16. 13C NMR Investigation on Lewis Base Activation Effect in High Yield Supported Ziegler-Natta Catalysts

185

16. 13C NMR Investigation on Lewis Base Activation Effect in High Yield Supported Ziegler-Natta Catalysts

Maria Carmela Sacchi, Incoronata Tritto, Chengji Shan Istituto di Chimica delle Macromolecole del CNR Via E. Bassini 15/A - 20133 Milano Italy Lucian0 Noristi Himont Centro Ricerche Giulio Natta Piazzale Donegani 12 - 44100 Ferrara Italy

ABSTRACT In this paper we will approach the study of the machanism of the Lewis base activation in high yield supported catalysts by

three different experimental

Ziegler-Natta

routes:

i) stereo-

chemical study, by 13C NMR analysis, of samples of

polypropene

prepared using selectively

13C

enriched AlEt3 as cocatalyst; ii)

GPC characterization of the most isotactic fractions; iii) study of

the exchange between

internal and

external

base6

by

GC

analysis of the base content of the solid catalyst. Despite the well known complexity of the problem, on the basis of all data

it

is possible to single out some general trends

Lewis base

activation

characteristics of

and

these trends depend

the specific pair of internal

more and

these

of

the

on

the

external

bases than on those of the single internal or external base.

186 M. C. Sacchi, I. Tritto, C. Shan and L. Noristi

INTRODUCTION In previous papers we reported a study of the mechanism the

Lewis base activation in high yield supported

catalysts known

the Lewis bases exert at least

concurrent effects: sites;

poisoning

or

two

It

activation

polymerization

is well

different

i) poisoning of both isotactic and

ii) activation of

the

isotactic

ones. A

is

observed

depending

and

atactic

dominating on

the

Our approach consisted in finding

the conditions

in which

various Lewis

internal and external bases, produced a effect.

Ziegler-Natta

for isotactic propene polymerizationl.2.

that

of

In such

bases, used prevailing

both

as

activation

conditions we studied the effect of the Lewis

bases on the steric structure of isospecific centers of different catalytic

systems. The method

we used

to obtain

structural

information on the active centers was the investigation, by

13C

NMR, of

the

the

presence

of

initiation step in propene polymerization the

selectively

13C-enriched

in

cocatalyst

A1(13CHzCH3)3. Indeed, taking into account only monomer insertion on the isotactic-specific centers, when polymerization starts on a

selectively enriched titanium-l'CHzCHs

detect and

bond it is possible

distinguish the two possible stereoisomers of

to

chain

end groupsa#g:

erythro Erythro

threo

(or isotactic) is the stereoisomer in which

the

two

16.NMR Investigation on hiBase Actioation Effed 187

first monomeric units have the same configuration and threo syndiotactic)

that

one

in which

they have

the

(or

opposite

configuration. If e and t are the integrated peak areas of

the

enriched methylene

the

resonances assigned

respectively

to

erithro and threo placements of the first propene unit, the e/t ratio represents the extent of the first step stereoregularity.

We observed that the increase of the isotactic productivity

due

to the presence of either the internal or the external bases, is accompanied

by

a

change

stereospecificity. On

in

the

extent of

the

first step

the basis of these findings and

of

our

previous data concerning conventional Ziegler-Natta catalystslo-12 we

deduced

present

that both the internal and the external bases

in

centers and partially

the environment of at least some of the consequently

are

isospecific

the activation derives,

at

least

from a direct effect of the Lewis bases on the active

sites. In this paper some insights arrived at thorough different experimental

investigations will

individuate some new

be summarized

aspects of the base

or

problem

effect

internal and

bases rather than the behaviour of the single

external base.

we will

activation

concerning the behaviour of the specific pairs of external

and

internal

In spite of the well known complexity of

the

an attempt will be made to find out about some general

trends in the Lewis base behaviour.

RESULTS AND DISCUSSION 1) Propylene polymerization in the absence of external base.

