Nylon-6,6 is produced by the reaction of hexamethylenediamine with adipic acid. What type of polymerization is this, and what small molecule is eliminated?
AChain-growth polymerization; no small molecule is eliminated
BStep-growth (condensation) polymerization; water is eliminated at each coupling step
CRing-opening polymerization; CO2 is eliminated
DFree radical polymerization; HCl is eliminated
Each amide bond forms by reaction of an amine (-NH2) with a carboxylic acid (-COOH), eliminating one water molecule. This is step-growth polymerization: any two oligomers with complementary end groups can react at any time, and molecular weight builds slowly until very high conversion. The distinction from chain-growth polymerization (where monomers add one at a time to an active chain end) has practical consequences: step-growth requires very high conversion (>99%) to achieve high molecular weight, making stoichiometric balance of the two monomers critical.
Question 2 Short Answer
A polymer sample has a number-average molecular weight (M_n) of 50,000 g/mol and a weight-average molecular weight (M_w) of 150,000 g/mol. What is the polydispersity index, and what does it indicate?
Think about your answer, then reveal below.
Model answer: PDI = M_w/M_n = 150,000/50,000 = 3.0. This indicates a broad molecular weight distribution — the sample contains chains of widely varying lengths. A PDI of 1.0 would mean all chains are identical (monodisperse). Step-growth polymers typically have PDI near 2.0 (the most probable distribution); a PDI of 3.0 suggests either a broad step-growth distribution at moderate conversion or a blend of fractions. Living polymerization techniques can achieve PDI < 1.1.
Polymers are fundamentally different from small molecules in that a sample is always a mixture of chains with different lengths. M_n weights every chain equally; M_w gives more weight to longer chains. Their ratio (PDI) measures the breadth of the distribution. This matters because many properties (melt viscosity, toughness, processability) depend on the full distribution, not just the average. Gel permeation chromatography (GPC) is the standard method for measuring both averages.
Question 3 True / False
Below its glass transition temperature, polystyrene is hard and brittle. Above T_g, it becomes soft and flexible. This transition involves melting of the crystalline regions.
TTrue
FFalse
Answer: False
The glass transition is NOT a melting transition. It occurs in amorphous regions (or entirely amorphous polymers like atactic polystyrene) and represents the onset of large-scale segmental motion of the polymer backbone. Below T_g, chains are frozen in place and the material behaves like a glass. Above T_g, chains gain enough thermal energy for cooperative segmental motion, making the material rubbery. Melting (T_m) is a separate, first-order transition that occurs only in crystalline regions at a higher temperature. Many polymers have both T_g (amorphous regions) and T_m (crystalline regions).
Question 4 Short Answer
Why does cross-linking a polymer prevent it from dissolving in solvents?
Think about your answer, then reveal below.
Model answer: Cross-links are covalent bonds connecting different polymer chains into a single network. Because dissolving a polymer requires separating individual chains and surrounding them with solvent, a cross-linked network cannot dissolve — the chains are permanently bonded together. The network can swell (absorb solvent and expand) but not dissolve, because breaking covalent cross-links requires energies far exceeding the thermal energy available in solution. The degree of cross-linking determines the swelling ratio: lightly cross-linked networks swell extensively, while heavily cross-linked materials barely swell at all.
This distinction between dissolving and swelling is fundamental. A linear polymer dissolves because individual chains separate into solution. A cross-linked polymer (vulcanized rubber, epoxy resin, hydrogel) forms a single macroscopic molecule — the entire sample is one covalently bonded entity. This is why cross-linked polymers are thermosets: they cannot be melted and reprocessed, unlike thermoplastic linear polymers.