Which of the following would you expect to be (-) elastomers and which thermosetting polymers at room temperature? Justify each choice. (a) Linear and highly crystalline polyethylene (b) Phenol-formaldehyde (c) Heavily crosslinked polyisoprene having a glass transition temperature of \(50^{\circ} \mathrm{C}\left(122^{\circ} \mathrm{F}\right)\) (d) Lightly crosslinked polyisoprene having a glass transition temperature of \(-60^{\circ} \mathrm{C}\left(-76^{\circ} \mathrm{F}\right)\) (e) Linear and partially amorphous poly(vinyl chloride)

Elastomers are soft, rubbery materials that can stretch and return to their original shape, while thermosetting polymers are rigid and hard materials that do not soften or melt when heated. Elastomers typically have lightly crosslinked structures, allowing them to stretch, and have a glass transition temperature lower than room temperature. Thermosetting polymers generally have highly crosslinked structures, which make them rigid and stable at high temperatures.

(a) Linear and highly crystalline polyethylene: This polymer is neither crosslinked nor has a glass transition temperature below room temperature, making it less likely to be an elastomer. Since it is linear and not crosslinked, it cannot be considered a thermosetting polymer either. (b) Phenol-formaldehyde: This polymer is a highly crosslinked material, making it rigid and resistant to heat, which are characteristics of thermosetting polymers. Thus, it can be classified as a thermosetting polymer. (c) Heavily crosslinked polyisoprene having a glass transition temperature of \(50^{\circ} \mathrm{C}\left(122^{\circ} \mathrm{F}\right)\): The heavy crosslinking makes this polymer rigid, and its glass transition temperature is higher than room temperature. These properties align with those of a thermosetting polymer, and thus it can be categorized as such. (d) Lightly crosslinked polyisoprene having a glass transition temperature of \(-60^{\circ} \mathrm{C}\left(-76^{\circ} \mathrm{F}\right)\): This polymer has a lightly crosslinked structure, making it flexible, and a glass transition temperature lower than room temperature, allowing it to stretch and return to its original shape. These properties make it an elastomer. (e) Linear and partially amorphous poly(vinyl chloride): This polymer has a linear structure and lacks the crosslinking necessary for a thermosetting polymer. It also lacks the properties necessary for an elastomer, such as a glass transition temperature below room temperature and a crosslinked structure. Therefore, it cannot be classified as either an elastomer or a thermosetting polymer. In conclusion, polymers (b) and (c) can be considered thermosetting polymers, while polymer (d) can be considered an elastomer. Polymers (a) and (e) are neither elastomers nor thermosetting polymers based on their properties at room temperature.