Preparation and characterization of polydimethylsiloxanes with narrow molecular weight distribution

European Polymer Journal. Vol. I I. pp. 663 to 667. Pergamon Press 1975. Printed in Great Britain.

PREPARATION AND CHARACTERIZATION OF POLYDIMETHYLSILOXANES WITH NARROW MOLECULAR WEIGHT DISTRIBUTION H. J. HOLLE* and B. R. LEHNEN Institute of Physical Chemistry, Joh. Gutenberg-University, Sonderforschungsbereich "Makromolekiile", Mainz/Darmstadt, West Germany (Received 17 January 1975)

Abstract--The preparation of polydimethylsiloxanes (PDMS) of narrow molecular weight distribution (MWD) by anionic polymerization of hexamethyicyclotrisiloxane (D3) in an improved polymerization apparatus is described. Using (CH3)2Si(OLih as initiator and (CH3hSiCI (TMCS) as terminating agent, polymers with only methyl groups were obtained with molecular weights ranging from 2 x 103 to 2 × 10 6. Kinetic investigations were performed only so far as necessary for controlling the polymerization under the chosen experimental conditions (solvent: n-hexane, solvating agent: hexamethylphosphortriamide (HMPT), polymerization temperature: 25°). The molecular weights of the polymers were determined by light-scattering and, after calibration, by viscometry and GPC. The non-uniformities of the samples with symmetrical MWD were estimated using the 4a-method. The GPC apparatus had been calibrated with polystyrene and poly-a-methylstyrene samples of extremely small non-uniformity. INTRODUCTION a highly polar solvating agent. Traces of water do Because of their unusual properties, polydimethylsi- not terminate the silanolate chain ends, but a fast loxanes (PDMS) become more and more interesting equilibrium is established between silanoles and for technical and scientific purposes. Especially valu- silanolate ions. Therefore the anionic polymerization able for physical investigations are samples of well- of D 3 is less sensitive to moisture than the "living defined structure and narrow molecular weight distri- carbanion" systems. bution (MWD) which until recently could be obtained only in small amounts by tedious fractionations [1,2]. In 1969 Lee et al. [3] reported that hexamethylcycEXPERIMENTAL lotrisiloxane (D3) can be polymerized anionically to n-Hexane, n-pentane, cyclo-octane (Merck. pure grade) products of narrow MWD. This can be explained by the ring-strain of Da, which enhances the reactivity were refluxed for 3 days o v e r C a l l 2 and fractionated using of the Si-O bonds. Later, siloxane-styrene and other a 2 m column. Hexamethylphosphortriamide (HMPT), dimethylsulcopolymers were made by this method [4,5], but there phoxide (DMSO), dimethoxyethane (DME) and trimethare almost no quantitative data concerning molecular ylchlorosilane (TMCS) were fractionated from p.A. grade weight, uniformity, etc. Merck materials. For our light- and neutron-scattering experin-Butyl-lithium (Bu-Li) was used as 20~g solution in nments [6], we needed well-characterized PDMS-sam- hexane (Merck) without further purification. ples over a large range of molecular weights (2 x Molecular sieves (3 A, Merck) were activated by immers103 < M < 2 x 106). This paper deals with the char- ing in THF for several days and then dried in vacuo at acterization of these samples and with the experimen- 130°. D3 (Wacker Chemic Ltd.) was dried in benzene solution tal techniques necessary for their preparation by by standing over molecular sieves for 3 days. To prepare anionic polymerization. We hope that our experience the stock-solutions, it was fractionated directly in n-hexane will help to show the possibilities and limitations of and stored over molecular sieves. this method. (CH3hSi(OLi)2 was prepared in solution by titrating Under "living end" conditions [7] the polymeriza- (CHa)2Si(OH)2 dissolved in THF, with Bu-Li in a dry tion of D 3 should lead to products with approximate argon atmosphere. The portion of higher oligomers was found, by GC after derivatization with TMCS, to be less Poisson-distribution. However, reaction of growing than 12~. silanolate-ends with Si---O bonds of already formed polymer chains causes redistribution; this effect SAMPLE CHARACTERIZATION becomes more important as the conversion increases. The selectivity towards ring- and chain-bonds Molecular weight and polydispersity of the samples respectively depends strongly on the counter-ion: of were determined mainly by the G P C method. The the alkali-metals, Li + forms the least reactive and G P C a p p a r a t u s was equipped with 6 columns therefore most selective ion-pair. In hydrocarbon (Waters Ass. Ltd.) of porosities of 5 × 106, 7 x I0 -~solvents, reaction takes place only after addition of 5 x 106, (1.5--7)10 s, 2.5 x 104, 2.5 x 103, 5 x 102 (length 1-2 m, ~b 0.78 cm). The solvent was toluene * Present address: DEGUSSA, FCP 6451 Wolfgang, with a flow rate of I ml/min. The column temperature West Germany. was 30 ° and the usual concentration 0-4wt ~o. The

