Copolymers Based on Dimethylsiloxane and Diphenylsiloxane Units

Chapter 7

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Copolymers Based on Dimethylsiloxane and Diphenylsiloxane Units Thomas M. Gädda and William P. Weber D. P. and Κ. B. Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, CA 90089-1661

A number of novel copoly(dimethylsiloxanes/ diphenylsiloxanes)s have been prepared and characterized. These copolymers have oligodiphenylsiloxane blocks located either alternatively in the polysiloxane main chain or at the chain ends of polydimethylsiloxane (PDMS). Alternatively, alkylheptaphenylcyclotetrasiloxanes were distributed either randomly as grafts along the PDMS backbone or at the chain ends of PDMS of different molecular weights. These copolymers were characterized by 1H, 13C and 29Si NMR, IR, and UV spectroscopy. Molecular weights were determined by GPC as well as by NMR end group analysis. Thermal transition temperatures and stabilities of copolymers were analyzed by DSC and TGA, respectively.

82

© 2007 American Chemical Society

In Science and Technology of Silicones and Silicone-Modified Materials; Clarson, Stephen J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

83

Introduction A variety of copoly(dimethylsiloxanes/diphenylsiloxanes)s have been prepared and can be found in the literature. These copolymers have been prepared to improve the thermal stability of polydimethylsiloxane (PDMS), alter its viscosity or to reduce its crystallinity. Triblock diphenylsiloxanedimethylsiloxane-diphenylsiloxane polymers, on the other hand, have been prepared in the hope that they would exhibit thermoplastic elastomeric properties analogous to those of polystyrene-polybutadiene-polystyrene (PS-PB-PS). Siloxane polymers, compared to polyolefins, possess higher thermal stability due to the strength of the siloxane bond, which does not undergo thermolysis easily. Thus, polysiloxanes are well known to undergo degradation by inter- and intramolecular cyclization reactions to lose volatile cyclosiloxanes. Polydiphenylsiloxane (PDPS) has been proven to exhibit very high thermal stability and does not decompose before 500 C . The bulky phenyl groups of PDPS may hinder depolymerization reactions. Thus, the thermal stability of PDPS is roughly 250 °C higher than PDMS in inert atmospheres. For this reason, aromatic or arylsiloxy units may be incorporated into PDMS to improve its thermal stability. Despite high thermal stability, PDPS has found little commercial use. This may be due to the difficult preparation of PDPS which results from its unfavorable ring-chain equilibria. In addition, PDPS requires very high temperatures for isotropic melt formation, which in turn results in serious problems related to its melt processing. ' Besides good thermal stability, polydialkylsiloxanes are also well known for their low temperature flexibility. This predominately results from the characteristics of the siloxane bond. However, polysiloxanes are semicrystalline and the formation of crystals has limited the low temperature use to their melting temperatures. Hence, arylsiloxane units have been incorporated into PDMS and polydiethylsiloxane to distrupt crystal packing and to prevent the undersirable crystallization. " These modified materials may retain rubbery properties at temperatures below their T and hence show significantly improved low temperature flexibilities. The dçsirable properties of the first thermoplastic elastomers, i.e. polystyrene-polybutadiene (or polyisoprene)-polystyrene triblock copolymers, have provided encouragement to prepare analogous siloxane based materials. PDPS-PDMS-PDPS triblock copolymers have been prepared by sequential anionic ring-opening polymerization (AROP) of hexamethylcyclotrisiloxane (D ) and hexaphenylcyclotrisiloxane (P ) at high temperatures. Elastomers may be obtained from PDPS-PDMS-polyvinyl-methylsiloxane triblock copolymers after peroxide initiated radical coupling. Diblock copolymers have also been prepared and their thermal properties thoroughly investigated. While these


1

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0

2-4

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84

materials demonstrate the expected thermal transitions by DSC, no spectral data in support of their structures have been reported. Herein, we report our preparation of copoly(dimethylsiloxanes/diphenylsiloxanes)s. The length of the diphenylsiloxane blocks were kept short to avoid the difficulties associated with higher molecular weight PDPS and its preparation. Never- the-less, we hoped that the copolymers would exhibit phase separation to yield new elastomers. While high molecular weight PDPS is practically insoluble, oligodiphenylsiloxanes with low degrees of polymerization (DP < 11-18) are sufficiently soluble in common organic solvents at room temperature for analysis. In addition, low molecular weight PDPS does not form liquid crystalline mesophases, which limits the processability of PDPS. 14,15