Propylene was polymerized with MgClz/TiClr, HgClz/EB/TiClr

and

188

M.C. Sacchi. I. Tritto. C. Shan and L. Noristi

MgC12/DEHP/TiC14 catalytic systems (EB=ethyl benzoate, DEHP=di(2ethylhexyl)phthalate), cocatalyst. All atactic

selectively 13C enriched AlEts

the polymers were separated into isotactic

fractions by extracting them with boiling

isotactic The

using

heptane.

fractions were further extracted with boiling

heptane

insoluble-octane soluble and

octane

as and The

octane. insoluble

fractions of all the samples were characterized by gel permeation chromatography

and

13C NMR

analysis.

The

polymerization

conditions and the results obtained are reported in Tab.1. The polymerizations

in

performed

two differently prepared MgClz supports (runs 1

and 2).

with

the absence of

the

internal base

were

The polymerization with the catalyst containing EB (run

3 ) was repeated under different conditions (run 4 ) .

In fact when

the catalysts containing an internal base are placed in contact AlEt3, they progressively lose the internal base

with

and

the

extent of this base extraction depends heavely on the time, the aluminum/titanium ratio and the temperatures. Therefore, in order to

analyze the catalyst's behaviour in the conditions in which

the

internal

base should be at least partially present on

solid catalyst, we

repeated

the polymerization

3

in milder

COnditiOn6, that 1s at lower time and temperature. In Tab.1 isotactic productivities of all the samples are shown.

the presence of the internal bases is not possible

case

in

due this

since the samples are performed with catalysts containing

different with

the

However a

correct evaluation of the increase of isotactic productivity to

the

amounts of fixed titanium (see note 12) observe that

the catalysts that do not contain any base

heptane

the

isotactic

insoluble fraction is nearly completely octane soluble

Table I

EFFECT OF DIFFERENT INTERNAL BASES ON MgC1z SUPPORTED CATALYSTS

Ti%

Y

MgClz/TiClr

0.34

MgClz/TlClr

1.1.

I.P.

22

49

3170

2.43

50

41

843

MgCl2/EB/TiClr

1.33

65

45

2199

HgCl2/EB/TiClr

1.33

31

72

1678

MgClz/DEHP/TiClr

3.04

62

73

1489

Catalyst

[mml

e/tc

S-8

42

216

3.9

0.95

1.7

1-8

7

489

2.6

0.98

-J

S-8

41

192

4.0

0.95

2.1

1-8

0

-

-

S-8

45

200

4.7

1-8

0

-

-

S-8

60

398

4.9

0.93

4.5

1-8

12

623

3.0

0.97

y8

S-8

49

250

4.5

0.94

2.0

1-8

24

497

3.0

0.95

“5

0.95

-

Cocatalyst: A1(13CH2CH3)3; Y: yield in grams of polymer/grams of catalyst-hour 1.1.: isotacticity index=weight percent of heptane insoluble fraction I.P.: isotactic productivity=grams of isotactic polymer/grams of Ti-hour S-8: octane soluble fraction; 1-8: octane insoluble fraction [mml: molar fraction of isotactic triads by N M R ; e/t: intensity ratio of resonances related to the isotactic (e) and syndiotactic ( t ) placement of the first propene unit 8 : T=room temperature, time=l hr; b: T=O ‘C; t=30’ c : see note 13

-9

3.9

-

190 M. C. Sacchi, I. Tritto, C. Shan and L. Noristi

and is characterized by a relatively low 'Hw. A

(run 2 )

small

octane insoluble fraction is present in run 1 (MgClz activated by milling), value.

and

by higher g w and

is characterized

lower E w / h

A greater octane insoluble fraction is obtained with

the

catalyst containing DEHP as an internal base (run 5 ) .

As to the

catalyst containing EB as an internal base, an octane

insoluble

fraction

is obtained only in the conditions in which the base is

less easily removed from the solid surface (run 4 ) . The NMR data show that all the octane insoluble fractions have a higher

stereoregularity of both propagation and initiation with

respect to the octane soluble ones. All

these data suggest that EB is present in the environment

the active titanium and its presence makes the

isotactic

of

sites

able to produce more stereoregular polypropene, characterized by

zw

higher

accounts

and lower Rw/Rn value. for:

conditions

i) the

Indeed only

fact that only

in

mild

this assumption polymerization

(that is only when the base is maintained

on

the

catalyst surface) the octane insoluble fraction is present;

