Prepolymer Crosslinking

FIGURE 19 Daily rabbit blood plasma levels of levonorgestrel from a crosslinked polymer prepared from a 3,9-bis(ethylidene-2,4,8,10-tetraoxaspiro[5,5Jundecane)/3-methyl-l,5-pentanediol prepolymer crosslinked with 1,2,6-hexane triol. Polymer rods, 2.4 x 20 mm, containing 30 wt% levonorgestrel and 7.1 mol% Mg(OH)2. Devices implanted subcutaneously in rabbits, (o) 1 device/rabbit, ( ) 3 devices/rabbit. (From Ref. 2.)... 

The predominant RO membranes used in water applications include cellulose polymers, thin film oomposites (TFCs) consisting of aromatic polyamides, and crosslinked polyetherurea. Cellulosic membranes are formed by immersion casting of 30 to 40 percent polymer lacquers on a web immersed in water. These lacquers include cellulose acetate, triacetate, and acetate-butyrate. TFCs are formed by interfacial polymerization that involves coating a microporous membrane substrate with an aqueous prepolymer solution and immersing in a water-immiscible solvent containing a reactant [Petersen, J. Memhr. Sol., 83, 81 (1993)]. The Dow FilmTec FT-30 membrane developed by Cadotte uses 1-3 diaminobenzene prepolymer crosslinked with 1-3 and 1-4 benzenedicarboxylic acid chlorides. These membranes have NaCl retention and water permeability claims. 

Radical polymerization techniques that are for example used for the crosslinking of methacrylate-modified polysaccharides [9] are not included in this chapter. Prepolymer crosslinking through radical polymerization techniques as well as non-covalent microgel formation has recently been reviewed in a number of excellent review articles [2, 4, 10-14] and is thus not discussed within this chapter. 

C. Prepolymer-Crosslinker Systems as Scaffolding Imprint Matrix... 

Unsaturated prepolymers crosslink with peroxides or sulfur. This unsaturation can be present in the backbone or in the pendant groups. Vulcanization or crosslinking of elastomers with sulfur or peroxides is discussed in Chapter 8. 

The fiber reinforcement was prepregged with a solution of the monomers in alcohol, followed by removal of the solvent. The prepared prepreg was then heated at 150-200°C involving an in situ condensation reaction to form the norbornene-endcapped imide prepolymer. Crosslinking was subsequently achieved by applying pressure and heating at 250-300° C. 

Quickset, Organic peroxide catalyst, Witco Corp., Polymer Chemicals Group Quidur, Urethane prepolymer crosslinking agents, Merquinsa... 

Chemical Composition. A list of the several reactants such as monomers, prepolymers, crosslinkers, and initiators will be provided, together with a step by step reaction scheme. Such a presentation provides the greatest detail, yet by itself only provides an empirical formulation, lacking greatly in capability of systematization. 

SINs. Five different polymer combinations were made, two in bulk and three in solution. They were made by combining the linear prepolymers, crosslinking agents and catalysts, casting films or molding the mixtures, and curing them in situ, thereby forming SINs. 

Bismaleimide PU/epoxy Simultaneous IPN, bismaleimide free radically crosslinked PU prepolymer crosslinked with epoxy 192,193... 

Gordon and Roe, ibid. 21 75 (1956)] which are statistically closely related to the case of prepolymer crosslinking. 

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

An EB-curable struetural adhesive formulation usually eonsists of one or more crosslinkable oligomeric resins or prepolymers, along with such additives as reactive diluents, plasticizers, and wetting agents. The oligomer is an important component in terms of the development of mechanical properties. The adhesive and cohesive properties depend on the crosslink density, chemical group substitution, and molecular organization within the polymer matrix. Adhesion is achieved... 

From these, prepolymers are prepared where the diisocyanates may be completely reacted as in the case of the urethane oils which resemble the oil-modified alkyds but have urethane (—NHCOO—) links in place of the ester (—COO—) links of the alkyds, or where one only of the isocyanate groups is combined, leaving the other to participate in crosslinking reactions. Such a reactive prepolymer is the biuret that may be prepared from hexamethylene diisocyanate, has the following structure ... 

