Synthesis of Organosilicon Linear Polymer with

Chapter 14

Downloaded by NORTH CAROLINA STATE UNIV on October 19, 2012 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch014

Synthesis of Organosilicon Linear Polymer with Octaorganooctasilsesquioxanes Structural Units in Main Chain N.

A.

Tebeneva, E. A. Rebrov, and A. M. Muzafarov

Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 117393 ProfsoyuznayaUl.,70, Moscow, Russia

A macromonomer T , 1,4-divinylhexamethyloctasilsesquioxane (3), was prepared by hydrolytic condensation o f vinyltris(diethoxy methylsiloxy)silane (2a). Hydrosilylation o f (3) by dimethylchloro silane, methylphenylisopropoxysilane and phenyldimethylsilane leads to functionalized derivatives 1,4-bis(2-chlorodimethylsilylethyl)hexamethyloctasilsesquioxane (4) and 1,4-bis(2-isopropoxymethyl phenylsilylethyl)hexamethyloctasilsesquioxane (6) and a nonfuncti onalized derivative 1,4-bis(2-phenyldimethylsilylethyl)hexamethyl octasilsesquioxane (5). Treating 6 with thionyl chloride yields 1,4bis(2-chloromethylphenylsilylethyl)-hexamethyloctasilsesquioxane (7). T w o approaches to the synthesis o f polymers with fragments T in the main chain have been proposed: (i) hydrolytic polycondensation o f 7 and (ii) direct hydrosilylation o f the starting macromonomer 3 by 1,3 dimethyl-1,3-diphenyldisiloxane. Data obtained by G P C , D S C and H N M R spectroscopy have shown the latter approach to be preferable over the former. Thermogravimetric analysis o f polymers with fragments T i n the main chain has provided evidence for their high thermal stability. 8

8

1

8

Polyhedral organosiloxanes are unique entities i n the chemistry o f organosilicon compounds. They represent a structural variety o f polyorganosilsesquioxanes along with other species with a cross-linked, ladder-type and dendritic structure. T o date, a range o f individual compounds o f this type o f the general formula ( R S i O i . 5 ) have been prepared, where R is an organic radical or a functional group, and n = 6, 8, 12, 14, 16 (7-7) ( F i g . 1). In this series, the cubic molecular structure o f octaorganooctasilsesquioxanes (Tg) made up o f six fused eight-membered organosiloxane rings was found to be the most favorable thermodynamically. For this reason, the polyhedra (Tg) (whose molecules may be imagined as representing a unit n

214

© 2000 American Chemical Society

In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


215

cell o f quartz) are produced i n highest yield by either hydrolytic condensation o f trifunctionalized organosilanes or thermal (8) and thermocatalized depolymerization of polyorganosilsesquioxanes i n the presence o f sodium or potassium hydroxides (7). A structural study o f polymeric organosilsesquioxanes has shown that, irrespective o f the reaction conditions, their molecules are composed o f incompletely fused fragments (T8) only linked to each other i n a random manner (9). Despite the apparent imperfection o f their molecular structure, polyorganosiloxanes in question rank among the most thermally stable polymers. Staying within the framework o f a classical method o f their synthesis, viz., hydrolytic polycondensation o f trifunctionalized organosilanes, one is rather limited i n the choice o f means that would enable one to control effectively structure and, consequently, properties o f this class o f compounds. For this reason, the development o f methods for synthesis o f linear polymers with a regularly distributed fragments (T8) in the main chain o f macromolecules is a challenging and, simultaneously, promising problem i n this field o f research. Within the framework o f such an approach, soluble thermally stable polymers can be prepared and their properties controlled at a molecular level by varying the length and nature o f the linear bridges between fragments (Ts) and placing various radicals at silicon atoms at the cube vertices. A t present, a number o f purely organosilicon and hybrid polymers o f this type have been obtained whose molecules contain incompletely condensed fragments (Tg) only in the main chain (10). Completely condensed moieties (Tg) could be inserted only i n the side chain o f polymer molecules (10-11). O n the other hand, a study o f synthesized polymers with incompletely condensed fragments (Tg) has lent support to the usefulness o f the aforementioned approach. It was shown that insertion o f (Ts) in a linear polymer chain brings about a significant improvement in the basic characteristics o f end polymer products: thermal and thermal oxidative stability and gas permeability were observed to increase, and specific weight, combustibility and heat conductance, to decrease. Viewed i n this aspect, synthesis o f polymers with completely condensed fragments (T8) seems to hold much promise. However, for realization o f this idea, a number o f basic problems are to be resolved, in particular, those concerned with preparation o f starting macromonomers whose molecules would contain two functional groups only at silicon atoms i n a specified fixed position the structure (Ts). Following the traditional method o f synthesis o f polyorganosilsesquioxane polyhedrals - hydrolytic polycondensation o f trifunctionalized organosilanes - either nonfunctionalized or fully functionalized (Ts) only can be prepared i n reasonably acceptable yield. Attempts to synthesize the desired macromonomers by cohydrolysis o f a mixture o f two differently trifunctionalized organosilanes, for example, n-C3H7SiCl3 and 3-ClC3H6SiCl3, have led, not unexpectedly, to a fairly complex mixture o f structural isomers from which, using a preparative H P L C method, only three target isomers (Ts) whose molecules contained two 3chloropropyl groups at silicon atoms in different positions o f the structure (T8) could be separated and characterized (13) (Fig. 2). With all respect due to the experimental skill o f the authors cited, it must be confessed, i n all candor, that synthesis o f the starting macromonomer by that procedure was not quite simple and sufficiently effective. In the present study, we



