Initiation of Polymerization - American Chemical Society

22 Viscosity and Aggregation of Alkyllithium Initiated Polymers

Downloaded by GEORGE MASON UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch022

H. L . HSIEH and A. G. KITCHEN Phillips Petroleum Company, Bartlesville, OK 74004

Utilizing a Dynatrol Viscosity System, viscosity response of polymer solutions can be continuously measured. A series of experiments were carried out to measure the change of viscosity response when we introduced a change in chain ends for an anionically polymerized polymer in hydrocarbon solution. It is concluded that the order of viscosity for either polybutadiene or polystyrene molecules prepared with butyl lithium is butadienyl end group > isoprenyl end group > styrenyl end group > terminated. Based on coupling experiments with chlorosilanes, i t is concluded that higher aggregates exist for polydienes, while polystyryllithium molecules are mostly dimeric. The cross-association between active polymer molecules and alkyllithium molecules forming mixed aggregates is demonstrated and their possible roles in affecting propagation rates and cross-propagation rates are suggested. It is fairly well understood that alkyllithiums form rather stable aggregates in which carbon-lithium bond order is maximized by the utilization of all valence orbitals of lithium.(1) Polystyryl lithium molecules are mostly dimeric in solution.(2.»3,4) This has been generally accepted by all the investiaators.(5j However, association numbers of two(D and four(Z>8J has been reported for polydienes. Two methods were used in determining these values, v i z . , light scattering measurements (in vacuo) and viscosity measurements. In the late fifties and early sixties, we, at the Phillips R&D laboratories, had an extensive research program on the copolymerization of dienes and styrene by direct reaction of the initiator with the monomer mixtures as well as by the incremental addition of monomers.(9.) The decrease in solution viscosity was qualitatively apparent in many cases when the active dienyl chain ends were converted to styryl chain ends. Until recently, how0097-6156/83/0212-0291$06.00/0 © 1983 American Chemical Society Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

292

INITIATION OF POLYMERIZATION

ever, we did not have quantitative measurements to substantiate this observation. Dynatrol Viscosity Systems are designed for continuous mea surement of viscosity in process streams. Dynatrol Viscosity Detectors are i n s t a l l e d d i r e c t l y in process vessels without the need for sampling or analysis. Response i s immediate and contin uous. U t i l i z i n g this system, we carried out a series of experi ments to study the change in viscosity of polymer solutions when we introduced a change in polymer chain ends.


Experimental Reactor. The polymerization tests were carried out in a stainless steel reactor with approximately 7 . 6 - l i t e r (2-gallon) capacity. The reactor was b u i l t by Bench Scale Equipment, Dayton, Ohio, s p e c i f i c a l l y for P h i l l i p s R&D to be used in anionic poly merization studies. The monomer and solvent tanks are connected d i r e c t l y to the reactor and form a closed system. The weights of monomers and solvent tanks can be read d i r e c t l y and the errors are less than ±1% (±40g.) for the solvent and ±0.1g. for the monomers. For this study, a nitrogen pressure of about 350 k.Pa (50 psig) in the reactor was maintained and the impeller mixing speed was 300 r.p.m.. The temperature was controlled by the automatically regu lated steam pressure in the reactor jacket. Materials. P h i l l i p s polymerization grade cyclohexane and butadiene were used. Styrene was a commercial polymerization grade. Solvent was dried over activated Alcoa H151 alumina, and monomers were dried over activated Kaiser 201 alumina before they were transferred to the charge tanks. η - B u t y l l i t h i u m and secbutyl lithium were purchased from Lithium Corporation of America. Chlorosilanes were vacuum d i s t i l l e d before use. Viscosity System. A Dynatrol viscosity system was purchased from Automation Products, Inc., Houston, Texas. The detector (Type CL-10DVT-4) i s inserted in the vessel with the probe f u l l y immersed in the process medium. The drive coil i s excited at a frequency of 120 cps. This produces a pulsating magnetic f i e l d which causes the drive armature to vibrate at a frequency of 120 cps. Mechanical vibration of the drive armature i s transferred along the attached spring rod, through a welded node point to the probe. The probe i s driven into mechanical vibration at the same 120 cps frequency. The amplitude of the vibration of the probe depends upon the viscosity of the process medium. The mechanical vibration of the probe is transferred through a second welded node point along the upper spring rod to the pick up end of the detector. The pick-up end consists of an armature and coil arrangement which i s similar to that of the driver end; one exception being that the stator of the pick-up c o i l i s a per-

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

22.