ii)

the fact that in the octane insoluble fraction so obtained,

the

e/t ratio is higher than in the octane soluble one. As to

the

catalyst containing DEHP as an internal base (run 5 ) ,

the

fact

that a noticeable octane insoluble fraction is already present at standard polymerization conditions can be in principle

accounted

for by two hypotheses: i) the diesters can be less easily removed from

the catalyst by AlEt313;

ii) the removal of

DEHP

leaves

active sites characterized by higher isospecificity and/or higher stability with respect to the catalyst without any base. The fact that

a

small octane insoluble fraction is present even

in

the

16. NMR Investi&~tion on h i s Base Activation Effect 191

absence of internal base (run 1) shows that this kind of

active

iw, low iw/Mn value

and

noticeable first step stereoregularity) may be present, on

the

sites

(which are characterized by high

catalyst surface, even without any base. 2)

Propylene polymerization with different pairs of internal and

external bases. Table I1

EFFECT OF DIFFERENT BASES ON THE CATALYST MgClz/EB/TiClr

Be

Y

0

65

1.1

45

2199

38

94

2685

TMPip 69

91

4721

PTES

94

3251

EB

46

-MW. 10-3

Rwlk [mml

eltc

200

4.7

0.95

3.9

-

-

-

-

S-8 4 0

218

5.7

0,96

4.0

1-8 5 4

491

2.9

0.99

y7

41

207

4.5

0.96

2.8

1-8 50

531

3.2

0.99

‘10

S-8 47

323

5.5

0.96

5.4

1-8 47

573

2.9

0.99

“/lo

Wt%

I.P.

E-8 45 1-8

5-8

0

Cocatalyst: A1(13CH2CH,), Be: external base Y: yield in grams of polymerlgrams of catalyst*hour 1.1.: isotacticity index=weight percent of heptane insoluble fraction I.P.: isotactic productivity=yield in grams of isotactic polymer/grams of Tiehour S-8: octane soluble fraction; 1-8: octane insoluble fraction [mml: molar fraction of isotactic triads by NMR elt: intensity ratio of resonances related to the isotactic ( e ) and syndiotactic (t) placement of the first propene unit c : see note 13

192

M.C. Sacchi, I. Tritto, C. Shan and L.Noristi

Three different bases, EB, TMPip (2,2,6,6-tetramethylpiperidine)

and PTES (phenyltriethoxysilane) have been used with

catalysts

both

containing EB and DEHP respectively as internal bases.

All the results are shown in Tab.11 (EB as an internal base) Tab.111

(DEHP as an internal base).

differences

between

the

two

and

some

Some similarities and

series

of

experiments

can

be

recognized. Table I11

EFFECT OF DIFFERENT BASES ON THE CATALYST MgClz/DEHP/TiClr

Be

Y

1.1.

0

62

73

1489

EB

75

76

1875

TMPip

66

91

1976

PTES

65

94

2010

I.P.

Wt%

-MU-10-3 -Mw/i?n

[mml

e/tc

S-8

49

250

4.5

0.94

2.0

R-8

24

497

3.0

0.95

55

S-8

66

195

3.5

0.93

2.2

1-8

10

449

2.8

0.98

%5

S-8

47

260

4.4

0.92

1.7

1-8

44

493

2.9

0.96

4.4

S-8

45

238

4.9

0.97

3.0

1-8

49

467

2.8

0.96

??a

Cocatalyst: Al(?’CH2CH,), Be: external base Y: yield in grams of polymer/grams of catalyst-hour 1.1.: isotacticity index=weight percent of heptane insoluble fraction I.P.: isotactic productivity=yield in grams of isotactic polymer/grams of Tiohour S-8: octane soluble fraction; 1-8: octane insoluble fraction [mm]: molar fraction of isotactic triads by NMR e/t: intensity ratio of resonances related t o the isotactic ( e ) and syndiotactic ( t ) placement of the first propene unit c : see note 13

16. NMR Investigufirm on h i s Base Activofirm Effect 193

to

As

the

external

former, if we take

base

the experiments without

as starting points, we can

observe

any

that

the

addition of the external base always produces an increase of

the

isotactic productivity, even if to a different extent depending on the base. Moreover, when the external base is added, an octane insoluble fraction is always present, than by