Hydrosilylation copolymerizations - PDPS-PDMS multiblock copolymers A series of copolycarbosiloxanes were prepared by Pt-catalyzed hydrosilylation co-polymerization of 1,9-divinyldecaphenylpentasiloxane (I, Scheme 1), 1,7-divinylocta-phenyltetrasiloxane (II), 1,5-divinylhexaphenyltrisiloxane (III) and 1,3-divinyltetraphenyldisiloxane (IV) with α,ω-dihydridodecamethylpentasiloxane (V). Oligomers I-III were prepared by treating a α,ω-dihydroxydiphenyloligosiloxane with butyl lithium and vinyldiphenylchlorosilane. A similar series of copolycarbosiloxanes was prepared by copolymerization of I with α,ω-dihydridooligodimethylsiloxanes. 16

17

The α,ω-divinyloligodiphenylsiloxanes were synthesized in good yields. The oligomers and the copolymers were highly soluble in common organic solvents at room temperature. The majority of diphenylsiloxanes and silanes described in the literature are solids with melting points above room temperature. The oligodiphenylsiloxane described herein, however, were viscous liquids at room temperature, except for IV which displayed a melting point at 80 °C. Never-the-less, melting like transitions between -35 °C and -14 °C were measured by DSC for oligomers I-III. The temperatures of these transi-ions increased with increasing oligomer molecular weight. A similar trend between the copolymer T 's was observed and a linear relationship was found between copolymer T and their diphenylsiloxy content (Figure la). Each copolymer exhibited only a single thermal transition by DSC. This is consistent with the lack of phase separation in the copolymers. These results and the low melting temperatures of oligomers I-III suggested that a longer oligodiphenylsiloxane segment may be required to achieve phase separation. The NMR spectra of oligomers and copolymers confirmed their structures. However, the absence of end groups in the *H or S i NMR spectra did not permit calculation of accurate number average molecular weights. In addition, no g

g

29



85

I t i

I é

I I

f

ιî ·«·*

1 1

ο

to



a)

20

ί

-50 Η

1

1

1

1

Γ-

45

55

65

75

85

95

wt-% diphenylsiloxy units

0 -I

1

r

1

1

r-

r-

r—

0

100

200

300

400

500

600

700

Temperature [°CJ Figure 1. a) Glass transition as a function of diphenylsiloxy content and b) thermal stabilities of alt-copoly(dimethylsiloxane/diphenylsiloxanes). Dx-Py where χ is the number of dimethylsiloxanes units (D) and y is the number of diphenylsiloxanes units (P).


87 Si-Η signals could be observed in the IR spectra. A l l of the copolymers displayed good thermal stability in N as expected for a copolymer with diphenylsiloxane units in the main chain (at least to 400 °G, Figure lb). Above this temperature a single weight loss process occurs. Clear differences between the copolymers were observed for the onset and the quantity of char residue when the TGA analysis was carried out in N . Longer oligodiphenylsiloxane segments lead to a higher onset temperature for the thermal decomposition as well as to higher char yields. 2