663

664

H.J. H6LLE and B. R. LEHNEN

calibration curve was established with PDMS samples, well characterized by light scattering and viscometry [9]. Figure 1 shows the PDMS calibration curve compared with narrow M W D PS-standards (Pressure Chemical Company and samples of D. Rahlwes [ 10]). The relationship is expressed by:

methylstyrene; this is justified at least approximately by the similar B'-values of PS and P D M S [13]. The real non-uniformities of these samples, kindly supplied by D. Rahlwes and L. L. B6hm, were known to be < O01 by means of Baker-Williams-fractionations. For 3 x 104 < M < 7 x 10s, the following relationship was obtained:

In M = A - B'VE

Aa,, = 1.264 -- 0.0172 VE.

where V~ = elution volume in counts. For A and B', the following values were obtained: PDMS, A = 34.98 and B' = 0.583 (104 < M < 1.7 x 106); PS, A = 33.16 and B' = 0-534 ( 4 x 103 < M < 2"0 x 106). The GPC-elution curves are determined by the M W D of the sample as well as by the hydrodynamic dispersion of the apparatus. Since the GPC-curves of most our samples were symmetrical and of Gaussian shape and the MWD's were expected to be almost symmetrical, we could assume that the hydrodynamic dispersion also has Gaussian character. Then one can calculate an apparent non-uniformity U,pp, which under these conditions is equal to the value defined by Wesslau [11]:

So U, and U could be calculated for each molecular weight; only small deviations were found for concentrations < 0.4 wt%. Due to the roughness of the procedure, the non-uniformities obtained by this method are only accurate within +0-03. The data for some samples are summarized in Table 1 together with their polymerization conditions. Some characteristic G P C curves are collected in the Appendix. POLYMERIZATIONS

To establish a connection to Lee's work [3] and to know the capacity of the method, we also started using a syringe-technique (ST). U,pp = exp [(B'ov) 2] - 1 The polymerizations were carried out under dry argon in flasks previously evacuated repeatedly at where a~ = variance of the GPC-curve. a, Is approximately equal to one fourth of the dis- 300-. The solvent was n-hexane and the initiator Butance between the points of intersection of the inflec- Li; HMPT, DMSO and DME were tried as solvating agents. After termination with TMCS, the D3-convertion-tangents of the G P C curve with the baseline. The real non-uniformity (U = M , d M , - 1) of a sion was determined by GC using cyclo-octane as inpolymer is always smaller than U,~,~,. According to ternal standard. The courses of some polymerizations were followed Berger and Schulz[12], we define the excessive by periodic removal of aliquots, which were derivanonuniformity U~: tized by addition of TMCS and analysed by GC. The U - U,pp U . . . .~, where U = polymerization was found to be 1st order with respect 1 +U~ to D3 up to 70% conversion. For comparison, acid- and alkali-catalysed rearranand U~. = expl'(B'Aa~)2] - 1 exp[(B'o'~,.o)"] - 1 gements of octamethyl-cyclotetrasiloxane (D4) were performed: Equilibration of D4 with 0"IVo K O H (150 °, 6hr) Since U~ is influenced by the G P C apparatus and by the sample in a very complicated manner, we did yielded a product with ~ , = 32,000 and U = 0"95, not aim at extremely accurate and absolute deter- whereas Mn = 6400 and U = 1.I was obtained using mination. Therefore the error is not too great if U~ 0"5°~ H,SO4 (80 ~, 3 hr). In contrast, the anionic procedure performed by is estimated empirically by comparison with very narrow M W D test samples of polystyrene and poly-~- ST improves the M W D remarkably, in particular in the lower range of molecular weights. Removal of aliquots during polymerization obviously broadens the × MWD (see Table 1). However, in polymerizations aiming at molecular weights of 200,000, the moisture content is inevitably of the same magnitude as the initiator concentration, causing bimodal distributions [3] (see P D M S 3660 and 7300). Due to these findings and due to the relatively high content of butyl endgroups in the low molecular weight polymers, we developed the following modified break-seal polymerization technique (BS): The polymerization flask A (see Fig. 2) was thoroughly cleaned and repeatedly evacuated at 300 °. Then a defined amount of (CH3)2Si(OLi), dissolved in n-hexane-THF was added under dry argon by syrvE inge. After evacuating for several hours on a high Fig. I. GPC--calibration curve of polysiloxane (PDMS) and polystyrene (PS) in toluene at 30°. Concentration 0.4 vacuum line, the nozzle C was melted off. The flasks wt ~. PDMS: A viscometric data using Haug's calibration D and E were prepared in the same way and filled curve, x light-scattering data. PS: • light-scattering data with molecular sieves. Then the D 3 stock solution (samples of Rahlwes and B6hm), x light-scattering data together with the solvating agent (mostly HMPT) was (Pressure Chemical Company). syringed in D, and pure solvent in E.

