Chain extenders reaction with prepolymers

The Michael addition reaction of amines and thiols with bismaleimides or functionalized monomaleimides is a versatile tool ia the synthesis of chain-extended maleimide-terroinated prepolymers. These prepolymers generally are soluble ia organic solvents from which they can be processed to prepreg and molded to high quaUty, void-free laminates. 

The principal of formation of this type of polyurethane elastomer medium, based on a cationic urethane latex, is where an isocyanate-terminated prepolymer derived from either a polyester or polyether diol and toluene diisocyanate is first chain-extended with an alkyl diethanolamine to yield a relatively low molecular weight urethane capable of further chain-extending reactions. Emulsification occurs when the partially extended urethane is added with high-speed mixing to 3% aqueous acetic acid. Curing of the latex takes place either by reaction of water with the terminal isocyanate groups or by reaction with water-soluble diamines. 

Blocked isocyanate, for our purposes, will refer to the reaction product of a diisocyanate or isocyanate-terminated prepolymer in which the isocyanate functionality has been reacted with a blocking agent . Once blocked , the diisocyanate can be added to polyols or certain chain extenders, and these materials will not react at room temperature. The concept is shown in the sixth item of Fig. 1. An adhesive formulated with a blocked isocyanate is basically a two-component adhesive that does not react until heated to the activation temperature. When an adhesive is made with a blocked isocyanate together with hydroxyl-containing curatives, the adhesive has a good long shelf life at room temperature. However, once heated... 

To obtain the polyurethanes, typically a prepolymer was first prepared by reacting the diisocyanate with various diols in dimethylformamide or dimethylacetamide in a two to one molar ratio at 100-110°C for two hours under nitrogen atmosphere. A solution of chain extenders, such as BEP, was then added to the prepolymer reaction mixture and further reacted another three hours. The polymer was isolated by quenching the reaction mixture in cold water. Fine white powder was obtained with a typical yield of around 90%. 

The above prepolymer on treatment with 2 as the chain extender in dry DMF did not proceed at ambient temperature. The mixture had to be heated to 60°C for 3 h before the reaction was complete. After curing at 60°C for 24 h, the yellow, translucent block polyurethane film (BPUR2) again showed the absence of the —NCO peak in the IR spectrum indicating that curing had been complete. The fact that a higher temperature had to be used in the case of 2 as the chain extender compared to 1 is in keeping with the lower order of reactivity of diols with diisocyanates as compared to diamines with diisocyanates. 

The block lengths and the final polymer molecular weight are again determined by the details of the prepolymer synthesis and its subsequent polymerization. An often-used variation of the one-prepolymer method is to react the macrodiol with excess diisocyanate to form an isocyanate-terminated prepolymer. The latter is then chain-extended (i.e., increased in molecular weight) by reaction with a diol. The one- and two-prepolymer methods can in principle yield exactly the same final block copolymer. However, the dispersity of the polyurethane block length (m is an average value as are n and p) is usually narrower when the two-prepolymer method is used. 

Curing Agents for Carboxyl-Terminated Polybutadiene Prepolymers. The types of curing agents used to prepare binders for CTPB propellants are the same as those for PBAN or PBAA. The bifunctionality of CTPB, however, requires that part of the curing agents be polyfunctional to provide for the formation of the tridimensional network. Almost without exception, the polyfunctional aziridines and epoxides used with CTPB undergo side reactions in the presence of ammonium perchlorate, which affects the binder network formation. Kinetic studies conducted with model compounds have established the nature and extent of the cure interference by these side reactions. The types and properties of some of the crosslinkers and chain extenders used to prepare solid propellants are summarized in Table IV. 

Polyurethanes offer a convenient method by which iimnobilization of enzymes can be affected. Prepolymers are polymers with active end groups. While the primary purpose of the isocyanate end groups is serving as chain-extending agents, they also react with the amines that characterize an enzyme backbone. Thus, as many of the studies cited will show, the reaction sequence is (1) preparation of an aqueous solution of the enzyme and (2) emulsification of the solution with the prepolymer. The reaction time is on the order of 0.5 hours compared to the 24 hours required by some methods. 

