on the model parameters for both systems are compared.

Experimental part

Cutulyst: Supported catalyst was prepared by comilling TiCI, with anhydrous MgCl, and ethyl benzoate (EB, internal base). The supported catalyst (0,Ol mmol of Ti) was mixed in situ with AEt, (1,5 mmol) and a desired amount of EB (external base) when copolymerization was started.

Polymerizatiorr Polymerization of propene was carried out in a 250 ml glass reactor in heptane solvent at 55°C for 10 min. Copolymerization w a s performed following an established method under the same conditions as homopolymerization. After polymerization, the products were poured into an alcohoVhydrochloride solution to precipitate the polymer and remove catalyst residues. The products were then washed with ethanol and dried for analysis.

13NMR spectra‘ They were obtained on a Bruker-200 pulse Fourier transform NMR spectro- meter operated at 50 MHz and a sample temperature of 120°C. Experimental procedure and instrument conditions have been described in a previous paper ”). Chemical shift assignments for 13C resonances were based on those proposed in literature 19). Pentad tacticity was determined from the area of the resonance peaks of the methyl region. Diad and triad concentrations of the copolymers were calculated from the equations reported la).

Results and discussion

Aolypropylene tacticity

The experimentally determined pentad tacticity values for three homopolymers prepared with different ratios of external ethyl benoate (EB) to AlEt, are shown in ’Itib. I. The trend of the increase in concentration of mmmm with the increase of this ratio is apparent.

The data were analyzed through the application of a suitable statistical model. There are two general alternatives in explaining the behaviour of stereospecific catalysts. The

Effect of ethyl benzoate on the microstructure distributions of olefins homopolymers . . . 91

’hb. 1. Observed and calculated values (E/B model, see text) of pentad tacticities for propene homopolymers

Mole ratio EB/AlEt, mmmm

mmmr

nnmr

mmrr

mmrm + rmrr

rmrm

rrrr

rrrm

mrnn

ss . lo4=)

0

0,512 0,513 0,098 0,085 0,027 0,017 0,080 0,099 0,084 0,082 0.030 0,034 0,060 0,058 0,055 0,062 0,054 0,050 7,29

020

0.692 0,693 0,081 0,066 0,018 0,008 0,057 0,073 0,040 0,041 0,015 0,016 0,035 0,035 0,03 1 0,032 0,03 1 0,037 6,27

0,30

0,857 0,858 0,034 0,032 O,OO!J 0,005 0,030 0,034 0,016 0,022 0,009 0,010 0,011 0,009 0,011 0,014 0,012 0,017 3900

Sample PPI PP2 PP3

obs. calc. obs. calc obs. CalC. obs. calc. obs. Calc. obs. calc obs. calc. obs. calc obs. calc.

a) Sum of the squares of deviations.

configuration of the adding monomer is determined by the asymmetry of either the growing chain end units or the catalyst m). Based on 13C NMR analysis2’), the isotactic propagation of the polypropylene chain is shown to be controlled by the asymmetry of the catalytic complexes; the syndiotactic propagation, however, by the asymmetry of the end unit in the growing chain. The isotactic propagation process can be described by the enantiomorphic model assuming the presence of an equal number of R- and S- preferring site, i.e. the “E-sites”. The atactic propagation can be described by the symmetric Bernoullian model with “B-sites” where the addition of the monomer to the growing chain end is assumed to be influenced only by the last unit of the growing chain. A two-site mixture model (E/B model) has been for describing the total behavior of stereospecific polymerization of propene over a heterogeneous Ziegler-Natta catalyst because the original polypropylene can be fractionated into at least two fractions with different tacticities. The examinations of a number of propene samples with widely different tacticities supported the two-site modelg).

Therefore, the two-site model was used in the present study of polypropylene. For this model, three parameters, ‘fh, a and 6, are needed. Here, *Ih is the mole fraction of propene polymerized at isospecific sites (enantiomorphic sites), and a is the probability to select an R-configuration at an S-preferable site and B is the probability to select a meso-diad configuration at the non-stereospecific site. These model

92 Q. Wu, N.-L. Yang, S. Lin

Mole ratio EB/AlEt, 9-a) a O C )

parameters can be obtained by fitting the experimental data of 13C NMR spectra to the theoretical expressions. The theoretical values for all of the pentad tacticities were compared to the observed values. If a discrepancy was found, the parameters were changed. This procedure was repeated iteratively until the minimum of the sums of the squares of deviations (SS) between the predicted and observed values for all of the pentads was obtained. For three samples prepared with and without external EB, the calculated values of pentad tacticities are shown in 'hb. I. The predicted values from the two-site model are in good agreement with the experimental values. The low values of the sums of the squares of deviations indicate validity of the two-site model for the catalytic system.