50%

and this can

reach more

of the overall isotactic polymer and is characterized

-

higher Mw

and

lower iiw/in value with

respect

to

the

corresponding octane soluble fractions. However the behaviour of every

base

same:

e.g. the greatest improvement of isotactic productivity is

produced with

by

with the two different solid catalysts is not

TMPip with the catalyst containing EB and

the catalyst containing DEHP; moreover EB is a

efficient

isotacticity

improver with

the former

by

the

PTES

relatively and

an

inefficient one with the latter. The NMR data show that the [mml contents are higher in the series of experiments with EB

as an

internal base than in that with DEHP. All the e/t ratios of the octane

insoluble fractions are higher

corresponding octane soluble ones. that all

than

those of

the

It is interesting to observe

these e/t values are widely different from each

other

and depend both on the characteristcs of the external base, as we have already shownl, and on those of the specific combination of external and internal base. In particular, with EB as an internal base, both

TMPip

and PTES produce catalytic sites having

an

initiation stereoregularity nearly as high as

the propagation

stereoregularity

e/t values

observed when internal base.

while

the same

different

and

lower

two base8 are used

with

are

DEHP as an

194

M.C. Sacchi, I. Tritto, C. Shan and L.Noristi

As

to

the

octane

are

insoluble higher

slightly

lower

fractions.

The

than

fractions,

they

are

roughly

by relatively low fiw and high fiw/Mn value and

characterized [mml

soluble

those

corresponding

of

than

in

the

corresponding

e/t values are different

the

octane

soluble

and

fractions

catalysts without external base.

It seems

the

octane mostly of

the

likely

that these fractions are produced by a mixture of active sites of different kinds containing and not containing the internal and/or the external base. The fact that TMPip produces a decrease of the e/t

values of the octane soluble fractions of both catalysts

is

not easy to be accounted for.

3)

Study of the interactions between the solid catalysts and the

external bases. In

order

to get a better understanding of

the

activation

mechanism we have compared the above stereochemical data with the results takes

coming from a study of the exchange of place

solution. contact base

between

the

solid catalyst

and

components the

cocatalyst

It is known that when the solid catalyst is placed with

a solution containing both AlEt3 and

that

in

an

external

a partial replacement of the internal base by the

external

one

occurs.’ Table IV shows the results obtained by

the

base

content

catalysts

after

of both MgClz/EB/TiClr treatment

with AlEt3

or

and

determining

MgClz/DIBP/TiCl4

AlEta/external

base

mixtures. The contact conditions were chosen as close as possible to

the

polymerization

conditions.

The

contact

procedure

is

described in the Experimental Part. Methyl-para-toluate (MPT) was

16.NMR Investigation on Lewis Base Activation Effect 195

used

instead of ethyl benzoate as an external base to make

it

possible to recognize the internal-external base exchange when EB is the

internal base.

isobutyl phtalate should not

The use of a

catalyst

(DIBP) as an internal base

containing di-

instead

of

DEHP

change the results, since it is likely that both

diesters have similar behaviour. The data of Table IV show that the diester can be removed from the catalyst by this treatment to a

higher extent than EB, either with and without external base. Table IV

Catalyst

BASE CONTENT OF THE CATALYSTS TREATED WITH AlEtj/EXTERNAL BASE MIXTURES Treatment

Base Content

Contact with

MgClz/EB/TiClr (Ti = 1.7 % )

MgClz/DIBP/TiClr (Ti = 2 . 4 % )

Interna1 mmol/lOOg

None

58

AlEt:,

10

External mmol / 1oog

AlEts/MPT (3/1)

27

21

AlEt 3 /PTES

I'

18

27

AlEt 3 /TMPip

"

15

34

None

44

AlEt 3

6

AlEt3/MPT (10/1)

8

6

AlEt 3 /PTES

'I

3

41

AlEt 3 /TMPip

I'

5

22

Contact conditions: T=50 'C; time=l hr; cat.conc.=4 g/1 Al/Ti=20 m.r.; solvent=hexane

196 M.C. Sacchi, I. Tritto, C. Shan and L. Noristi

We

can also observe that in both series of experiments the best

catalytic

systems

(that is those

that

give

the

highest

isotacticity index and isotactic productivity) are those in which the external base is able to be absorbed on the solid catalyst to the

largest extent.

behaviour with.

