Novel Methods of Catalyzing Polysiloxane Syntheses - American

Chapter 3

Novel Methods of Catalyzing Polysiloxane Syntheses 1

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C. J. Embery , J. G. Matisons , and S. R. Clarke Downloaded by UNIV OF ARIZONA on September 22, 2015 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch003

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Polymer Science Group, Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Current address: School of Chemistry, Physic and Earth Sciences, The Flinders University of South Australia, G.P. Office 2001, Adelaide, South Australia 5001, Australia

Abstract Lewis Acid catalysis has also been presented, including a review of acid and base catalyzed equilibration polymerization of siloxane monomers has been presented, including the current Lewis acid perspective on equilibration polymerization by phosphoronitrile chloride catalysts. An overview of our research demonstrates that phosphoronitrile chloride equilibration catalysts occurs by an acid catalytic role, in preference to the commonly accepted Lewis acid pathway.

Introduction Siloxane polymers, which are commonly referred to as 'silicones', are commercially prepared by an equilibration polymerization reaction ; where hydrolyzate monomers are catalyzed with either strong bases (such as ammonia or alkali metal hydroxides) or strong acids (such as mineral acids or Lewis acids). Equilibration polymerization offers control over the resulting conformation and molecular weight of the polymer. Disiloxanes, such as hexamethyldisiloxane, are usually used as 'end blockers' in this equilibration polymerization, resulting in inert trimethylsilyl end groups for each siloxane polymer chain. When 1,3-difunctionaldisiloxanes are used as end blockers, then α,ω end-chain functionality can be incorporated into the polymer chain. 1 - 4

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© 2003 American Chemical Society

In Synthesis and Properties of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

Downloaded by UNIV OF ARIZONA on September 22, 2015 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch003

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Figure 1. Reaction for the acid catalyzed condensation of organodichlorosilanes to prepare hydrolyzate monomer 1,2

Commercial hydrolyzate monomer is a mixture of low molecular weight silanol end terminated linear siloxane oligomers (siloxanediols) and cyclic siloxane species, which is predominantly D (with some D and D ) ; the hydrolyzate being obtained by pouring the respective organohalosilane into ice water . (see figure 1). Chain extension of hydrolyzate monomer in the equilibration polymerization reaction has often been attributed to a relatively rapid catalyzed ring-opening polymerization (ROP) reaction, which is then followed by a slower re-equilibration reaction. It is for this reason that the catalyzed ring-opening polymerization reaction of cyclic siloxanes, and in particular, octamethylcyclotetrasiloxane (D ) has been extensively studied. 4

3

5

1,2

4

Base (Anionic) Catalyzed Ring-Opening Polymerization Strong bases, such as alkali metal hydroxides, alkali metal silanolates and amines will catalyze the ring-opening polymerization of cyclic siloxanes, such as D . Commercially, this is the most commonly used reaction to prepare poly(dimethylsiloxane) (PDMS), which is without a doubt, the major poly(siloxane) in global production. For the purposes of this article, a very simplified base catalyzed mechanism, involving anionic attack on the siloxane bond to open the cyclic ring has been shown (see figure 2). However, Chojnowski and others ' have studied the mechanism for this reaction, and have shown it to be far more complicated, involving the complexation of silanolate end-groups to form clusters, which influence the overall reaction. 4

1 - 6

5

6


28

OH" ÇH

s

a r t O hcH3

3

γη

•^HO-HSi-O-j—î-0~

\ I

CHj '2 CH

Λ 3

3

/

^CH, '

3

CH CH


3

CH, I . -Si-0

η ΉΟ-i-Si-O-

I n + 2

CE

3

Figure 2. Proposed Mechanism ofBase (Anionic) catalyzed ring-opening polymerization ofSiloxane Polymerizations ' 1,2