The optimized model parameters are listed in 'Islb. 2. As shown by the model parameters, the fraction of isotactic polypropylene, ' f h , increases with enhancing EB/AlEt, ratio. In the meantime, the probability selecting isotactic configuration at isospecific site, a, and the probability selecting meso-diad configuration at nonstereo-

0 0,20 430 0,712 0,855 0,916 0,935 0,959 0,986 0,337 0,308 0,401

Tab. 2. Calculated parameters from E/B model for propene homopolymers

Sample PP2 PP3

specific site, a, increase. This indicates that external EB selectively inactivates the non- stereospecific sites and participates directly in the formation of a part of the isospecific sites"). In addition, the external EB can stabilize the structure of isospecific sites. As proposed by Busico et al. 23) and Soga et al. external EB can decrease the extent of the reaction of alkylaluminium with the internal base. This reaction can lead to the formation of non-stereospecific site during the polymerization.

Effect of external EB on EP copolymerization

As shown in the previous section, the addition of external EB alters the concentration ratio between propene homopolymers having different tacticities. Therefore, the additon should affect the compositional distribution of copolymerization products. In a previous paperlS), the suitability of two-site model with individual r,rz = I (BB model, i. e. both sites Bernoullian) was established for the EP copolymerization over the supported titanium catalyst without external EB. In the present study, four EP copolymers prepared with different EB/AIEt, ratios are used for analyses. The triad


ratios, EEP/EPE and PPE/PEP, for all the samples are close to 2, characteristic of multisite Bernoullian model. Therefore, it is reasonable to fit the experimental data of diad and triad distribution of the EP copolymers to this model. According to the two- site model, three parameters, %, 'E and 'E, are needed to decribe the diad and triad distribution. Here, *fC is the mole fraction of monomers polymerized at low- ethylene-activity site and 'E is the polymerization probability of ethylene monomer at site i. 'hb. 3 shows the calculated values and observed values of diad and triad

Tab. 3. Observed and calculated values (BB model) of diads and triads for ethylene-propene (EP) copolymersa)

Sample I 3 El E2 E3

Mole ratio EB/AIEt, Conc. of E in mol-Vo EE

EP

PP

EEE

EEP

EPE

PEP

PPE

PPP

ss.

obs. Calc. obs. calc. obs. calc. obs. calC. obs. calc. obs. Calc. obs. calc. obs. calc. obs. Cdc.

0

45,4 0,271 0,279 0,366 0,360 0,362 0,361 0,193 0,187 0,176 0,183 0,098 0,092 0,101 0,088 0,162 0,177 0,270 0,273 6,lO

0,lO

43,6 0,257 0,262 0,358 0,355 0,385 0,383 0,184 0,180 0,160 0,165 0,088 0,082 0,101 0,095 0,182 0,190 0,286 0,288 2,19

0,20

41,7 0,248 0,249 0,337 0,335 0,414 0,416 0,176 0,176 0,143 0,147 0,077 0,073 0,095 0,094 0,187 0,189 0,322 0,321 0,61

0,30

38,4 0,227 0,225 0,3 14 0,314 0,459 0,461 0,156 0,159 0.127 0,133 0,073 0,067 0,095 0,090 0,177 0,180 0,372 0,371 1.30

a) All samples were prepared with a monomer mole ratio E/P of 0,107 in the polymerization

b, Sum of the squares of deviations. medium.

distributions, indicating a good fit to the model. Therefore, it is appropriate to compare the results from the homopolymers and the copolymers based on the model parameters obtained.

The optimized model parameters for four copolymerization samples are shown in lhb. 4. Increase in EB/AIET, ratio from 0 to 0,3 decreases the total ethylene incorporation into the copolymerization product from 45,4 mol-% to 38,4 mol-% and alters the relative portion of the copolymers produced at different sites. With an

94 Q. Wu, N X . Yang, S. Lin

Mole ratio EB/AIEt3 Cone of E in mol-% p i a)

1j)) ZEC)

Sample 3 El E2 E3

0 0.10 0,20 0,30

2,93 3909 3,62 4,23 0,452 0,544 0,608 0,630 0,696 0,732 0,758 0,750

45,4 43,4 41,7 38,4

increase in EB concentration, the fraction of copolymer with low ethylene content increases. A comparison of the tendencies in the change in 'fvalues with the EB/A1Et3 ratio between homopolymerization and copolymerization suggests that the isospecific sites have a lower activity toward ethylene monomer while the non-stereospecific sites give the copolymer with a high ethylene content.