2

PDMS with terminal and pendant 2-(heptaphenylcyclotetrasiloxanyl)ethyl groups In general, there has been considerable interest in modified polymers and their preparation. " Recently, a,co-bis(pyrene)PDMS was reported to exhibit thermoplastic elastomeric properties and reduced crystallinity. Similarly, we have modified PDMS by end terminating and randomly modifying it with heptaphenylcyclotetrasiloxy moieties (Ph D -moieties, Scheme 2). Vinylheptaphenylcyclotetrasiloxane (VI), prepared from vinylphenyldichlorosilane (VII) and hexaphenyltrisiloxane-l,5-diol (VIII), was introduced to various Si-Η functional PDMS polymers by Pt-catalyzed hydrosilylation. The chemical modification significantly altered the physical properties of the material. End group analysis by H and S i NMR was used to calculate molecular weights and polymer compositions. The molecular weights, obtained from the ratios of the integrals of repeating dimethylsiloxy units to the integral of the SiH(CH ) or - S K C H s H C ^ ^ P l ^ units, calculated before and after modification were in very good agreement with each other (Table 1). Also, the molecular weights obtained by GPC are in reasonable agreement with NMR end group analysis. These results indicated that a possible side reaction involving the Pt catalyzed oxidation of Si-Η to Si-OH by water, followed by condensation of the Si-OH did not occur. Thus, the PDMS block before and after modification is constant and changes in the material properties are due to the chemical modification. The measured T 's for the modified polymers were very close to that of PDMS (Table 1). On the other hand, a significant reduction in crystallinity of the PDMS was observed for all modified samples when compared to their unmodified counterparts. In fact, some samples were completely amorphous materials and showed no signs of melting or cold crystallization, while higher molecular weight samples displayed reduced crystallinities (Figure 2a). Crystal formation was inhibited, when the bulky Ph D unit is introduced at the ends of a low molecular weight PDMS or randomly as grafts to the PDMS main chain. No melting transition from a Ph D phase was observed, consistent with a homogenous material that does not undergo phase separation. 18

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In Science and Technology of Silicones and Silicone-Modified Materials; Clarson, Stephen J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007. 4

7

7

7

4

3% Ph D -PDMS

Scheme 2. Preparation ofvinylheptaphenylcyclotetrasiloxane (VI) and modified PDMS.

4

4

7

Ph D -XI Ph D -XII Ph D -XIII


89 Thermal and thermo-oxidative stabilities of these modified materials were comparable or higher than those of a,œ-bis(trimethylsiloxy)PDMS ( M / M = 17400/8300 by GPC). The modified polymers started to thermally degrade in N at -350 °C. While the onset of thermal degradation was nearly the same for all modified materials, significant increase in char yield was observed as the ratio of dimethylsiloxane units to diphenylsiloxane units decreased. This is consistent with a different degradation mechanism for phenyl containing siloxanes compared to PDMS. Higher char yields for PDPS has been attributed to formation of cross linked networks by way of radical coupling. ' Although phase separation was not detected, relatively large increases in melt viscosities were measured when the viscosities of unmodified a,co-bis(hydrido)PDMS is compared to the corresponding viscosities of modified materials (Table 1). Since the average molecular weights of the polymers have not drastically changed, the observed increases in viscosities may be due to intermolecular attactive interactions of the terminal and pendant Ph D groups. w

n

2


1 2

7

4

Table 1. Molecular weights, viscosities and crystallinities of materials before and after modification. Ph D modified PDMS

Si-Ηfunctional PDMS

7

4

29

Si NMR M„ (g/mol)

Viscosity (CPs)

IX

1570

X

29

(%)

Si NMRM„ (g/mol)

Viscosity (CPs)

(°C)

(%)

10

79

2620

1920

-57

0

3000

34

60

4750

720

-119*

0

XI

4300

80

64

7500

611

-121**

0

XII

14800

463

56

15400

1330

-123

40

XIII 3% SiHPDMS CROP of XIV AROP of XIV

28300

773

52

30500

2140

-123

44

13300

234

+

15300

3870

-123

0

—

...

...

13700*"

++

4

0

—

...

...

10500

-1

0

CCcrysl

•H-

&cryst

*T = -120 °C and **T = 123 °C after 2 h equilibration at -60°C. "'measured by *H NMR. AHf° unknown ^not measured g

g

+


90


a)

-150

-130 -110

-90

-70

-50

-30

-10

10

30

50

Temperature [°C]

Figure 2. a) Normalized DSC thermograms ofstarting and modified materials; b) thermal decomposition profiles in N of modified polymers and a PDMS standard. 2


91 b)


100

100

200

300 400 500 Temperature [°C]

600

700

800

Figure 2. Continued.