The reaction undergoes a rapid, anionic initiated ring-opening reaction, which gives chain extension, followed by a slower, back-biting, re-equilibration reaction, resulting in increased polydispersity. From the literature, " ' one particular group of basic catalysts has been particularly well documented. From the patent literature such catalysts include oligomers of neutral phosphoryl phosphazenes of the general formula, such as Cl[P(Cl) N] P(0)Cl and HO[P(Cl )N] P(0)Cl , (where 'n' is an integer). Hager and Weis have recently proved such catalysts to be extremely effective in promoting the polycondensation reaction. Research has also been carried out using oligophosphazenium salts of the general formula [Cl P(NPCl ) Cl] X\ where X can be a chlorine atom or a complex ion such as PC1 \ SbCl " or A1C1 * ; examples of such compromising hexachloro-1-λdiphosphaza-l-enium hexachloroantimonate salt [Cl PNPCl ] [SbCl ]" and ptrichloro-N-dichlorophosphoryl phosphazene (C1 PNP(0)C1 ) " . 2

5

79

9

1 0 - 1 5

9

2

n

2

2

n

2

1 6

+

3

2

6

n

6

4

+

3

3

7

3

6

8

2

1 7 - 1 9

It has been shown that the activity of this ring-opening of cyclic siloxane monomers increases strongly down the series Cs > Rb > K > Na > L i , which has been attributed to ion-pairing interactions becoming weaker as the cation size increases. As it stands, current commercial polymerizations of cyclic siloxanes by alkali metal hydroxides are disadvantageous from the viewpoint that the polymerization process is of a low polymerization rate at low reagent concentrations or temperature, and that the polymers thus obtained have poor thermal stability. This has been attributed in the literature to association of the active catalyst (which is the alkali metal silanolate) with itself ; this 'self association' needing to be broken before ring-opening polymerization can occur . Such bases are similar in structure, but undergo a different mechanism to the phosphoronitrile halides covered later. In a recent development , phosphazene bases of the form ((NMe ) PN) PNi-2ta were found to be strongly basic, with ΡΚ*, values of up to 10 times stronger than most other commercially available bases, such as +

+

+

+

5 , 7 - 8

1 7 - 1 9

1 7

2

3

3

18


+

29

diazabicycloundecene. It has also been proposed that with the new phosphazene materials, the catalytic activity being much greater because of the larger, and hence softer cation exhibiting little or no self-association. Research conducted with such bases has shown that upon contact with even trace amounts of water, the catalyst is activated and the highly active species [((NMe )3PN) PN^Z?w] [OH]" is formed ; such water being contained within all silicone hydrolyzate feed stocks. It has been shown that such phosphazene and phosphoronitrile chloride catalysts can be used at sufficiently low concentrations, over a broad temperature window and yet, yield high molecular weight polymers - the molecular weight of which may be controlled by end-capping or end-blocking with monofunctional halosilanes. +


2

3

Acid (Cationic) Catalyzed Ring-Opening Polymerization 20

21

2 2

Strong acids ' , such as mineral acids and acid modified clays have been used to catalyze the ring-opening polymerization of cyclic siloxanes. Commercially, this process is used to a lesser extent than anionic polymerization, but is important when hydrido cyclic siloxanes need to be polymerized by a ring-opening reaction. This results in hydrido functionality along the backbone. Base catalyzed polymerization of hydrido cyclic siloxanes results in unwanted hydrolysis of the silicon-hydrogen bond, causing silanol formation and ultimately, cross-linking across these silanol groups. The acid catalyzed ringopening procedure does not result in this ; however, the reaction is much less understood because it also involves not only a polymerization step and reequilibration (the reaction involving a polymerization reaction as shown in figure 3), but also, a slower, condensation step (involving the polymer chain ends) of the terminal silanol groups. Chojnowski and others ' have also studied this reaction in some detail in recent years. 1,2

5,6

2

However, after further research, it was proposed that the reaction also includes two very different and distinct pathways ; those of equilibration (figure 4) and condensation (figure 5). u

5

,

6

Lewis Acid Catalyzed Ring-Opening Polymerization 23

35

Lewis acids " , particularly transition metal complexes, organometallics and metal halides have also been used to catalyze the ring-opening polymerization of cyclic siloxanes. The actual mechanism of catalysis was studied and proposed as far back as the late 1960's. Lewis acid catalysis differs from other, conventional methods of siloxane polymerization in that no silanol species are generated - instead, donation of a lone pairfromthe oxygen atom to the metal occurs ; in the mechanism below, iron being an electron deficient atom, and chloride providing the leaving group. 2