HSIEH AND KITCHEN

Alkyllithium

Initiated

Polymers

293


manent magnet. The vibration of the pick-up armature in the f i e l d of this permanent magnet induces a 120 cps A-C voltage in the pick-up coil which i s proportional in magnitude to the amplitude of the pick-up armature vibration. Since the pick-up armature i s being driven by the probe, the magnitude of the voltage generated in the pick-up coil i s a measure of the viscosity of the process media. The detector we used had a range of 10 to 1000 c e n t i poises. The 120 cps output signal from the detector i s fed to the converter, where i t i s converted into a 0-10 MVDC signal compat i b l e with 0-10 MVDC recorder. Span and zero controls are located at the convertor. The laboratory set-up i s shown in Figure 1. Procedure. Cyclohexane solvent (3.8 kg.) was introduced to the reactor f i r s t , heated to 50°C and 338 grams of monomer then added. I n i t i a t o r was added at 50°C and the polymerization a l lowed to proceed adiabatically until a peak temperature (65°C to 70°C) had been observed. This generally took about 30 to 40 minutes. A second increment of the monomer (338 grams) was added and allowed to polymerize to quantitative conversion. Peak tem perature was reached generally in 10-15 minutes. Cooling was ap plied slowly with constant mixing to 78°C. This generally took 20 additional minutes. This temperature was then held constant (±0.2°C) by controlling the steam pressure in the reactor jacket. The response of the viscometer probe immersed in the polymer solu tion was recorded continuously. A typical response from the v i s cometer during the polymerization and the termination of the ac tive ends i s shown in Figure 2. Extreme care was taken to main tain a constant temperature, pressure, and solids concentration while recording the viscometer response. For example, when mono mers were used for capping, appropriate amounts of solvent were introduced at the same time to maintain the solids concentration. Because the entire set-up i s a closed system, the control and reproducibility of the polymerization are remarkably good. Since the polymer solution can be drained and rinsed out by the dried solvent from the bottom of the reactor, the closed system rarely needs to be opened and exposed to the atmosphere. For this study, the overall scavenger levels (the difference between levels of added RLi and effective RLi) were in the range of less than 10-4, but more than 10-5 mole/liter,representing 2 to 6% of the total i n i t i a t o r added. Typically, the alkyllithium i n i t i a t o r concentra tion i s around 1 χ 10-3 mole/liter. The response of the viscometer represents a relative change. An increase (+) in response (a higher number) indicates that the vibration of the probe has been dampened by a more viscous solu t i o n . A decrease (-) in response (a lower number) indicates a less viscous solution. The Dynatrol system has shown i t s e l f to be r e l i a b l e , sensitive, and provides reproducible information. This i s true when the conditions are carefully controlled.



294


ο 0

SPAN ZERO

CONVERTER 4-20 MA DC

REACTOR

VISCOMETER PROBE

Figure 1.

Figure 2.

ELECTROPNEUMATIC TRANSDUCER 3 - 1 5 PSI

• RECORDING CHART

Dynatrol viscometer installation.

A typical viscosity response curve.


22.