Attempts to carry out similar Pt-catalyzed hydrosilylation of copoly(hydridomethylsiloxane/PDMS) materials with Si-Η content higher than 3 mol-% gave crosslinked materials. Such crosslinking may be due to the undesired Pt-catalyzed oxidation of Si-Η groups by water followed by condensation, if reactions are not carried out under strictly anhydrous conditions. To avoid crosslinking and to allow preparation of copoly-mers with higher percent of modification, we prepared l-P'-iheptaphenylcyclotetrasiloxanyl)ethyl]-l,3,3,5,5-pentamethylcyclotrisiloxane (XIV) and studied its ringopening polymerizations. Analogous bicyclic siloxanes and a number of 1,2bis(cyclotri- and cyclotetrasiloxanyl) substituted ethanes to XIV have been prepared. " Attempts to selectively polymerize these by ring opening of the more reactive cyclotrisiloxane to yield linear materials have been unsuccessful. To our knowledge, ROP of XIV is the first example of a regioselective cationic ring opening of a cyclotrisiloxane in the presence of a cyclotetrasiloxane (Scheme 3). The polymer properties of such a material may be determined by the bulky heptaphenylcyclotetrasiloxanyl group which is attached to every third silicon in the polymer back-bone. Both acid (CROP) and base (AROP) catalyzed ring-opening polymerization of XIV gives highly viscous, transparent polymers. Based on S i NMR spectroscopy, the polymers appear to result exclusivelyfromring opening of the cyclotrisiloxane ring. No evidence for ring-opening of the cyclotetrasiloxane ring was observed. T s of 4°C and -1 °C were measured for the polymers prepared by CROP and AROP, respectively. This is significantly higher than the T observed 25

26

31

29

g

g


92 , 1)

AROP

I

I

©Θ Θ© Li O - S i - O - S j - 0 Li THF, rt. 2h

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R - S i - OV- f S i - O - S i - O - S i - O j - S i - O - S i - O - S i - O - S i - R n ;

s-o, W

XIV

SUP

R = HorVi

I

I

HSj-O-Sj-H

CROP

CF3SO3H (cat.) CH C1 , -40 °C, 5 min 2

2

Scheme 3. AROP and CROP of XIV.

for the 3 mol-% randomly modified PDMS reported earlier, which had a T of-123 °C. This suggests that bulky side groups reduce the free segmental mobility of the flexible Si-0 bond. However, evidence for phase separation as a result of increased amounts of the pendant heptaphenylcyclotetrasiloxane groups was not observed. g

PDPS-PDMS-PDPS triblock copolymers PDPS-PDMS-PDPS triblock copolymers were prepared by sequential monomer addition of hexamethylcyclotrisiloxane (D ) and hexaphenylcyclotrisiloxane (P ) to a difunctional anionic initiator, or by a convergent method (Scheme 4). The convergent method uses Pt-catalyzed hydrosilylation to couple known blocks. This was used to confirm the properties and structure of the polymers prepared by sequential monomer addition. Starting materials are shown in Table 2. Diphenyl ether has been the usual solvent used for anionic polymerization of P . We found that o-dichlorobenzene is a suitable solvent for AROP of P . This and the use of lower reaction temperatures allowed us to prepare well defined triblock copolymers. As previously mentioned, PDPSPDMS block copolymers have been prepared but no spectral evidence in support of their structures has been provided. 3

3

3

3


In Science and Technology of Silicones and Silicone-Modified Materials; Clarson, Stephen J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007. Λ

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ΡΠΡ^-ΡΗΜ^-ΡΠΡ^ rurz-ruMb-rurb triblock copolymer

Scheme 4. Sequential monomer addition and convergent route to PDPS-PDMS-PDPS triblock copolymers.