Λ

0

CH, CH,

/

H j C

CH, CHj

A-Si-O-SÎ-O-î-OH

0

CHj

^

-HA

HA 0

CHj

CH,

CHj

A Si—O-Si-O-Si-OH

CHj CHj

/

SiC, "CH, ÇHj

CH,

CH,

CH. CH 3

A-Si-O-Si-O-î-OH

ÇHj

1,2

5

HjC

CH, CH,

A-Si-O-Si-O-î-OH

Figure 3. Proposed Mechanism of Acid (Cationic) catalyzed ring-opening polymerization of Siloxane Polymerizations ' '

^

0

A-


31


s

s/ si Figure 4. Equilibration Mechanism Pathway of Acid/Base Catalyzed Reaction

\ „ / —Si-O—SÎ—

-^Si-OH +

/

HX

\ ... / - —Si-O-SH- • X 7 H

—Si-X /

\ \ —Si-OH + — S i - X (1)

—Si-O—Si— + HX (2) / " \

—Si-OH + -^Si-OH ^ 7 - ^

-^Si-X + H 0 ^ _

^ i-o-s(^ S

• H 0 (3) 2

. ^ s i - O H + HX (4)

2

Figure 5. Condensation Mechanism Pathway of Acid/Base Catalyzed Reaction

—Si-O-Sî—

,

\3 .. 4 / —Si-O-Sl—

+

\ 1 . —Si—O:

+

—Si-O

2 Si—

^4 / SÙ—

Si—

V

,

\

_

+

\ 3 —Si-O-Si—

* S +

Si—

1 4/ S i - O — SI— + FeClj

Fe-CL Figure 6. Proposed Mechanism of Lewis Acid catalyzed ring-opening polymerization of Siloxane Polymerizations . In Synthesis and Properties of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Purpose of Our Research - to Investigate whether Lewis Acid Catalysis or Protonic Catalysis by Phosphoronitrile Halides? This use of the trimeric form of phosphoronitrile chloride in the ringopening polymerization of cyclic siloxanes by Wacker-Chemie has resulted in two patents being lodged, with respect to claims that the catalyst offers a number of advantages over conventional acid or base catalysis. These patents not only claim that the polycondensation product does not contain cyclic oligosiloxanes (due to the catalytic redistribution of linear polysiloxanes ; i.e. formation of a significant amount of cyclics), but also take account that the polymer had improved wettability characteristics and more uniform viscosities. The aim of our research was to investigate the nature of the industrial polymerization of commercially available silicone hydrolyzate by phosphoronitrile chloride type catalysts. It has been proposed that the catalysis of polymerisation by phosphoronitrile chloride oligomers occurs by a Lewis acid mechanism (figure 7), that is analogous to the mechanism detailed previously in figure 6 ; with phosphorous being an electron deficient atom, and chloride providing the leaving group. The overall intention of our studies was to investigate the validity of the proposed Lewis acid catalysis mechanism.


1 ( M 1

3 4 , 9

c CI

ci I

α I

ci I

I

I ci

I

C1-P=N-P=N-P-C1

I CI—P=N

Monomer

CI

CI

Linear Trimer

K

/ C

1

PCL< P

P

CK %/ ^ci Cyclic Trimer

Figure 7. Phosphoronitrilic Chloride catalystforms. The polymeric form selected for this research was the linear trimer Cl PNPCl NPCl3.PCl6, because it has the highest catalytic activity of all forms, even when compared with the monomer. Presumably, the ionic nature of the trimer promotes the activity of this Lewis acid catalyst . 3

2


33

Synthesis of Phosphoronitrile Chloride Type (Lewis Acid) Catalysts. The synthesis (and subsequent use) of Phosphoronitrile chloride type catalysts has been well documented throughout the literature in various patents and journals . Detailed below is a typical synthetic procedure. The synthetic procedurefromthe patent literature " was used to prepare the phosphoronitrile chloride " ' ; Polymerizations of commercially available hydrolyzate were carried out, samples were removedfromthe reaction mix and quenched with zinc oxide at pre-set intervals, and the resultant polymers characterized by Gel Permeation Chromatography (GPC). Gel permeation chromatography was used to determine molecular weights and molecular weight 36_44