Polyester-based networks are typically prepared from polyester prepolymers bearing unsaturations which can be crosslinked. The crosslinking process is either an autoxidation in the presence of air oxygen (alkyd resins) or a copolymerization with unsaturated comonomers in the presence of radical initiators (unsaturated polyester resins). It should also be mentioned that hydroxy-terminated saturated polyesters are one of the basis prepolymers used in polyurethane network preparation (see Chapter 5). 

Crosslinking in phenol-formaldehyde resins is carried out on essentially linear prepolymers which have been formed by having one of the components in sufficient excess to minimise crosslinking during the initial step. These prepolymers may be one of two kinds the so-called resoles or the so-called novolaks. 

Having made the prepolymer, a reactive diluent, usually styrene, is added, together with an appropriate free-radical initiator system in order to bring about crosslinking. This cure reaction can be made to occur at either ambient or elevated temperatures and, depending on conditions, is complete in anything from a few minutes to several hours. 

In order to bring about crosslinking of polyesters with styrene one of two types of initiator systems is used, which differ in the temperature at which they are effective. For curing at elevated temperatures, peroxides are used which decompose thermally to yield free radicals. Among those peroxides employed are benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, di-t-butyl peroxide, and dodecyl peroxide. Mixtures of polyester prepolymer, styrene, and such initiators are reasonably stable at room temperatures but undergo fairly rapid crosslinking at temperatures between 70 °C and 150 °C, depending on which particular peroxide is used. 

As is usually characteristic of crosslinked polymers of commercial importance, epoxy resins are prepared in two stages, with the initial reaction leading to a linear prepolymer and the subsequent reaction introducing the crosslinks between the molecules. The prepolymers from which epoxy resins are prepared are diglycidyl ethers with the structure shown in Figure 4.2. 

To prepare crosslinked material, 2 eq of the diketene acetal is reacted with 1 eq of the diol and the resulting prepolymer is then reacted with a triol or a mixture of diols and triols. 

Because the ketene acetal-terminated prepolymer is a viscous Liquid at room temperature, therapeutic agents and the triol can be mixed into the prepolymer at room temperature and the mixture crosslink id at temperatures as low as 40°C. This allows incorporation of heat-sensitive therapeutic agents into a solid polymer under very mild conditions of thermal stress. However, because the prepolymer con-tedns reactive ketene acetal groups, any hydroxyl groups present in the therapeutic agent will result in the covalent attachment of the therapeutic agent to the matrix via ortho ester bonds (16). 

Different types of gel materials, such as polysaccharides, proteins and synthetic polymers, are now used to entrap biocatalysts. Among them, photo-crosslinkable resin prepolymer ENTP-4000 as shown in Eig. 7 is more useful compared to others. Entrapment of biocatalysts should be carried out under the illumination of near ultraviolet hght within 3-5 min, by which high temperatures, shifts of pH to extremely alkahne or acidic sides are avoided. ENTP-4000, hydrophobic photo-crosslinkable resin prepolymer, is one of the most suitable prepolymers for entrapment of p-glucosidase. Molecular weight of its main chain is about 4000. 

Immobilization of (3-D-glucosidase from almonds on photo-crosslinkable resin prepolymer (ENTP-4000) was carried out by the following procedure. One gram of ENTP-4000 was mixed with 10 mg of a photosensitizer, benzoin ethyl ether, and 110 mg of (3-D-glucosidase from almonds (3.4 units mg ). The mixture was layered on a sheet of transparent polyester film (thickness, ca. 0.5 mm). The layer was covered with transparent thin film and then illuminated with chemical lamps (wavelength range 300 00 nm) for 3 min. The gel film thus obtained was cut into small pieces (0.5 x 5 x 5 mm) and used for bioconversion reaction. 

Figure 7 Structure of photo-crosslinkable resin prepolymer ENTP-4000.

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