Siloxane-Urethane Copolymers

Synthesis of hydrolytically stable siloxane-urethanes by the melt reaction of organo-hydroxy terminated siloxane oligomers with various diisocyanates have been reported i97,i98) -yhg polymers obtained by this route are reported to be soluble in cresol and displayed rubber-like properties. However the molecular weights obtained were not very high. A later report56) described the use of hydroxybutyl terminated disiloxanes in the synthesis of poly(urethane-siloxanes). No data on the characterization of the copolymers have been given. However, from our independent kinetic and synthetic studies on the same system 199), unfortunately, it is clear that these types of materials do not result in well defined multiphase copolymers. The use of low molecular weight hydroxypropyl-terminated siloxanes in the synthesis of siloxane-urethane type structures has also been reported 198). 

Synthesis of siloxane-urethane copolymers from various hydroxyalkyl-terminated PDMS oligomers and aliphatic diisocyanates, such as tetramethylene- and hexame-thylene diisocyanate and HMDI was reported 333,334). Reactions were conducted either in chloroform or 1,4-dioxane and usually low molecular weight, oily products were obtained. No data were available on the molecular weights or the thermal and mechanical properties of the copolymers obtained. These products were later cross-linked by a peroxide. Resulting materials were characterized by IR spectroscopy and water contact angle measurements for possible use as contact lenses. 

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In this study siloxane-urethane copolymers were prepared by the reaction of a,(o-hydroxyhexyl terminated PDMS oligomers, 4,4 -isocyanatocyclohexylmethane and 1,4-butanediol, which was used as the chain extender. Influence of the reaction solvent, PDMS molecular weight and hard segment content on the overall molecular weight and thermal and mechanical properties of the products were investigated. Performance of PDMS based polyurethanes were compared with those of PTMO based systems, with similar structures and compositions. 

J. Alternate Siloxane-Urethane Copolymer by Three-Step Reaction 194... 

D. Synthesis of Siloxane-Urethane Copolymer from BHTD... 

Finally, the siloxane-urethane copolymer (22) was obtained from BHTD and 2,5- tolyene diisocyanate (2,5-TDI) in dry DMF at 80°C. The CAChe representations of model MDI-PDMS and model TDI-TMDS structures are presented in Scheme 36 and Scheme 37, respectively. 

Siloxane-urethane segmented copolymers, which have very good mechanical, fatigue and surface properties 370,377 or their blends with conventional polyurethane-(ureas) have been successfully used in the production of blood pumps, intra-aortic balloons and artificial hearts 200,332,370,376,377). 

High purity and high molecular weight siloxane-urea/urethane copolymers, (1), were prepared by Kuepfer [ 1] at ambient temperature using dibutyl tin diacetate as catalyst under anhydrous conditions. Additional siloxane-urea/urethane copolymer derivatives were prepared by Schafer [2]. 

A typical procedure followed in the preparation of siloxane-urethane segmented copolymers is as follows Calculated amounts of HMDI and PDMS oligomer are introduced into the reaction flask, stirred and heated up to about 60°C. This mixture is not miscible. However, when 0,5 mL of 1.0% DBTDL solution in toluene is added, the mixture turns clear in about one minute indicating a fairly fast reaction between PDMS and HMDI. There is also a dramatic increase in the reaction temperature from 60 to about 90°C, typical for very exothermic urethane formation reaction. Prepolymer formation is followed by FTIR, monitoring the disappearance of the broad hycfroxyl peak centered around 3300 cm Prepolymer obtained is then diluted with THF to about 50% solids and heated to reflux temperature of 64.5 C. Chain extender, BD, is dissolved in THF and added dropwise into the reaction mixture. As the system became viscous as a result of the chain extension reaction, it is diluted with THF and DMF to prevent the premature precipitation of the copolymer formed, which is indicated by the formation of a cloudy solution. Reaction was continued until the complete disappearance of sharp isocyanate peak around 2250 cm in the FTIR... 

Table II. Characteristics of Siloxane-Urethane and Polyether-Urethane Segmented Copolymers...

Scheme 29 illustrates the synthetic routes to three classes of silicone-urethane block copolymers that were reported by Benrashid et al. It is interesting that the solvents from which the siloxane-urethane block coplymers were cast have a significant influence on microphase segregation. For example, films cast from THF had a higher siloxane concentration at the surface relative to polymers cast from DMAc/ CH2CI2 or dioxane. 

Scheme 29 S3Uithetic routes for three classes of siloxane-urethane block copolymers.

The synthesis of siloxane urea urethane copolymers through step-growth polymerization has been explored previously,(2 - 5) and work performed before 1988 has been reviewed.(6) We have used a "hard segment first" two-step polymerization procedure related to that of Harrell(7) to achieve hard segment compositional control. The synthesis of these polymers(8) and initial studies of surface characterization(9) have been reported. 

One of the major drawbacks to many promising copolymers is their unsatisfactory electrochemical stability. Carbonyl groups which feature in many of the back-bone/chain linking groups are likely to cause stability concerns. Likewise, urethane, alcohol, and siloxane functions are sensitive to lithium metal. With this in mind, a recent trend has been to find synthetic routes to amorphous structures with... 

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

Careful XPS analysis of a series of poly(dimethylsiloxane-urea-urethane) multiblock copolymers demonstrated that, as well as a surface layer of siloxane, there was a layer enriched in the hard block immediately beneath this. The thickness of both these layers depended on the molecular weights of the soft and hard block segments, respectively112. Annealing of these copolymers increased the thickness of both layers. The same authors have also shown that the thickness of these layers of hard and soft blocks could be modified by use of solvent mixtures which selectively precipitate the polar hard block during film formation by solvent casting113. 

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