I0


3

u

4 9

distributions, Mw I M , of polymer samples with respect to polystyrene standards, as well as the relative masses of cyclic oligomeric and linear polymeric species. n

Polymerization via Phosphoronitrile Chloride Catalysts. Hydrolyzate was weighed into a steel container, then heated to 75°C, with mechanical stirring. Phosphoronitrile chloride catalyst was added to the reaction vessel (see Table i) as defined in Table i. Zinc oxide (2:1 ratio with respect to the phosphoronitrile chloride catalyst) was used to neutralize the catalyst and subsequently terminates the reaction ; samples being taken at required time intervals (seefigure8). From figure 8, In-situ molecular weight measurements made during the reaction shows that from the start, the amount of D falls because it is being incorporated into the polymer, due to the onset of the equilibration reaction. This is also reflected in figure 9, at Stage 2 (discussed later). As the reaction 9

4

Table i. Masses of Starting Materials Employed in Polymerization Reactions. One Polymerization Catalyst (ppm) 432 Hydrolyzate mass 1946 (grams) 0.841 Catalyst mass (grams) Zinc Oxide mass 1.219 (grams) 44.1 %D 4

Two 800 1000

Three 1503 1252.5

Four 3000 2000

0.770

1.883

6.00

1.553

2.683

8.430

40.5

31.6

40.7


34 proceeds, the amount of D in the system decreases exponentially as it is reacting, with a corresponding increase in the percentage of linear polymer in the system. From the molecular weight analysis shown in Figure 9, it is seen that Stage 1 is representative of the initial condensation reaction, which consumed all of the linear oligomers at the start of the reaction, leading to a high molecular weight polymer. Stage 2 shows a large number of D molecules being incorporated into the polymer, due to the onset of the equilibration reaction, but this number (as well as the molecular weight of the polymer chains) decreased upon reaching Stage 3. The equilibration process resulted in the most stable system at Stage 4, however a minimum molecular weight for the polymer was observed. The subsequent increase in molecular weight is due to the continued, but slow inclusion of D into the polymer ; current thinking assumes that a final equilibrium can be seen, accounting for the plateau at Stage 5. 4


4

9

4

2

Noll proposed catalysis of the ring-opening polymerization reaction of cyclic siloxanes via Lewis acids such as phosphoronitrile halides was not possible. Conversely, the patent literature ~ , independently suggests that this is necessarily be the case. It was found that during the polymerization, a definite decrease in the D content of the system occurred (as in figure 8), which points towards it being ring-opened, and included in the final product (see also figure 8). This is in agreement with the patent literature " . We believe that the polymerization of low molecular weight siloxanes by phosphoronitrile chloride acts via two different pathways - equilibration and condensation. During the course of the reaction, Hydrogen Chloride gas was evolved, and we are of the opinion that the catalyst is hydrolyzed by water and that H is actually catalyzing the reaction. I0

15

4

10

15

+

Our Proposed Mechanism

The alternate, and less predominant pathway of catalysis is that of direct Lewis acid catalysis, which is a reaction sequence analogous to that mentioned previously from figure 6. It should be clearly noted at this stage, that the reaction pathways provided show two very distinct ends of a spectrum of proposed mechanisms. There is some experimental evidence to suggest that the hydrolyzed form of the catalyst (figure 10) was involved in the reaction, but this role is currently open to speculation.



1000

2000

3000 40000

5000

6000

Reaction Time (m ins)

Figure 8. Schematic Representation of changes in percentages of cyclic siloxane / polymeric siloxane product with time

τ

1

Reaction Time Figure 9. Diagram of Molecular Weight versus Reaction Time from GPC analysis of quenched polymer samples.


