Relationships Among Ideas and Processes Practice Test

Chemical Reaction: free-radical polymerization of ethylene

Many plastics are produced by chain-growth polymerization, in which small molecules (monomers) add sequentially to a growing radical. In free-radical polymerization of ethylene to polyethylene, an initiator such as an organic peroxide decomposes thermally to form radicals. During initiation, a radical adds to the C=C double bond of ethylene, creating a new carbon-centered radical. In propagation, this radical adds to more ethylene monomers, lengthening the polymer chain.

Polymer growth ends by termination, commonly via radical-radical combination (two radicals join) or disproportionation (hydrogen transfer yields two non-radical products). The average polymer chain length depends on the relative rates of propagation and termination. Conditions such as temperature, initiator concentration, and the presence of chain-transfer agents influence molecular weight distribution. A chain-transfer reaction occurs when the growing radical abstracts an atom (often hydrogen) from another molecule, creating a dead polymer chain and a new radical that can start another chain; this lowers average molecular weight.

Industrial low-density polyethylene (LDPE) production often uses high pressure and temperature, which increase radical formation and allow branching through backbiting reactions, affecting material properties like density and flexibility. By contrast, controlling radical concentration and transfer reactions can yield polymers with different mechanical strength and melting behavior.

What is the relationship between chain-transfer reactions and average molecular weight in the polymerization described?

Medical Innovation: insulin analogs and glycemic control

Insulin therapy aims to mimic physiological insulin secretion, which includes a basal level and meal-related spikes. Native human insulin tends to form hexamers in solution stabilized by zinc; hexamer dissociation into monomers is required for absorption after subcutaneous injection. Insulin analogs modify amino acids to alter self-association and pharmacokinetics. Rapid-acting analogs (e.g., with substitutions that reduce dimer/hexamer formation) absorb quickly, better matching postprandial glucose rises. Long-acting analogs prolong action by promoting depot formation or albumin binding, providing steadier basal coverage.

Clinical implementation balances efficacy with risks such as hypoglycemia. Intensive regimens combine rapid-acting boluses with long-acting basal insulin, improving HbA1c (a measure of long-term glycemia) but requiring careful dosing and monitoring. Continuous glucose monitors and insulin pumps further refine delivery by providing real-time feedback and programmable basal rates. However, cost, device training, and access can limit adoption.

At the molecular level, changing absorption kinetics changes the timing of insulin availability relative to glucose excursions. If insulin peaks too late, post-meal hyperglycemia persists; if it peaks too early or lasts too long, hypoglycemia may occur. Thus, analog design links protein chemistry to clinical outcomes.

What is the relationship between insulin hexamer dissociation and rapid-acting insulin analog absorption described?