Urethane reaction copolymer

The end hydroxy groups of the polymer are secondary groups and are ordinarily rather unreactive in the urethane reaction. Initially, this limitation was overcome by the preparation of isocyanate-terminated prepolymer and by the use of block copolymers with ethylene oxide. The latter products are known as tipped polyols and are terminated with primary hydroxy groups of enhanced activity. 

An ideal PU primary structure assumes a perfect alternating block copolymer. However, under practical conditions, the SS structure, as well as the urethane reaction, follows a statistical Flory distribution, which is why the HS formed is not perfectly alternating and the block length may vary [28]. 

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... 

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. 

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. 

One of the most important uses of end-functionalized polymers is the preparation of block copolymers.73,74 The reactions are identical to the chain extensions already mentioned, except that the sequences being joined are chemically different. In the case of the -OSilCR Y chain ends mentioned above, R is typically (CH2)3 5 and Y can be NH2, OH, COOH, CH=CH2, and so on The siloxane sequences containing these ends have been joined to other polymeric sequences such as carbonates, ureas, urethanes, amides, and imides. 

In the present work we have used, therefore, dextran as the water-soluble macromolecule to be coupled and a film of an ethylene-vinyl alcohol copolymer (EVAL) as the substrate material. This copolymer has hydroxyl groups but is insoluble in water and can be injection-molded to yield a material with good mechanical properties. The coupling reaction of dextran onto the film surface will be achieved by using diisocyanate, since dextran, as well as EVAL, has hydroxyl groups which readily react with isocyanate under formation of urethane linkage. 

This reaction is often employed in the production of flexible urethane foams, which are frequently block copolymers of polyether or polyester segments joined to polyurea segments. (The polyester or polyether segments terminate in urethane segments resulting from reaction of polyether or polyester hydroxyl end groups with the isocyanate.)... 

This longer time is also a result of the very low heat of reaction of caprolactam, only 3-4 Kcai/mole versus 18-20 kcal/mole for urethanes. A typical reaction exotherm is shown in Figure 11 for NYRIM 2000 nylon block copolymer. 

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