Chain extenders agents

Woodhouse described a degradable polyurethane prepared with a novel amine chain extending agent. As with the insertion of lactic acid into the backbone, bio-degradability was achieved, but the fragments appeared problematic. Thus, while biodegrading polyurethanes are fairly simple to prepare, one must be aware of the product to which a polyurethane degrades. The use of TDI as the isocyanate would clearly raise concerns from the FDA. 

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. 

SP-C Chain-extended product from SP using phosgene as chain-extending agent... 

Two different types of chain-extending agents were used, namely 4,4 -di-isophenylmethane diisocyanate (MDI) and phosgene, which lead to two different classes of copolymers, poly(ester-urethane)s and poly(ester-carbonate)s respectively. High molecular weight poly(ester-carbonate)s have earlier been prepared by the dichloroformate synthesis [12-13]. 

The kinetics of simultaneous IPN formation was studied [172] for the system comprising PU prepared with hydroxy-terminated polybutadiene (HTPB), diisocyanate, and chain-extending agent and styrene-DVB or PMMA cross-linked with EGDMA. The authors obtained the curing curves for individual networks and IPNs, and established that the formation rate of the PU component was higher than that for the other two networks under the chosen conditions. The reaction orders for PU and IPN are close to 1, whereas for cross-linked copolymers it is about 0.5. They concluded that the formation of IPNs is dominant. 

Fillers (qv) are occasionally used in flexible slab foams the two most commonly used are calcium carbonate (whiting) and barium sulfate (barytes). Their use level may range up to 150 parts per 100 parts of polyol. Various other ingredients may also be used to modify a flexible foam formulation. Cross-linkers, chain extenders, ignition modifiers, auxiHary blowing agents, etc, are all used to some extent depending on the final product characteristics desired. 

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... 

The six-position may be functionalized by electrophilic aromatic substitution. Either bromination (Br2/CH2Cl2/-5°) acetylation (acetyl chloride, aluminum chloride, nitrobenzene) " or chloromethylation (chloromethyl methyl ether, stannic chloride, -60°) " affords the 6,6 -disubstituted product. It should also be noted that treatment of the acetyl derivative with KOBr in THF affords the carboxylic acid in 84% yield. The brominated crown may then be metallated (n-BuLi) and treated with an electrophile to form a chain-extender. To this end, Cram has utilized both ethylene oxide " and dichlorodimethyl-silane in the conversion of bis-binaphthyl crowns into polymer-bound resolving agents. The acetylation/oxidation sequence is illustrated in Eq. (3.54). 

The future direction of polyester R D efforts is likely to involve further progress in polyester synthesis given the wide range of potential monomer combinations, new blending technology and the use of advanced functional additives such as nanoclay reinforcements, reactive impact modifiers, anti-hydrolysis agents and chain extenders. 

Another highly effect chain extender is trimellitic anhydride (TMA) which gives rise to branching of the PET structure. Note that the multifunctional epoxies (see Table 14.2) react quickly with the terminal carboxylic acid groups of PET but can also react with the film former and the silane coupling agent on glass fibre reinforcements. 

Since chemical nucleating agents actively scission PET chains as part of their action, a drop in PET intrinsic viscosity can result. One strategy to offset this accompanying molecular weight reduction is to use both a nucleating agent and a chain extender in combination. 

The application of this method is visualized using the example of a tricomponent system composed of a telechellc polymer with end groups of independent reactivity, a trlfunctlonal crosslinking agent and a blfunctlonal chain extender where the latter two compounds... 

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. 

Flexible PU foams are prepared from basically the same raw materials as PU elastomers. Instead of a short-chain extender, water is used. Water reacts with isocyanates to form an amine and C02 (Eq. (2.22)), which results in foaming. The diamine thus created can then react with isocyanate groups giving polyurea short segments. A surfactant is also introduced as a cell control agent. 

Thermoset polyurethanes are cross-linked polymers, which are produced by casting or reaction injection molding (RIM). For cast elastomers, TDI in combination with 3,3,-dichloro-4,4,-diphen5lmethanediamine (MOCA) are often used. In the RIM technology, aromatic diamine chain extenders, such as diethyltoluenediamine (DETDA), are used to produce poly(urethane ureas) (47), and replacement of the polyether polyols with amine-terminated polyols produces polyureas (48). The aromatic diamines are soluble in the polyol and provide fast reaction rates. In 1985, internal mold release agents based on zinc stearate compatibilized with primary amines were introduced to the RIM process to minimize mold preparation and scrap from parts tom at demold. Some physical properties of RIM systems are listed in Table 7. 

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