Resins, ’ popcorn

Choice of Support. The most commonly used organic support is polystyrene (cross-linked with DVB) in its microp-orous (1-2% cross-linking) form, although it has also been used in its macroporous and popcorn form (Ford et al, 1982 Shan et al.,1989). Various other polymeric catalysts have been used like polyvinylpyridine resins, commercial ion-exchange resins (Arrad and Sasson, 1989), modified dextran anion exchangers (Kise et al, 1981), and macroporous glycidyl methacrylate-ethylene dimethacrylate resins (Hradil and Svec,... 

Decomposition of a -labeled mixed anhydride (DF 0.13, 1.0 mmol/g) in a popcorn polystyrene produced polymer-bound carbonate and benzoate esters (Scheme 10) (US). At 25 C for one year or at 100 C for 10 h the dry resin gave 20% or 25% intermolecular decomposition. The resin in dioxane at 90 <>C for 10 h or at 37 °C for 15 h with added triefliylamine gave S4% or 82% intermolecular decomposition. The swelling solvent provided the polymer chain mobility needed for reaction between two polymer-bound functional groups. 

A fourth generation of electrically conductive adhesives based on cyanate esters and modified cyclo-olefin thermoset (MCOT) resins was introduced in the 1990s. These resins were specially formulated for low moisture absorption, high thermal stability, and low stress to be compatible with solder-reflow conditions. They were developed to prevent or reduce the so-called popcorn effect, a failure mode attributed... 

The final resin bead structure of a macroreticular resin contains many hard microspheres interspersed with pores and channels. Because each resin bead is really made up of thousands of smaller beads (something like a popcorn ball), the surface area of macroporous resins is much higher than that of microporous resins. A gel resin has a (calculated) surface area of less than 1 m g". However, macroporous resin surface areas range from 25 to as much as 800 m g". ... 

The most commonly studied resin support is polystyrene crosslinked with divinylbenzene (DVB) (1-2% crosslinking) in its microporous form, though it has also been used in its macroporous and popcorn forms. The PT catalyst can be physically adsorbed or chemically bound on the support with or without a spacer chain between the support and the PT catalyst. These supported catalysts are active in a variety of reactions but find limited commercial applications due to lower reactivity than the soluble analogs, which is mainly due to diffusional limitations in the... 

For this reason, rather irregular, porous beads are desired. The shapes of the resin spheres have been variously described as shrunken orange, popcorn, or modified popcorn with dull, spongy surfaces [85]. 

Today, almost 50% of the semiconductors encapsulated in plastic packages are made for surface-mount assembly that subjects the devices to a considerable thermal shock during the soldering process. Within a few seconds, the internal package temperature rises to 215-260°C and the moisture absorbed by the plastic encapsulant and the organic adhesive evaporates explosively. This sometimes results in package cracks that start at the interface between the chip and the die pad or in delamination within the die attachment layer. To investigate the relationship between the chemical structure of epoxies and this so-called popcorn elfect , a series of polyfunctional resins has been evaluated [5]. They include new experimental epoxy novolacs whose chemical formulae have been previously displayed [4]. 

A large variety of different materials have been tried as insoluble supports during the evolution of oligonucleotide synthesis, and the major materials are described in Table 1. The first solid-phase oligonucleotide syntheses were performed using the same nonpolar popcorn polystyrene resin (3,4) used for peptide synthesis (Fig. 5). Later, polar polyamide resins were developed that were more suitable for the polar solvents used in early phosphodiester (5)/triester (d) methods (Fig. 5). 

Fig. 5. Solid-phase supports made of synthetic resins. Figs 1 and 2 The nucleoside linkages used on the early polystyrene popcorn supports (3,4). Fig. 3. The nucleoside linkage used on a polyamide based resin (5). Fig. 4 The structure of a polyamide/silica gel composite developed for continuous-flow synthesis (8) Fig. 5. The structure of a PEG-polystyrene grafted copolymer, tentacle support (25). The mean mol wt of die PEG side chain is 3000 daltons. Fig 6 The nucleoside linkage used on the recently introduced highly crosslinked polystyrene support (27).

---