Polydimethylsiloxane preparation

665

Table 1. Polymerization conditions and data for characterization for polysiloxanes (PDMS). II1: concentration of Bu-Li for ST-polymerizations and concentration of (CH3JzSi(OLi)2 for BS-polymerizations tD31

P~techsample nique

Ill

[HNPT]

(tool I I ) (melll) 400*

(molll)

reactiontime {rain)

converGPC sion (%) count

Uapp

I~ U = t~ n- I

M n

MGPC

ST

0.57 1.3.10.3

1.5 "tO"2

102 h

30

42.35

o. Ig

0.09

30 000

1100

ST

0.57 1.2-10-3

3.18.10-2

210

75

40.6

0.41

0.30

83 000

1350 1500

ST ST

0.57 9.0.10 -4 2.7 .10-2 0.57 1.1.10-4 0.12

400 30

65 I00

4 0 . 3 5 0.64 40.15 0.74

0.52 0.62

97 000 II0 000 280 135 340 270

MLS

periodic removal of Miquots during polymerization

3600

ST

0.57 4.0. I0-4

3.18.10"2

360

65

7300

ST

0.57

2.0.10 -4

3.18-10 "2

500

70

38.5 39.8 37.4 38.6

40 135 200

BS BS 85

0.39 2.7"10-2 0.71 1.2"10-2 0.52 2.0.10 "3

7.50.10-2 5.70"10-2 3.05.10 -2

45 35 120

98 75 25

46.8 44.6 43.8

0.18 0.14 0,12

0. I I 0.06 0.03

3 000 10 000 16 000

13 000

800

BS

0.52

2.0.10 -3

7.0 .10 "2

20

9g

41.4

0.19

0.09

59 000

57 000

880 2000

BS BS

0.52 0.49

1.0"10 -3 3.18"10 -2

56 g8

41.15 39.7

0,02 0.16

68 000 150 000

63 000 142 000

54 000

4.75-10 -2

g5 450

0.13

?.5-10 -4

2100

BS

0.23

1.7.10 -4

4.75.10 -2

50

50

39.6

0.14

0.03

160 0(30 149 000

153 000

2700

BS

0.32

1.8-10 -4

3,18.10 -2

265

SO

39.2

0.14

0.03

200 000

0.27

remarks

000 000 000 000

BimodH distribution (see GPC curves in appendix) 2 800 12 500

197 000

5200

BS

0.26 1.0-10-4 4.75"10"2

I05

65

38.I

0.21

0.0g

365 000 403 000 380 000

15000

BS

0.43 6.7.10 -5 4.75.10-2

130

70

36.2

0.30

0.16

1050000 1040 000 1150 000

25000

BS

0.32 2.4.10.5

200

60

35.2

0.29

0.14

1800000 1900 O00 1750 000

4.75-10-2

* Solvating agent DME instead of HMPT. ** Resulting from U = (U~pp- U~)/(t - U,); U, is obtained by using well defined PS-test samples.

Before starting the polymerization, the solid (CHs)2Si(OLi)2-salt was dissolved by breaking the glass-ventil GI. After breaking the ventil G2, the monomer solution shot into the evacuated flask A within a few seconds. This procedure provided rapid and simultaneous initiation. For all polymerizations the amounts of molecular sieves and solution were kept constant; the portion of the solution remaining with the molecular sieves was determined to be 15%. (CHa)Si(OLi) 2 does not introduce any end-groups in the polymer molecules. Furthermore, it was expected to yield unimodal distributions even in the presence of traces of moisture. The polymerizations were terminated by addition of TMCS. The solution was extracted repeatedly with ~o!9 ~.

