Pre-polymer

An organic isocyanate (MDI or a pre-polymer) with a hydroxy compound. The isocyanates are potent respiratory sensitizers, the risk increasing with volatility... 

Store in safest form, e.g. as pre-polymers, as chemical for generation of requirements (e.g. hypochlorites for chlorine) in dilute form... 

A typical maleimide resin is synthesized by the Michael addition of MDA and BMI (Fig. 4). If the stoichiometrically equal amounts of MDA and BMI are added into the reaction solvent under controlled temperature, linear, high molecular weight polyaminoimide (PAI) results. To obtain crosslinkable oligomer (pre-polymer) with maleimide end groups, a calculated 1.1-1.8 times an excess... 

Another original approach is exemplified by Ciba-Geigy s XU292. BMI is mixed with o,o -diallylbisphenol A (DAP, 2,2-bis(3-allyl-4-hydroxyphenyl)pro-pane Fig. 9) followed by heating to 110-125°C to obtain a liquid pre-polymer. The pre-polymer could be cured in the same way as common maleimide and epoxy resins are done [ 16,17]. 

The GGM-rich hemicelluloses, isolated from water-impregnated spruce chips by heat-fractionation [218], has been used as pre-polymers after modification with methacrylic functions [439]. Radical polymerization of the modified hemicelullose with 2-hydroxyethyl methacrylate in water yielded elastic, soft, transparent, and easily swollen hydrogels. 

The PMMA bone cement is formed from a mixture of prepolymer PMMA powder, which contains a free-radical initiator, and liquid MMA monomer. In the operating theatre the powder and liquid are mixed, causing the initiator to dissolve and bring about polymerisation in the monomer component. The powder pre-polymer does not dissolve in the monomer but remains in the newly polymerised materials as a kind of reinforcing filler. 

The final structure of resins produced depends on the reaction condition. Formaldehyde to phenol (F/P) and hydroxyl to phenol (OH/P) molar ratios as well as ruction temperahne were the most important parameters in synthesis of resols. In this study, the effect of F/P and OH/P wt%, and reaction temperature on the chemical structure (mono-, di- and trisubstitution of methyrol group, methylene bridge, phenolic hemiformals, etc.) was studied utilizing a two-level full factorial experimental design. The result obtained may be applied to control the physical and chemical properties of pre-polymer. 

Many polymers have been processed by casting, e.g., acrylics, polystyrene, polyamide (nylon 6), phenolics, PVC/plasticiser. Many of these are used in a pre-polymer form, which polymerise on the casting belt, or the polymerisation can be completed later by application of heat. 

Both of the simple polyurethanes (TDI-PU and MDI-PU) were synthesized according to a well known solution polymerization technique (10). The polyurethane elastomer (MDI-PUE) was prepared by a pre-polymer method (11). 

This process, commonly used with thermosets, is sometimes used for specific thermoplastics such as monomers or pre-polymers of PMMA or polyamide (Nyrim). For the latter, a mix is made just before casting with ... 

Aciylamide is required in very large quantities as the pre-polymer of the polyacrylamide that is very widely used in polymer and flocculant apphcations. The chemical manufacture of acrylamide has been estabhshed for a long time. The original process involved treatment of acrylonitrile with sulphuric acid at 90°C. More recently processes have been introduced that require the use of copper catalysts and high temperatures (80-140°C), but result in the formation of large quantities of toxic waste, including HCN. The expensive copper catalyst used is difficult to regenerate. In addition the chemical process produces aciylamide that requires considerable purification, for instance because the... 

Some of the products formed are functional ingredients in their own right, such as high fmctose syraps, cefuroxime etc. Others are intermediates that can be either freely traded, such as acrylamide as a pre-polymer and 6-APA and p-hydroxyphenylglycine as precursors of semi-synthetic antibiotics. Alternatively some intermediates tend to be used in-house by the company producing them such as fS)-2-chloropropanoate and L-tert-leucine. 

Macromers with propylene oxide. Hydroxypropyl (organosolv) lignin (HPL) is a low molecular weight (pre)polymer with a number average molecular... 

The only other study considered for deriving a chronic inhalation MRL was the epidemiological study by Alexandersson et al. (1987). Although the study showed promise as a hmnan epidemiology study, these workers were exposed to the HDI monomer as well as to the HDI pre-polymers (biuret trimer). The study did not distinguish between the effects produced by the monomer versus the polymer therefore, since this was a combination/mixture exposure, it was not considered appropriate for use in determining a... 

An interesting approach to maleimide-terminated phenoxy resin has recently has described (42). para-Maleimidobenzoic acid was reacted with diglyci-dylbisphenol-A epoxy resin in the presence of catalyst to provide the bismale-imide of Fig. 13. Instead of diglycidyl bisphenol-A, linear epoxy resin pre-polymers can be used in this reaction to form a maleimide terminated phenoxy resin. Another suitable functionalized monomaleimide is m- or p- N-(hydroxyphenyl) maleimide which is synthesized from maleic anhydride and m-aminophenol in DMF as a solvent at 70 °C. The purified hydroxyphenyl maleimide was reacted with epoxy resin to form novel BMIs as outlined in Fig. 14. The new BMI and phenoxy oligomers polymerize at temperatures of 200-220 °C, but the cure temperatures can be significantly lowered when catalysts such as imidazoles or triphenylphosphine are added. The cured homopolymers show Tg of 140 and 230 °C for the n = 2 and the n = 1 polymer, respectively(43). 

In RIM processes, two or more reactive components are mixed together, starting the reaction between the components before the mixture is dispensed into the mold. This tends to increase the viscosity of the liquid that is dispensed due to an increase in molecular weight of the polymers or pre-polymers formed in the initial reaction. An increased viscosity can prohibit complete filling of the mold and permeation of the preform. This tends to decrease the adhesion between the matrix and the fibers. Poor interfacial adhesion between the reinforcement and matrix phase can cause a material to have less than desirable stiffness and strength. 

In the multiple zone process an olefin mixture is polymerized in a first reaction zone to produce a pre-polymer that has a lower molecular weight up to 35-65% the final conversion. After this step, volatile materials, such as hydrogen, are removed. The polymerization continues in a second reaction zone by adding more of the olefin mixture to produce the final polymer with a higher molecular weight (6). 

Processing of polyurethanes is different from other elastomers in that most polyurethanes are cast into a mold. The mixing of the pre-polymer and curative can be done with specially designed machines in large shops or by hand in small shops or for small runs. 

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