Moreover the external bases have

different

on

contacted

depending

the solid catalyst they

are

In fact MPT, that can be noticeably absorbed on the first

catalyst, is hardly absorbed on the other one and PTES and TMPip show

opposite

trend

of

absorption

in

the

two

series

of

experiments.

CONCLUSIONS The

results mentioned above allow us to

individuate

some

general trends in the Lewis base activation mechanism: i)

The

due

either

active base

isotactic activation effect of the internal base may to the presence of the base itself on the

sites

or to the fact that the removal of

isotactic

the

leaves active sites characterized by higher

be

internal

isospecificity

and/or higher stability with respect to the catalyst without

any

base. The latter effect should prevail in the catalyst containing a diester as an internal base since a noticeable octane insoluble fraction

is observed

despite

the fact that

the diester

is

strongly removed by AlEts. ii) The variation of the extent of the first step rity

(e/t) with the external base confirms that

activation effect interaction with

of the external base derives the active sites. However

stereoregulathe

by

these

isotactic

its direct e/t

values

16.NMR Investigation 011 L.awis Rase Activation Effeci 197

depend

not only on the characteristics of the.externa1 base

but

on those of the internal base too. iii) The isotactic activation effect of the external base has been shown to be proportional to the base capability of being fixed on the

solid catalyst by replacing the internal base.

Therefore

the

fact that with a diester as an internal base the e/t values

of the octane insoluble fractions are lower than with EB could be accounted

for by the larger room due to the replacement of

diester. However

it is not possible to distinguish whether

extremely high e/t values observed with EB as an

the replacement of the monoester or

the

internal base

are due to the smaller room left in the active site by

the

environment

to the effect

of

both

internal and external base on the same active site. iiii) From

all

the data observed it seems evident

that

effectiveness of a catalytic system depends more on the

the

specific

pair of internal and external base than on the single internal or external base. Moreover, while the amount of activation effect clearly depends on the choice of the external base, the internal base

seems to affect prevailingly the stereoregularity of

both

initiation and propagation.

EXPERIMENTAL

Reasents. The MgClz/TiClr catalyst containing Ti = was

obtained

milling

0.34%

starting from MgClz activated by 1 0 days of ball

in a roller-type milling machine.

The MgClz

containing Ti=2.43% was obtained sterting from MgClz

catalyst

synthetized

198

M.C.Sacchi, I. Tritto, C. Shan and L. Noristi

by

chlorination of the Grignard compound n-C4H9MgCl as described

in

the

patent

benzoate

literature.14

The

catalyst

as an internal base (Ti = 1.33%,

containing

=

E.B.

ethyl was

10.5%),

kindly supplied by dr. Albizzati of Istituto G. Donegani, Novara. The

catalyst containing di(2-ethylhexy1)phtalate as an

internal

base (Ti = 3.04%, DEHP = 17.9%), was prepared from soluble MgClz, 2-ethyl hexanol,

phtalic anhydride and TiClr according to patent

1iterat~re.l~

A1(13CHzCH3)3

was

prepared

by

reaction

of

CH313CHzLi with AlC13 as reported in literature.16 pola

All the polymerizations were carried out in

glass

reactor

containing

50

(Al/Ti = 20 m . r . ) ,

Al(13CHzCH3)s

mL

heptane

as

a

solvent.

the Lewis base (base/Al =

0.3

with the catalyst containing EB and base/Al = 0.1 m.r. with

m.r. the

catalyst

were

added

in

propylene

conditions shown

the said order.

and

atmospheric

(0.2 9.)

containing DEHP) and the solid catalyst

the

pressure

The

reactor

polymerizations for 1 hr at

room

were used for run 4 of Tab.1.

on the Table.

were

was

filled

performed

temperature. These

with under

Different

conditions

are

The polymers were fractionated with boiling

solvents by conventional methods.