CONVERGENT METHOD:

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Ο" Ο ο) U«K-vOl */^0*\

©Θ . „„ U O-Si-0 U

SEQUENTIAL MONOMER ADDITON:


94 The PDPS-PDMS-PDPS copolymers (XV-XVII) prepared by sequential monomer addition were obtained in relatively low yields and molecular weights. This is may be largely due to the nature of the ring-chain equilibria for diphenylsiloxanes. It is well known that the equilibrium for siloxanes bearing bulky substituents are strongly shifted towards cyclics as the conformational freedom for the linear polymer is reduced when compared to dimethylsiloxanes. Despite this, they behaved like chewing gum or hard, slightly elastic waxes at room temperature. Polymers prepared by the convergent method were similar in physical appearance. As expected for triblock materials thermal transitions for both phases can be observed for all copolymers (Figure 3, Table 3). A T (~-120 °C) and a T (-50 to -40 °C) for the PDMS phase was seen, while the PDPS phase of the triblock copolymers displayed two melting endotherms, T i and Tntf (160-190 °C), as observed by DSC. T i's and T^'s measured are in the temperature range in which melting transitions of low molecular weight diphenylsiloxane oligomers have been reported to occur. However, these melting transition of the PDPS phase are somewhat lower than those reported. ' On the other hand, the T of PDPS (40 °C) of the highly crystalline PDPS blocks could not detected. XVII displayed a partially random structure and hence its melting temperatures were lower than for the other copolymer prepared. The PDPS oligomer used in the preparation of copolymers XVIII and XIX displayed its highest melting peak at -200 °C. Heats of fusion obtained for PDMS and PDPS crystal melting in copolymers X V and XVI were determined. X V displayed a A / / = 9.3 J/g and a AH = 3.5 J/g while corresponding values for XVI were A// p s 10.0 J/g and AHf 3.7 J/g. The A// p ps is the total heat of fusion over the whole melting transition and is close to values previously reported. On the other hand the enthalpies measured for the PDMS phases were lower than those of PDMS homopolymer. This suggests that the PDPS blocks restrict mobility of PDMS chains and in this way reduces their ability to form crystals. Degrees of crystallinities were 15 % and 16 % for X V and XVI, respectively, using A// ° = 61,19 J/g in calculations for a fully crystalline PDMS. Thermal transition temperatures were confirmed and mechanical properties of the materials were evaluated by DMA. Similar storage and loss moduli profiles were obtained for all copolymers.


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The regular structure of our triblock copolymers was confirmed by H , C , Si NMR and DSC. In the H NMR spectra, a complex phenyl region was observed. The triblock structure can be confirmed form the H and C NMR spectra since only a sharp singlet for the -Si(CH ) 0- group can be observed in the dimethylsiloxy region of the spectra (Figure 4a). If randomization occurs, several additional peaks will be visible in this -(CH ) SiO- region (Figure 4b). Similarly, for a regular triblock copolymer, a single peak at —21.8 ppm should be observed in the -(CH ) SiO- region and at —46.5 ppm in the -Ph SiOregion of the S i NMR spectra. In the in the -(CH ) SiO- region a single peak is observed (Figure 5a). However, two relatively broad peaks are observed in the 29

!

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2

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95

Table 2. Starting materials used and molecular weights of copolymers.


Sample

GPC Molecular Weights

Theoretical Mn:s

Feed

Initiator PDMS PDPS M„ total ,^ , MJM mmol kg/mol kg/mol kg/mol kg/mot kg/mol w

n

g

g

XV

0.38

0.6

0.017

22.5

36

58.6

15

8.8

1.7

XVI

0.5

0.6

0.017

30

36

66

18

10.3

1.8

XVII

0.5

0.5

0.023

22

22

44

16.1

11.8

1.4

χνπΓ

1

0.62

-

14

3.8

21.6

36.4

24.9

1.5

XIX*

1

0.31

-

28

3.8

35.6

52.7

31.3

1.7

*Feed in grams of polymer.

-150

-100

-50

0

50

100

150

200

250

Temperature [°C] Figure 3. DSC thermograms of PDPS-PDMS-PDPS triblock copolymers.


96 triblock

a)

paitially random

b)

4

il Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0964.ch007

140

120

100

I 80

60

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·1

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140

I120

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Figure 4. C NMR of a PDPS-PDMS-PDPS triblock copolymer XV (a) and partially random copolymer XVII (b). -Ph SiO- region in the Si NMR spectra instead of one signal as expected (Figure 5a). In addition to the signal at —46.5 ppm, a signal of lower intensity at —47.5 ppm in the -Ph SiO- region is observed. This signal results from Ph SiO- units that are adjacent to -(CH ) SiO- unit. 2

15,36

2

2

3

2

29

Figure 5. Si NMR of a PDPS-PDMS-PDPS triblock copolymer XV (a) and partially random copolymer XVII (b).