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CI 9 CI C I — P N P - P N - C I + HCI θ / d)H C I 1

N


H

H Figure 10. Schematic Diagram ofHydrolysis of a Phosphoronitrile Chloride Catalyst Molecule ' ' 3 4

Results and Discussion It seems that all of the species concerned in the catalysis reactions were involved in competing reactions, and one pathway was dominant in catalyzing the entire reaction ; alternatively, the catalyst may involve catalyzing just one of the reactions (equilibration or condensation). A third possibility is that one of the species involved may catalyze a reverse reaction, thus reducing the overall reaction rate. In this situation, hydrolysis of a catalyst molecule (as detailed above in figure 8) leads to the release of a proton, and thus protonic acid catalysis is much more likely to be the dominant pathway. This may be more clearly understood when we consider that the rate of reaction with respect to protonic acid catalysis is several times higher than that with Lewis acid catalysis . Recent literature suggests that a 'back-biting' reaction may be occurring, thus cyclic molecules are produced from the polymer during the course of the equilibration, but further P NMR spectroscopy could provide further information on this. This is actually in good agreement with work done by Chojnowski et al., who showed that in the first stage of the polymerization process, linear oligomers were formed as almost the sole product of the reaction (condensation), then consecutively, a redistribution reaction (equilibration) took place. It was also noted that the formation of cyclic oligomers was considerably delayed with respect to linear species. Thus, Chojnowski's kinetic analysis was based on the logical assumption that the redistribution step leads exclusively to the equilibrium between linear species, and hence neglects the formation of cyclic species. Current research by Dow Corning " has steered away from the more common practices of using compounds such as barium hydroxide, acid clays and potassium and ammonium catalysts to drive their reactions ; however, such 10

1 5

31

5 , 7 - 8

12

14, 1 7


37

catalysts have been found to be non-specific with respect to the reaction pathway of condensation. Such phosphoronitrile halide catalysts have been extensively studied, particularly by Dow Corning, " ' and it has been acknowledged from such studies that the active species for silanol condensation is species '3' in the reaction scheme proposed below. In this mechanism, the species is being formed by hydrolysis of the precursor complex Γ through to the oxo derivative '2', followed by further hydrolysis to the amidic acid complex '3'. 12

14

17

4

Cl ,C1 Downloaded by UNIV OF ARIZONA on September 22, 2015 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch003

x

ci c.

p

x/

CI

Ν

/

ci—P-ci

2H 0

Cl^ * /

2

Cl

-

Cl Cl

+ poclj

N

c i - p = o

+ HC1

CI

Cl 1

Cl

Cl

Cl'

N

CI

N

2H,o

/ ci—p=o

ci , 0 H cr ^ + HC1

i —

N

/

c i - p = o

Cl

Figure 11. Proposed Dow Corning mechanism of Phosphoronitrile halide catalysis ' n 14,

17

Conclusions It has been shown that the mode of catalysis achieved by phosphoronitrile chloride is a combination of condensation and equilibration. Therefore, it may be concluded that the active catalyst exists as the hydrolyzed form; and as a result, the actual catalytic species is the hydrogen ion. This may be properly understood when it is taken into account that the rate of reaction with respect to protonic acid catalysis is several times higher than that with Lewis acid catalysis. The effect of the concentration of this catalyst has also been ascertained, and it appears to have little effect on the condensation process, but a definite effect on the equilibration process. This dependence was not linear, but a definite correlation has been shown to exist.


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The difference in results between hydrolyzate batches shows that the system is complicated and there are many factors affecting the reaction that are yet to be discovered, or at least quantitated. Both H and P NMR spectroscopy may provide valuable information about the active form of phosphoronitrile chloride in this system. 1

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Acknowledgements. The authors of this paper would like to thank Dow Corning Proprietary Limited, our industrial sponsors, for their much appreciated help and financial support in this investigation.

References 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Brydson J.A, Plastics Materials ; Butterworth-Hall : New York, 1996. Noll W., The Chemistry and Technology ofSilicones ; Academic Press : 1968. Fielden, M., Embery, C.J., Matisons, J.G., RACI Polymer Division, 24 RACI Australasian Polymer Symposium, Beechworth, Victoria, Australia. Fielden, M., Embery, C.J., Britcher, L.G., Clarke, S.R., Matisons, J.G., ACS Spring Meeting, San Diego, California. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2001, 42 (1), 165. J. Chojnowski, Siloxane Polymers ; Prentice-Hall, Englewood Cliffs, New Jersey, 1993 ;p1. S. Boileau, Ring-opening Polymerization ; American Chemical Society : Washington, DC, 1985 ;p23. th