I'] ~.rqo._B_~n

D

twice distilled water, dried over molecular sieves and purified by chromatography using neutral A120 3. Then the polymers were isolated at a rotatory evaporator. Further purification could be achieved by several reprecipitations from T H F solutions with methanol. Best results were obtained when the final isolation was done from n-pentane solutions. Purities of the samples were checked by i.r., u.v. and NMR spectroscopy. KINETICS

A rough kinetic investigation was performed in order to obtain the desired molecular weights in further experiments. The results are only for guidance because the system is very complicated and our polymerization procedure (BS) is not well suited for kinetic measurements. The reaction is 1st order with respect to D a and so we can write: diD3[ dt=

E

klDalllrmlZ[""

(1)

After integration 2"303 log 100 t 1 0 0 ~ - -X = k IIImIZl".

Fig. 2. Polymerization apparatus.

(2)

Ill = concentration of (CH3)2Si(OLi)2; IZI = concentration of solvating agent, X = conversion of D 3 in %. With n-hexane as solvent, H M P T as solvating agent and Li + as counterion, Eqn. (2) is satisfied best by the experimental data if m = 1/2 and n = 4 (see Fig. 3). The rll values are calculated as well from the amount of added initiator as from the conversion of D 3 and the molecular weight of the polymer. With

666

H.J. HOLLEand B. R. LEHNEN I

I

I

I

1,4x10"2

1.2xl0 "2 0

o

f xlO"2

I 8"

•

8x I 0 "~

6 x l O "~ 4 x l O "~

2 x l O 4'

,

I

2

3

4

( I)"~H~.JIPT)~ 10emot s'2 x (9,2

Fig. 3. Polymerization runs of D3 in n-hexane with (CH3)2Si(OLi)2 as initiator and HMPT as solvating agent. Temperature 25°. Conversion X determined by GC after derivatization with TMCS.

regard to the errors in III, IZI and in the conversion X determined by GC, we obtain for k at 25°:

DISCUSSION

A slightly higher value was obtained for perdeuterated D 3. A reaction order of 1/2 with respect to Ill has already been postulated by Shinahara [14] for the anionic polymerization of D,. However, Fig. 3 only contains polymerization runs with an III:IZI ratio < 1:80. At higher initiator concentrations, Eqn. (2) could not be verified: for example, at constant HMPT concentration of 5 x 10 -2 mol/l one even finds "negative" reaction-orders with respect to III, if II1 > 5 × 10- a mol/l. These results easily can be explained by the following reaction scheme:

As Table 1 shows, our BS-technique is superior to the usual syringe-technique in many respects. (a) Because of the improved exclusion of water, polymers with very narrow and unimodal MWD can be prepared up to molecular weights of 2 x 106. (b) By using (CH3)2Si(OLi)2 as initiator and TMCS as terminating agent, only methyl-groups are incorporated into the polymer molecules. (c) The polymerization apparatus can be such that large amounts of polymer can be prepared; for example 42 g of polymer PDMS 800 were obtained. The best results with respect to MWD are achieved for molecular weights between 2 x 10'L and 2 x 10s, if the conversion is less than 50%. For example an increase of conversion from 50 to 98% raises the nonuniformity of PDMS 2100 and PDMS 2000, which

Dissociation of dimeric active sites:

A2

k = (2-4 + 1-3) 105 min-l mo1-9/2 19/2.

Stepwise addition of 4 HMPT-molecules: Addition of D3 to A Z4 :

.... A + 4 Z ...... A Z4 + D3

Neglecting other propagation reactions, the rate of polymerization is given by: v = k, IDal IAZ, I = k, KIDal IAI IZl4 = k , K K~/2 IDal IA211/21ZI" ",, kDallll/21Zl'*

for

IA21~ III.

(3)

IZI is the concentration of the free solvating agent and can only be equated with the concentration of added solvating agent (IZol), if IZo] ~> III. According to our results, a propagating chain can bind 2 x 4HMPT-molecules; therefore an increase of 1II causes a greater decrease of IZI. Due to the high reaction order with respect to IZI, this occasions a reduction in the rate of polymerization.