29

Although the S i NMR confirms distinct blocks for PDMS and PDPS, the complete analysis of the oligodiphenylsiloxane blocks by S i N M R is problematic. Integration of the -(CH ) SiO- and -Ph SiO- regions should give information on the composition of the copolymers. However, the integration results from *H and S i NMR are different and *H NMR gives higher -Ph SiOcontent than that obtained from S i N M R integration (Table 3). This discrepancy probably results from different relaxation times that S i nuclei have depending on their chemical environment. In general, S i NMR spectra are acquired with a similar pulse sequence used to acquire C N M R for which it is known that adjacent protons enhance the relaxation of the nuclei during NMR experiments. Thus, the lower signal intensities for -Ph SiO- units compared to -(CH ) SiO- units, is probably due to the fact that the closest proton is three 29

3

2

2

29

2

29

29

29

l 3

37,38

2

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2


97 bonds away in a phenyl substituted Si atom, whereas it is only two bonds away in a methyl substituted Si atom. Although integration of C spectra is often thought to be impossible, scientists appear to believe that Si NMR can be integrated. ' In our experience, this is not always fully possible. To solve similar integration problems paramagnetic impurities have traditionally been added to NMR samples. These provide a different relaxation mechanism for the Si nuclei. ' Addition of Cr(acac) increases the -Ph SiO- Si signal intensity by -30 % relative to the (CH ) SiO- signal. For instance, SiMe 0/SiPh 0 ratio for copolymer XVII decreased from 7.07 to 4.89. However, attempts to achieve comparable Si integration results to *H NMR or to sharpen the broad -Ph SiO- peaks by variable temperature NMR were unsuccessful. Similar inconsistencies in the integration results of H NMR and Si NMR of copolymers XVIII and XIX to those of copolymers XV-XVII were obtained. 13

29

36 39

29

37 38,40

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2

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Table 3. Polymer yields, NMR integration and thermal properties of copolymers. Theoretical Integration DSC Polymer Ratio Integration Thermal Properties yield SiCHj/SiPh SiCH/SiPh HNMK* / HSi Τg Τ m, PDMS Τ m, PDPS Sample [%] Si NMR NMF? NMR* [°C] [°C] [°C] 161, XV 3.79 -121 -51 51 1.67 3.06 181 176, XVI -47 5.34 -117 58 2.23 3.43 193 116 XVII -123 -37 4.89 51 2.68 3.23 l

l

29

29

χνπΓ

93

4.97

4.79

6.78

-120

-43

XIX*

94

9.95

6.01

9.12

-119

-43

a

161, 206 184, 199

b

ratio = ( W 6 ) / ( W H > ) ratio = W 4 w *

The presence of 2,2-dimethyl-4,4,6,6,8,8-hexaphenylcyclotetrasiloxane (P ) in supernatant liquids from polymer precipitations confirmed that intramolecular backbiting (reversion) accompanies the polymerization of P even at the very beginning of the reaction. Me2

4

3

Conclusions Block copolymers of AB type were prepared from oligodiphenylsiloxy and oligodimethylsiloxy segments by hydrosilylation. A single glass transition was


98


observed indicating materials that do not phase separate. These materials were highly thermally stable, as expected. In a similar manner, PDMS polymers bearing alkylheptaphenylcyclotetrasiloxane moieties did not undergo phase separation, but displayed significantly increased viscosities and somewhat improved thermal stability compared to PDMS. These results suggest that longer diphenylsiloxy segments are required to prepare thermoplastic elastomers. Triblock copolymers were prepared by sequential addition of monomers and by a convergent approach. Spectroscopic analysis by NMR confirms their triblock structure. These block copolymers showed the expected phase separation.

Acknowledgement T. M . Gâdda thanks fellowship support from the Foundation of Technology and the Swedish Academy of Engineering Sciences in Finland.

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