HSIEH AND KITCHEN

Alkyllithium Initiated Polymers

295


Results and Discussion In the f i r s t series of experiments, polybutadiene samples of about 110,000 molecular weight were prepared in cyclohexane solu tion at 15% concentration. The viscosity response of the solu tions was continuously measured and changes were recorded when terminated with isopropyl alcohol or when capped with isoprene or styrene (Table I ) . In this table and a l l subsequent tables, B* and S* mean active ("living") polybutadiene and polystyrene mole cules, respectively. Terminated polymers are denoted simply as Β and S. BS* and BI* mean polybutadiene molecules with active sty rene and isoprene end groups (capped), respectively. SB* and SI* mean polystyrene molecules with active butadiene and isoprene end groups (capped), and BSB* means polybutadiene i n i t i a l l y capped with styrene was capped again with butadiene, etc. To convert B* completely to BS* i s d i f f i c u l t due to an unfavorable cross-propa gation rate.(5.) We found that by using 5% of the styrene monomer and adding i t in 8 increments resulted in the maximum observed changes. In the same series of experiments, a polybutadiene sam ple of half of the molecular weight was also prepared for compari son purposes (Table I ) . Figures 3, 4, and 5 show typical v i s cosity response charts. They correspond to the data in Table I. After the completion of polybutadiene experiments, similar experiments were carried out on polystyrene. The results are shown in Table II and Figures 6 and 7. They are consistent with the findings of similar experiments based on polybutadiene. The viscosity i s very different depending upon the nature of the end group. This i s true whether the polymer i s polybutadiene or polystyrene. Coupling reactions of polybutadienyllithium (B*) with chlorosilanes are well known and well established(10.) and are used for the production of controlled long-chain branched polymers. (JJL) In Table I I I , the peak molecular weight and viscosity response change, before and after termination and coupling reactions, are reported (also see Figures 8-12). We used mono-, d i - , t r i - , and tetrachlorosilanes to produce l i n e a r , t r i chain and tetrachain polybutadiene molecules. Generally, about 2-8% of the precursor, for reasons such as premature terminations, incomplete l i n k i n g , e t c . , remained uncoupled. Therefore, the peak molecular weight from the GPC curves was used here to i l l u s t r a t e the degree of coupling. Figure 13 shows the GPC curves of the precursor and SiCl4-coupled product which includes the minor amount (~6%) of the uncoupled (precursor) material. The presence of such small amounts of the uncoupled material should have very l i t t l e effect on the solution viscosity. It i s significant that the viscosity of the Si d e c o u p l e d polymer did not change within experimental error, in spite of the four-fold increase in actual molecular weight. The R2 SiCl2-coupled polymer with doubling the actual molecular weight of the precursor dropped the viscosity response by 19 units. This decrease in viscosity response i s nearly the


296


TABLE I VISCOSITY RESPONSE CHANGE OF POLYBUTADIENE 3

5

BY TERMINATION

WITH ISOPROPYL ALCOHOL OR CAPPING WITH ISOPRENE OR STYRENE c

Viscosity Response Change

Action


Bj* + ROH

—+· B

-30

l

Βχ* + I

—• Β Ι*

-10

Bi* + S

— • BjS*

-20

ΒχΙ*

-12

χ

'Βχ* + I l ΒχΙ* + Β

+9

—-+ ΒχΙΒ*

ΒχΙΒ* + ROH — + ΒχΙΒ

-30

Βχ* + S

-21

- — • BjS*

Bj^S* + ROH

S* + B

B2* + ROH

-10

-> BiS

Βχ* + S B l

d

B^*

-19

B^B*

+14

---+ B

-30

2

Βχ* versus B *

-21

2

a.

Span 550, zero 420

b.

(Mw/Mn) χ 10" = 111/103 for B\, a typical value 61/57 for B 3

2

c.

About 5% in 2 increments

d.

About 5% in 8 increments

{

Denotes a single experiment


22.

HSIEH A N D

KITCHEN

UJ100 CO Ο Û.

CO UJ

oc

Initiated

60

297

Polymers

ALCOHOL TERMINATION

70

H OC
BS* BS).


298


TABLE II VISCOSITY RESPONSE CHANGE OF POLYSTYRENE BY CAPPING WITH ABOUT 256 BUTADIENE OR ISOPRENE

Experiment


A A


Action

9

S* + B S* + I

— + SI*

Β Β Β Β

ÎS* + B ISB* + ROH [S* + I ^SI* + ROH

— * SB* — + SB —-»· SI* 4- SI

c

fs* + B ISB* + ROH fs* + I ι SI* + ROH

+ * + +

C C C

+30 +17 +39 -68 +22 -46 +17 -35 +9 -25

—-»• SB*

SB* SB SI* SI

Average a.