I
2A

K

.....A

k,

Z,

•..... D3--A Z4.

are similar in molecular weight, from 0.03 to 0.16. Compared with polymers made by equilibration reactions, a non-uniformity of 0'16 at almost complete conversion is still surprisingly small. The polymers of higher molecular weight show their still relatively small non-uniformity also in light scattering experiments. Here the characteristic curvature of the P'ffo~ function shows up in the Zimm plots [ 15]. Despite their errors the kinetic data were very helpful for choosing the right polymerization conditions. Usually the desired conversions and molecular weights were obtained with less than 2~,,; deviation. It remains to be mentioned that, by a slightly modified procedure, we succeeded in preparing a "co"polymer with a deuterated middle- and non-deuterated endblocks. The composition of the sample was as expected.

Polydimethylsiloxane preparation

Acknowledgements--We thank Prof. Dr. R. G. Kirste for his great interest in this work. Dr. K. C. Berger for some helpful discussions concerning GPC and our excellent glass-blower Mr. M. Grimm. Furthermore. we are indebted to Wacker Chemie Ltd. for providing us with some chemicals.

REFERENCES

I. D. W. Scott, J. Am. chem. Soc. 68, 1877 (1946). 2. J. F. Brown and G. M. J. Slusarczuk, J. Am. chem. Soc. 87, 931 (1965). 3. C. L, Lee, C. L. Frye and O. K. Johannson, Polym. Preprints 10(2), 1361 0969). 4. J. C. Saam, D. J. Gordon and S. Lindsey, Macromolecules 3(1), I 0970).

667

5. J. C. Saam, A. H. Ward and F. W. G. Fearson, Polym. Preprints 13, 1,524 (1972). 6. R. G. Kirste, H. J. H611e and B. R. Lehnen, Colloid Polym. Sci. in press. 7. M. Szwarc, Carbanions, Living Polymers and Electron Transfer Processes. Interscience, New York (1968). 8. P. C. Juliano. W. A. Fessler and J. D. Cargiolo, Polym. Preprints 12, 150 and 158. 9. A. Haug and G. Meyerhoff, Makromolek. Chem. 53, 91 (1962). 10. D. Rahlwes, Ph.D. thesis, Mainz (1974). I1. A. Wesslau. Makromolek. Chem. 20, 111 (1956). 12. K. C. Berger and G. V. Schulz, Makromolek. Chem. 136, 221 (1970). 13. F. R. Jones, Europ. Polym. J. I0, 249 (1974). 14. M. Shinahara, Polym. Preprints 14, 2, 1209 (1973). 15. G. S. Greschner, Makromolek. Chem. 170. 203 (1973). 16. S. W. Kantor, J. Am. chem. Soc. 75, 2712 (1953).

APPENDIX

40

37

Counts

Counts

45

413 37 Counfs

36

44

I

43

I

33

42 41

Counts

46 32

38

36

42

Counts

GPC curves of: (a) PDMS from catalytic rearrangement of D4 with KOH; (b) PDMS 1500; (c) PDMS 7300; (dl PDMS 200; (el PDMS 2000: (fl PDMS 2100: (g) PDMS 25000: (h) polystyrene with M. = 195 000 and U = 0.005 [10].

Zusammenfassung--Es wird die Darstellung yon Polydimethylsiloxanen (PDMS) enger Molekulargewichtsverteilung (MWD) durch anionische Polymerisation von Hexamethylcyklotrisiloxan (D3) beschrieben. Bei Verwendung yon (CH3)2Si(OLi)2 als Initiator und (CHs)3SiCI als Abbruchsmittel wurden ausschliel31ich Methylgruppen enthaltende Pdiparate mit Molekulargewichten zwischen 2 x l0 s und 2 x l06 erhalten. Kinetische Untersuchungen wurden nur insoweit durchgeftihrt, als sic zur Beherrschung des Polymerisationsvorgangs unter den gewiihlten Bedingungen (LiSsungsmittel n-Hexan. Promoter Hexamethylphosphortriamid. Temperatur 25:) n6tig waren. Die Molekulargewichtsbestimmungen erfolgten durch Lichtstreu- bzw. nach Eichung durch Viskosit~ts- und GPC-Messungen. Die Uneinheitlichkeiten wurden aus den GPC-Kurven nach der 4a-Methode abgesch~itzt; die Excessgr6Ben der benutzten GPC-Anlage wurden mit Hilfe yon Polystyrol- und Poly-:t-methylstyrol-Pr~iparaten bekannter, extrem kleiner Uneinheitlichkeit bestimmt.