Epoxy terminal group

The degree of polymerization is dictated by the ratio of Hquid resin (cmde DGEBPA) to bisphenol A an excess of the former provides epoxy terminal groups. The actual molecular weights attained depend on the purity of the starting material. Reactive monofunctional groups act as chain terrninators. 

Dicyandiamide (DICY) is a solid latent catalyst that reacts with both the epoxy terminal groups and the secondary hydroxyl groups. DICY has the advantage that it only reacts with the epoxy resins on heating beyond an activation temperature, and once the heat is removed, the reaction stops. It is widely used with epoxy resins where long shelf life (up to... 

Ihe literature " describes a number of dendrimers and the closely related star-like 120-123 polysiloxanes. Ihe hyperbranched polysiloxanes are the primary example of more random structures. Although the emphasis has been on synthesis and characterization,i i modeling on hyperbranched polymers has also been carried out. Some of the most interesting species involve polysiloxane chains. Star polymers, some with nanosized silica cores, have also been synthesized.i °-i i Hyperbranched polysiloxanes have been prepared with controllable molecular weights and polydispersities,i -1 - with epoxy terminal groups some are UV-curable ° and some serve as a source of molecular silica. Hyperbranched polysiloxanes have also been used in the sol-gel preparation of polypropylene/silica nanocomposites. ... 

Another example of modified epoxy resin is the area of cathodic electrodeposition whereby a low molecular weight epo) resin is reacted via the epoxy terminal groups with a primary or secondary amine to form an adduct which is solubilized with an acid (16) ... 

Poly(unsaturated ester)-siloxane segmented copolymers have been prepared by the polycondensation of epoxy-terminated polydimethylsiloxanes and carboxy-terminated poly(ethylene adipate-co-maleate) oligomers 243). Reactions have been conducted in cellosolve solvent, at 140-150 °C, in the presence of 2% by weight potassium hydroxide catalyst. The molecular weights reported were fairly low. The same group has also prepared poly(hexamethylene adipate)-polydimethylsiloxane copolymers con-... 

A special form of homofimctional hnking utihzes so called dendrimers. Dendrimers are nanospherical structures for which the exact size depends on the number of branching points and which carry reactive functional units in their periphery (for example aldehyde-, thiol-, epoxy groups etc). The structure of dendrimers is similar to a tree, and their ramifications consist of repetitive units. It should be noted that their size is limited due to the fact that the packing density of their terminal groups increases. With increasing size, their macroscopic structure approximates the form of a sphere. 

Macrocyclization. Trost and Warner17 have effected efficient cyclization to ten-and fifteen-membered rings by use of Pd(0) supported on a polystyrene bearing phosphine ligands. Precursors with an epoxy vinyl terminal group proved particularly suitable. Thus, 1 in the presence of such a Pd(0) catalyst cyclizes to 2 and 3 in 71% yield. The products are isomeric at the double bond, since both are oxidized to the ketone 4. The same catalyst system converted 1 (n = 9) into two macrocyclic isomers in 66% yield. Both cyclizations are concentration dependent. The temperature is also critical, the reaction being particularly clean at 65°. 

Epoxy acrylate resins or vinyl esters are made from the esterification of epoxy resin via their terminal group with an unsaturated acid, such as methacrylic acid derived from epoxy resin. A typical reaction sequence is illustrated in Fig. 4.3. The resultant polymer is usually dissolved in a reactive monomer such as styrene. 

The photoinduced cationic crosslinking of a, co-terminated disiloxanes containing vinyl ether (VEl and VE2) and epoxy flmctional groups (Scheme 1) in dependence of the light intensity and the type of photosensitizers was investigated by means of ceal rime (nfraced (RTIR) spectroscopy and calorimetric measurements. 

Acid proteases are inactivated by active-site specific reagents, diazoacetylnorleucine ethyl ester and other diazo compounds, and epoxy (p-nitrophenoxy)propane. Covalently labelled aspartic acid peptides have been isolated from pepsin, chymosin (= rennin), and penicillopepsin. The peptides labelled with the diazo compounds have similar sequences and differ from the epoxy (p-nitrophenoxy)pro-pane labelled peptides. These results indicate two aspartic acids at the active site and suggest homology between the enzymes. The latter is confirmed by a comparison of the sequence data. Studies of the action of porcine pepsin and penicillopepsin on some dipeptides with free N-terminal groups show transpeptidation involving a covalent acyl intermediate. It is proposed that there are differences in the mechanism of action of pepsin which are determined by the nature of the substrate. 

C HOCH,. Appears as the terminal group of epoxy resin structures resulting from reaction of epichlo-... 

A similar structure to the one in Figure 6.6 is obtained by using instead of epoxy resins diphenylmethane diisocyanate (MDI pure or crude ). For example by using crude MDI with the functionality of around 2.2-2.5 -NCO groups/mol, by the reaction with terminal groups of high molecular weight polyether triols (5000-6500 daltons), nonreactive NAD... 

The elastomeric materials most commonly used for this purpose are carboxyl-terminated acrylonitrile-butadiene copolymers. The carboxyl groups react with the epoxy group to produce an epoxy-terminated rubber that promotes interfacial bonding in two-phase systems (9). By controlling the concentrations of M, Xj, X2, Z, and U, a broad range of compatibilities can be achieved. Impact resistance and fracture toughness are Increased with minimal sacrifice in molulus and elevated-temperature performance. 

Brown [1997] has reported that polyester-PPE blends may be compatibilized through block copolymer formation between epoxy-terminated PPE and carboxylic acid end-groups on such... 

An additional way to produce a secondary amine group utilized the reaction of 8 moles of piperazine with 1 mole of the epoxy terminated siloxane oligomer. This reaction was conducted in di-oxane at 60° for about 24 hours. Excess piperazine was removed by washing the oligomer extensively with distilled water. Since the piperazine is water soluble and the oligomer is not, the final system was easily purified to very low levels of residual piperazine in this manner. The product was characterized by titration of the amine endgroups. 

---