The prepolymer is formed by addition polymerization, and there are no by-products formed that have to be removed. The prepolymer chains extend by the reaction of terminal isocyanate groups (-NCO) with the diol. The mole reaction ratio of the diol to diisocyanate is normally kept in the range of one mole diol to 1.6 to 2.25 moles of the diisocyanate. A reaction ratio of one mole diol to one mole diisocyanate will produce a linear material. This type of polyurethane is normally thermoplastic. 

The elastomeric polymer is obtained by extending the prepolymer through its reaction with short-chain diols such as butanediol or diamines such as ethylene diamine, thus completing the formation of hard groups between soft, flexible chains. When amines are used, the final step is typically done in a polar solvent such as dimethyl acetamide. The conversion of these polymers into usable fibers may be accomplished by wet-, dry-, or melt-spinning operations, depending on the polymer. Additives to impart whiteness or improve resistance to ultraviolet radiation and... 

Reaction with difunctional amines (see the next paragraph) allows the formation of poly (urethane-co-urea) and further extends the versatility of the segment architecture. For further control of the structure, a prepolymer is formed. The reaction, such as the polyurethane reaction shown above, is carried out with excess di-isocyanate so that an isocyanate-terminated prepolymer is obtained. The isocyanates used are typically aromatic, such as toluene di-isocyanate. This prepolymer is then reacted with a short-chain diol or diamine (for a polyurea) to form the final polymer. 

Preparation of Cast Elastomers. The cast elastomers were prepared in a two-step procedure. First prepolymers were made from one polyether polyol (poly(oxy-tetramethylene) glycol of 1000 M.W., (POTMG)) and two polyester polyols (adipate polyester of 2000 M.W. (PAG) and polycaprolactone of 1250 M.W. (PCL)) by reaction with the corresponding diisocyanates (MDI, PPDI, CHDI or NDI) at an NCO/OH ratio of 2/1. The temperature was maintained at 80°C and periodic samples were withdrawn to determined the isocyanate content. When the isocyanate content of the mixture reached within 0.3% of the calculated value, the reaction was stopped by cooling. The prepolymer could be kept for a period of six months in the absence of moisture. The isocyanate-terminated prepolymers were then chain-extended with... 

By the reaction of a prepolymer with a chain extender such as ethylene glycol, diethylene glycol, 1,4 butane diol or a diamine, the high molecular weight polyurethanes are formed (see Equation 2.5). 

In the special case of monocomponent polyurethanes, the single partner of the reaction is the prepolymer. The prepolymer is extended to a high MW polymer by reaction with water present in the atmosphere. Water, is in fact, a chain extender and the resulting high MW polymer has both bonds urethane and urea bonds ... 

The soft prepolymer segment from Reaction 14 is in turn condensed with a low molecular weight chain extender (HO-R2OH) and excess diisocyanate (Reaction 15) to give the block polymer (A-B) which possesses soft polyether segments and hard crystalline (high MP) polyurethane segments ... 

The above-described method can be simplified by masking the diamine with a ketone to form a diketimine. Ketimines can be mixed with NCO-prepolymers without a reaction occurring. This mixture can be mixed with water to form a dispersion subsequently the ketimine hydrolyzes to form a diamine that reacts with the NCO-prepolymer to form a chain-extended, high molecular weight polyurethane dispersion. Similarly, NCO-prepolymers can be mixed with a... 

A hydroxyl-terminated prepolymer was synthesized by Hiltunen et al. with 1,4-butanediol isocyanate was then used as a chain extender to Increase the molecular weight. According to their studies, the polyaddltlon reaction was Influenced by the structure and functionality of the monomers, the characteristics of the hydroxyl group, polymerization temperature, polymerization time and -OH/-NCO ratio [12]. 

Immediately add the chain extender (ethylene diamine), which has been diluted in water slowly to the aqueous polyurethane/urea dispersion. Typically chain extend only 80-90% of the actual free NCO groups because some will be lost due to reaction with water. As an alternative, the amine extender may be added to the water prior to dispersing the neutralised prepolymer. 

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