216

Figure 1. The first members o f polyhedral organosilsesquioxane homologous.

8

7

6

5

4

3

2

1

ppm Figure 2. Structural isomers Tg with two functional groups.


0

217

propose an alternative approach to the synthesis o f (Ts) which makes use o f individual hexafunctionalized organotetrasiloxanes with branched structure rather than monomeric organosilanes, at the step of hydrolytic polycondensation :

R2-S-4 0

x


Ri_Si_0-s1-R

Rl,

2

being organic radicals, and X = O R , C I

The molecular organization o f the starting compounds (whose structure features one completed vertex o f the cube and provides for the required positioning of substituents at silicon atoms) is expected to be best suited to tailoring molecules (Tg) with desired properties. It is pertinent to note that targeted synthesis o f the starting organosilanes with branched structure was the subject o f a separate study whose guiding principle was the use o f new high-quality reagents (l)-sodiumoxyorganoalkokxysilanes (14-15). Scheme 1 R2 2

NaO-Sif 1

R c"\^ci

1

I

-NaCI

°

R

R-Si(

n R

O OR iR.sVo-si-R* \j

^

Development o f reliable methods for preparing these reagents has allowed, i n the final analysis, successful implementation o f the goals outlined in the present work. Discussion of results A study o f the reaction o f hydrolytic polycondensation o f hexafunctionalized organosiloxanes (2) has shown that this reaction is predominantly intermolecular even in dilute (~ 3%) solutions i n the presence o f catalytic amounts o f HC1 or A c O H . This fact is favorable to the formation o f cubic molecules v i a interaction o f two molecules o f the starting oligomers 2. Thus, i n the course o f hydrolytic polycondensation o f vinyl-tris-(diethoxymethylsiloxy)silane (2a) whose molecules bear a vinyl radical at the central silicon atom, a target macromonomer, 1.4divinylhexamethyloctasilsesquioxane (3), was obtained. T w o reactive vinyl groups i n the molecule o f macromonomer 3 are attached to the silicon atoms that are located i n one o f the major corporal diagonals o f the cube. Formation o f the structure (Tg) during the course o f hysrolytic polycondensation is not a process sufficiently explicit in all particulars. Therefore Scheme 2 presents, in some detail, one o f the possible routes o f chemical transformations leading to the


218

occurrence o f macromonomer 3 in the reaction system. The yield o f target product was 23%. This value is sufficiently high for the cascade o f condensation processes involved i f one takes into account that the end molecules ( T s ) can form only from all-cis isomers o f cyclic hydroxy 1-containing intermediates (Scheme 2). Scheme 2

-*2 Downloaded by NORTH CAROLINA STATE UNIV on October 19, 2012 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch014

2

o CH =CH-Si-0-Si-Me ~ OH

H 0 2

CH =CH-Si-0-Si-Me

2

2

%

O Me-SiC

0

E

-EtOH

t

-H 0 2

OEt

2a HO ,Me M e , O H 2 Si—O—Si 3

H O , M e Me fiH -O—Si 3 v

CH =CHO O CH=CH «- /* n Me -O. Me Me \ N i i

CH =CH O

2

2

H > 6 ° - \ Me

v

S\^°\i/\

Synthesis of Organosilicon Linear Polymer with

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