GEGAnalvsls, The polydispersity and E w of all the heptane '

insoluble/octane determined

by

dichlorobenzene

soluble gel

and

permeation

at 135 'C,

octane insoluble chromatography

fractions (GPC)

using a Waters 150-C gel

were

in

0-

permeation

chromatograph equipped with a Ultrastyragel column (106, 105, 104 and l o 3 A ' pore size).

-.

ca.

The NMR samples were prepared by dissolving

100 mg of polymer in 1 mL of lr2,4-trich1orobenzene in a

10

16.NMR Iuvestigntion on h i s Base Actiwtion Effect 199

mm-0.d.

tube.

solvent,

One

half

mL

of

was added a s a

CzDzC14

and 1% of hexamethyldisiloxane was used

chemical shift reference.

lock

as an internal

All the spectra were obtained by using

a Bruker AM-270 spectrometer operating at 67.89 MHz in PFT

mode,

at a temperature of 115 'C.

Analvsis nf catalyst

were

solid W v s t s , Four

placed

in the reactor

temperature was raised to 50 'C. solution added

of

AlEt3

under

grams

nitrogen

the

Then 950 mL of hexane and

the

reaching one liter total

reaction mixture was stirred for one hour at 50 'C, washed dried

several

times with hexane at the

under vacuum.

solid

and

or of the AlEt~/externalbase

in the said order,

of

same

mixture volume.

were The

filtered and

temperature

and

The amount of base contained in the samples

so obtained were determined by GC.

REFERENCES Sacchi,M.C.; Shan,C.; Locatelli,P.; Tritto,I. Macromolecules, 1989, in press. Sacchi,M.C.; Tritto,I.; Locatelli,P. in "Transition Metals and Organometallics as Catalysts for Olefin Polymerization" W.Kaminsky, H.Sinn (Eds), Springer-Verlag Berlin, 1988, p.123. Soga,K.; Sano,T.; Yamamoto,K.; Shiono,T. Chem.Lett., 1982 , 425. Kashiwa,N. in "Transition Metal Catalyzed Polymerization: Alkenes and Dienes" ( R.P. Quirk, ed.), Harwood Acad. Publ., New York, 1983, p.379. Barbe',P.C.; Cecchin,G.; Noristi,L. Adv. Polym. Sci. 1986, 81, 1. Sacchi,M.C.; Tritto,I.; Locatelli,P. Eur. Polym. J., 1988, 24, 137.

200

M.C. Sacchi, I. Tritto, C. Shan and L. Noristi

(7) Tritto,I.; Locatelli,P.; Sacchi,M.C.

"Int. Symp. on Transition Metal Catalyzed Polymerization", R.P.Quirk (Ed.), 1988, p.255.

(8) Zambelli,A.; Sacchi,M.C.; Locatelli,P.; Zannoni,G. Macromolecules 1982, 15, 211. (9) Zambelli,A.; Locatelli,P.; Sacchi,M.C.; Tritto,I. Macromolecules 1982, 15, 831. (10) Tritto,I.; Sacchi,M.C.; Locatelli,P. Makromol.Chem. 1986, 187, 2145. (11) Sacchi,M.C.; Locatelli,P.; Tritto,I. Makromol. Chem. 1989, 190, 139. (12) In fact it is well known that, while the activity referred

to the entire catalyst increases with the titanium content, the activity expressed as the amount of polymer produced per titanium unit increases as the titanium content decreases.

(13) It must be said that the e/t values of the octane insoluble fractions are evaluated with a higher error than those of the octane soluble ones and of the heptane insoluble fractions.1.2 Indeed, due to the high molecular weight and to the high first step stereoregularity of these fractions, the smaller peak of the erithro resonance, in some cases, can be hardly detected. (14) Soga,K.; Shiono,T.; Doily. Makromol.Chem. 1988, 189, 1531. (15) Luciani,L.; Kashiwa,N.; Barbe',P.L.; Toyota,A.; German Patent 26431436, 1977.

(16) Blg. Pat., 895019, Mitsui Petrochem. Ind., C.A. 98, 2162126, 1983. (17) Mole,T.; Jeffery,E.A. "Organoaluminum Compounds", Elsevier, Amsterdam, 1972.