Experiment

Span

Zero

A B C

550 550 550

490 830 965

Initiation

TWWn χ 10-

3

157/140 116/109 149/129

n-BuLi sec-BuLi sec-BuLi

{

Denotes a single experiment

ALCOHOL TERMINATION

Figure 6. Viscosity response curve of butadiene-capped polystyrene (S* -> SB* -+SB).

10

20

30

MINUTES


40

22.

Alkyllithium

HSIEH AND KITCHEN


100 90 80 70 60 50 40 30 20 10 0

Figure 7.

Initiated

ISOPRENE ADDED

ALCOHOL TERMINATION

/

20 30 MINUTES

10

299

Polymers

40

50

SI*

Viscosity response curve of isoprene-capped polystyrene (S*

TABLE III VISCOSITY RESPONSE CHANGE OF POLYBUTADIENE BY TERMINATION 3

WITH H 0 OR COUPLING WITH CHLOROSILANES& 2

(Peak Mol.Wt.)C χ 10" Betore After 3

Action


-

118

-68

B* + S i C U

110

431

-2

B* + MeSiCl3

113

283

-5

134

257

-19

113

117

-64

B* + H 0 2

B* + Me Si Cl 2

B* + Me SiCl 3

2

a.

Span 550, zero 420

b.

Added in 4 increments - 60%, 20%, 20% and 10% of the calculated stoichiometric amount to allow maximum coupling

c.

By GPC


300

POLYMERIZATION


INITIATION OF

Figure 9.

Viscosity response curve of polybutadiene coupled with chlorotrimethylsilane.


22.

HSIEH AND KITCHEN

Alkyllithium

100 UJ

(Me) SiCI 2

on

/

CO

Ο

S ce


oc
(sec-BuLi)^ > (n-BuLi)*. It is also known that the rate of propagation is in the order: styrene > isoprene > butadiene. The possible relationship between the rate and degree of aggregation cannot be ignored. The differences in aggregation may also be the key to the mechanism of copolymerization. The so-called "inversion" behavior in copolymer!zations of diene and styrene may well be caused by the differences in degrees of aggregation, which in turn, control the cross-propagation rates. Literature Cited 1. 2. 3. 4.

Brown, T.L. in "Advances in Organometallic Chemistry", Volume 3, F. G. A. Stone; R. West Ed. Academic Press, New York, N.Y., 1965, p. 365. Morton, M.; Bostick, E.E.; Livigni, R. Rubber and Plastics Age 1961, 42, 397. Morton, M.; Fetters, L.J. J. Polym. Sci. 1964, A2, 3311. Johnson, A . F . ; Worsfold, D.J. J. Polym. Sci. 1965, A3, 449.


306 5. 6.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.


Hsieh, H.L.; Glaze, W.H. Rubber Chem. & Technol. 1970, 43, No. 1, 22. Morton, M.; Fetters, L.J.; Bostick, E.E. J. Polym. Sci. 1963, C1, 311. Worsfold, D.J.; Bywater, S. Can. J. Chem. 1964, 42, 2884. Sinn, H.; Lundord, C.; Onsanger, O.T. Macromol. Chem. 1964, 70, 222. Hsieh, H.L. Rubber and Plastics Age 1965, 46, No. 4, 394. Zelinski, R.P.; Wofford, F.F. J. Polym. Sci. 1965, A3, 93. Hsieh, H.L. Rubber Chem. & Technol. 1976, 49, No. 5, 1305. Fetters, L.J.; Morton, M. Macromolecules 1974, 1, 552. Al-Harrah, H.M.F.; Young, R.N. Polymer 1980, 21, 119. Szwarc, M. J. Polym. Sci., Polym. Lett. Ed. 1980, 18, 499. Fetters, L.J.; Morton, M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 199. Hsieh, H.L. J. Polym. Sci. 1965, A3, 153· Selman, C.M.; Hsieh, H.L. Polym. Lett. 1971, 9, 219.

RECEIVED September 28, 1982


Initiation of Polymerization - American Chemical Society

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