Chojnowski, J., Cypryk M., Fortuniak, W., Kazmierski, K., Taylor, R.G., J. Organomet. Chem., 1996, 526 (2), 351. Chojnowski, J., Fortuniak, W., Habimana J., Taylor, R.G., J. Organomet. Chem., 1997, 534, 105. Fielden, M. : Applied Chemistry Project, University of South Australia. Burkhardt J., US Patent No. 4,053,494. Burkhardt J., US Patent No. 4,203,913. Bischoff R., Currie J., Herron W., Taylor, R.G., Dow Corning Ltd., European Patent No. EP 0 860,461 A2. Harkness B., Taylor, R.G., Dow Corning Ltd., European Patent Number EP 0 860,459 A2. Bischoff R., Taylor, R.G., Dow Corning Ltd., European Patent Number EP 0 860,460 A2. Nitzche S., US Patent No. 2830967.



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16. Hager R., Weis, J., Ζ. Naturforsch., 1994, 49, 1774. 17. Taylor, R.G., 2001, 43 (ISPO Presentation 2001 ; Conference on silicon containing polymers at Canterbury, Kent, England). 18. Melikyan N.O., Tergazarova D.A., Arm. Khim. Zh., 1969,, 22, 82. 19. Kuznetsova, A.G., Ivanov V.I., Golubtsov S.A., Zh. Obsch. Khim., 1970, 40, 706. 20. Adrianov, K.A, Shkol'nik M.I., Kopylov V.M., Baravina N.N., Vysokomol. Soedin Ser. B., 1974,.16, 893. 21. Voronkov, M.G., Sviridova N.G., Zh. Obshch. Khim., 1976, 46, 126. 22. Bennett D.R., Matisons J.G., Netting A.K.O., Smart R.St.C., Swincer A.G., Polym. Int. 1992, 27, 147 23. Mingotaud A.F., Cansell F., Gilbert N., Soum Α., 1999, Polym. J. (Tokyo), 31 (5), 406. 24. Yoshioka H., German Patent No. DE 3932231. 25. Ohba T., European Patent No. EP 0393954. 26. Andrianov K., Izmailov B., Zh. Obshch. Khim. (Journal of General Chemistry), 1965, 5, 35 (2), 333. 27. Vdovin V., Nametkin N., Finkelshtein E., Oppengeim V., Izv. Akad. Nauk SSSR, Ser. Khim. (Bulletin of The Academy of Sciences of The USSR, Chemical Series), 1964, 3, 458. 28. Omietanski G., British Patent No. GB 1066574. 29. Scott D.W, J. Am. Chem. Soc., 1946, 68, 2294. 30. Klebansky A.L., Fikhtengolts V.S., Karlin A.V., 1957, J. Gen. Chem. USSR, 27, 3321. 31. Prut Ε.V., Trofimova G.M., Yenikolopyan N.S., 1964, Vysokomolekul. Soedin. 6, 2102 32. Prut E.V., Trofimova G.M., Yenikolopyan N.S., 1964, Polymer Sci. USSR, 6, 2331. 33. Borisov S.N., Sviridova N.G., J. Organometal. Chem., 1968,11,27. 34. Tabuse Α., Minowra Y., Kogyo Kagaku Zasshi, 1969, 72, 2133. 35. Pace, S.C., Riess J.G., J. Organometal. Chem., 1976, 121, 307 36. Van Dyke M.E., Clarson S.J., Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.), 1996,37(2), 668. 37. Van Dyke M.E., Clarson S.J., J. Inorg. and Organomet. Polymers, 1996, 8 (2), 111. 38. Schwesinger R. et al., Liebigs Ann., 1996, 7, 1055. 39. Schwesinger R. et al., Chem. Ber., 1994, 127, 2435. 40. Schwesinger R. et al., Angew. Chem.Int.Ed. Engl., 1993, 32, 1361 41. Schwesinger R., Tech. Lab.,1990 38,1214. 42. Schwesinger R. et al., Angew. Chem., 1987, 99 (11), 1210. 43. Esswein B., Molenberg Α., Möller M., Macromol. Symp. 1996, 107 (International Symposium on Ionic Polymerization) 44. Molenberg Α., Möller M., Macromol. Rapid Commun. 1995, 16, 449.


Novel Methods of Catalyzing Polysiloxane Syntheses - American

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