The above suggestion is also supported by considering the results from the triad tacticity data of the copolymers. The triad tacticity index, expressed as the concentration ratio of the (PPP),, methyl, 6 = 22,O - 21,l to the PPP methine, 6 = 28,7 - 28,3, are shown in lhb. 5. For the multiple site model with individual r,rz = 1, the triad PPP concentration in the copolymer sample can be expressed as follows:

PPP = C yC*(1 - iE)3

and the isotactic triad concentration

(PPP),, = C yc * ( I - 'E)3 .iPm

where iP,,,,,, is the probability of forming an mm triad in site i. The triad tacticity index is given by

(PPP),, - C yc * (1 - 'Ep * iP,, PPP C y C - ( l - iE)3

-

According to our suggested correlation between tacticity and relative activity toward monomers, iPm can be obtained from the following equations:

'P,, = 1-3 - (a-a')

Thus, the ratio can be predicted by substituting the values of the model parameters, a, a, lE, 'E, and 'fc, into EQ. (I). As shown in W. 5, the calculated values of the triad


f- PPP,, a)

'hb. 5. Observed and predicted triad tacticity indexes for ethylene-propene copolymers

Sample I 3 El E2 E3

0,707 0.811 0.890 0.954 _,___ _,__ .

(-)& I 0.799 I - I - - -

0,867 0,945

a) Conc. ratio of the (PPP),, methyl (S = 22,O - 21,l) to the PPP methine (S = 28,7 - 28,3) (triad tacticity index).

tacticity index are in essential agreement with the observed ones for the copolymer samples.

Similar results from the copolymerization of propene with a small amount of ethylene over a TiCl,/MgCl, -AlEt,/EB catalytic system were obtained by Kashiwa et al. 22). The addition of EB enhances the isotactic fraction having low ethylene content and distinctly decreases the yield of the atactic fraction having high ethylene content. Recently, Kakugo et al. 25) used a temperature-programmed elution column technique to fractionate an EP copolymer containing 0,41 mol-Vo I3C-enriched ethylene prepared by TiCl, * 1/3AlC1, -AlEt,Cl. 13C NMR analysis of the fractions also showed that the ethylene content decreases with increasing isotacticity.

The relative changes in 'jvalues reflect the alteration of the relative polymerization rates. The rates of copolymerization at site i, 'R, can be expressed as:

. 'R =

'r1 * M2 + 2M + . 'C . [MZ] M * 'r,/'k, + 'r2/'kz2

where 'rl, 'r2, 'kl1 and ik22 represent reactivity ratios and rate constants; the subscripts refer to comonomers with "1" being ethylene; M is monomer ratio of ethylene to propene; [M,], propene monomer concentration; 'C, the concentration of site i. The mole fraction of the monomers polymerized at site i, if,, is given by

The ratio of the amounts of the copolymers synthesized at two different sites is:

For the propene homopolymerization, the ratio between mole fractions of polymers with different tacticities can be expressed as:

% Q. Wu, N.-L. Yang, S. Lin

If the addition of external EB does not change the rate constants or the relative reactivities of the monomers at each site, the 'fc/yc ratio for the copolymerization should change by the same factor as for the propene homopolymerization. However, the result from the present study obviously deviates from this prediction. The ratio of isotactic to atactic polypropylene increases by a factor of 4,4 (see Tab. 2) as EB/A1Et3 increases from 0 to 0,3, whereas the ratio of the low-ethylene-content copolymer to the high-ethylene-content copolymer increases only by a factor of 2,l (see Tab. 4). One plausible explanation is that the effect of EB on the reactivities at isospecific site for the two monomers is essentially the same, but is different at the non-stereospecific site. This is supported by considering the resulting 'E values. The value of 'E does not change with the concentration of EB, while the value of 'E increases with increase in EB concentration. This indicates that the relative reactivity of ethylene compared to propene is increased at the nonstereospecific site. At this site, polymerization rate increases due to the increase in ethylene reactivity. Consequently, the increase in the ratio !fJ2fC for the copolymerization is lower than that for propene homopolymerization. This is consistent with the observationz) that the additon of EB decreases the amount of atactic fraction but clearly increases the ethylene content in this fraction.

In conclusion, the microstructure distributions of polypropylenes and EP copolymers, prepared by the catalytic system MgCl,/EB/TiCl,-AlEt,/EB, can be described by a two-site E/B model and a two-site Bernoullian model, respectively. A comparison of the effects of external EB on the microstructure distributions leads to the conclusion that the isospecific site produces lower ethylene content copolymer and the nonstereospecific site, high ethylene content copolymer.

A scholarship in partial support of Q. Wu from the National Education Committee, China, is greatly appreciated. We thank Dr. H. N. Cheng, Hercules Research Center, for a program, EPCO, on which a modified program for this work is based.

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effect of ethyl benzoate on the microstructure distributions of olefin homopolymers and copolymers...

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