Polymerization may be initiated by ionic species. Depending on the charge on the end group of a prop — Analytical Chemistry Chemistry Question
Ionic polymerization
Polymerization may be initiated by ionic species. Depending on the charge on the end group of a propagating chain, cationic and anionic polymerization types are distinguished. Ionic as well as radical polymerization involves the stages of initiation, propagation, termination and chain transfer. Cationic polymerization is initiated by strong acids and other electrophilic compounds, whereas anionic by strong bases and electron donors.
For each monomer given below, choose polymerization type(s) (radical, anionic, cationic) which it can be involved in.
[VISUAL]
Model Answer
Radical polymerization is possible for compounds: a, b, c, d, e, f, h, j, k, l
Anionic polymerization is possible for compounds: a, d, f, g, k
Cationic polymerization is possible for compounds: d, h, j
Explanation:
- Radical mechanism: All compounds containing double bonds (including cyclic unsaturated compounds thiophene (e) and pyrrole (l)) can be polymerized. In the case of aromatic heterocycles, the propagating radical is stabilized by conjugation with the ring system.
- Anionic mechanism: Monomers containing electron-accepting groups, such as nitrile (a), carbonyl (f), or nitro (k), stabilize the negatively charged macroions. Additionally, highly strained epoxides (g) can undergo anionic ring-opening polymerization.
- Cationic mechanism: Monomers with electron-donor substituents adjacent to the double bond, such as isobutylene (j), form stable carbocations. Vinyl ethers (h) are active due to the resonance/mesomeric stabilization of the macrocation by the alkoxyl group. Strained epoxides (g) and tetrahydrofuran (i) also undergo cationic ring-opening polymerization via protonation and subsequent cleavage of the C–O bond to form carbocations.
- Styrene (d): The phenyl group stabilizes both carbocations and carbanions, making styrene polymerizable by both anionic and cationic mechanisms (as well as radical).
Anionic polymerization initiated by metal alkyls can be described by the following kinetic scheme, which includes stages of initiation, chain propagation and chain termination. The latter occurs as a result of carbanion reaction with a terminating agent, acid HA.
[VISUAL]
Write down the rate equation for monomer consumption, expressing concentrations of monomer and active chains (macroanions) as [M] and [M–], respectively.
Model Answer
– d[M]/dt = kp [M–] [M]
Anionic polymerization allows synthesis of nearly monodisperse polymer. Based on this fact, compare qualitatively rate constants of initiation and chain propagation.
Model Answer
All chains of a monodisperse polymer are of equal length, which is only possible if all the chains are initiated at the same time and propagate simultaneously. Therefore, initiation must occur much faster than chain propagation: kin >> kp.
Calculate molecular mass of the polymer obtained as a result of polymerization of 100 g of styrene in 600 cm3 of 1,4-dioxane in the presence of 0.234 g of naphthalene and 0.042 g of metallic sodium, if 58.9 % of the monomer was consumed during polymerization.
Model Answer
Interaction of naphthalene and sodium in dioxane gives rise to the anion-radical of sodium naphthalenide, which produces styrene anions via one-electron reduction of styrene. This process initiates the anionic polymerization of styrene.
1. Monomer concentration [M]0:
[M]0 = (100 g) / (104.15 g mol-1 * 0.600 dm3) = 1.60 mol dm-3
2. Initiator concentration [In]0:
[In]0 = (0.234 g) / (128.17 g mol-1 * 0.600 dm3) = 3.05 * 10^-3 mol dm-3
3. Relationship between polymerization degree (Pn) and fraction of monomer consumed (q = 0.589):
Pn = [M]0 / [In]0 * q = (1.60 mol dm-3 / (3.05 * 10^-3 mol dm-3)) * 0.589 = 309
4. Molecular mass of the synthesized polymer:
M = Pn * M(styrene) = 309 * 104 g mol-1 = 32100 g mol-1.
Polymerization is a perspective approach towards design of chain molecules of various shape and size. Still chain termination can be regarded as a drawback of the method, since it leads to species not capable of attaching new monomer units.
What chain termination processes are probable for radical and anionic polymerization? Fill in the table.
[VISUAL]
Model Answer
The filled classification table is as follows:
- Disproportionation: Probable (+) for Radical polymerization; Improbable for most monomers for Anionic polymerization.
- Recombination: Probable (+) for Radical polymerization; Not probable (–) for Anionic polymerization.
- Chain transfer to solvent: Probable (+) for Radical polymerization; Possible in some solvents, e.g. in liquid ammonia (trace amounts of water and acids in the reaction mixture may also terminate chain propagation) for Anionic polymerization.
- Chain transfer to monomer: Probable (+) for Radical polymerization; Not probable (–) for Anionic polymerization.
Explain why a polymer obtained by anionic polymerization has narrower molecular mass distribution than that obtained by radical polymerization.
Model Answer
Unlike radical polymerization, anionic polymerization can proceed as a 'living' system without significant chain termination. Active anionic centers on the chain ends are preserved until the monomer is fully consumed. Consequently, all polymer chains propagate for the same duration and achieve nearly identical lengths, resulting in a very narrow molecular mass distribution.
The following solvents are used as a medium for anionic polymerization: (a) benzene; (b) 1,4-dioxane; (c) tetrahydrofuran; (d) 1,2-dimethoxyethane. Arrange the solvents in the order of increasing polymerization rate.
Model Answer
The order of increasing polymerization rate is:
benzene < 1,4-dioxane < tetrahydrofuran < dimethoxyethane
Explanation:
The rate of anionic polymerization depends on how strongly the propagating carbanion interacts with its alkali metal counterion. Solvents that coordinate/solvate the counterion more effectively separate the ion pair, increasing the exposure of the active carbanion and speeding up propagation.
- Benzene solvates alkaline metal ions poorly.
- 1,4-Dioxane is symmetrical and has zero dipole moment, solvating only slightly better than benzene.
- Tetrahydrofuran (THF) is polar and coordinates alkali cations with higher efficiency.
- 1,2-Dimethoxyethane is a flexible bidentate ether that can form stable chelates with alkali metal cations, yielding the highest concentration of free or solvent-separated ion pairs and the highest rate.
Compare the rates of anionic polymerization with sodium, potassium and cesium naphthalenides used as initiators.
Model Answer
The rate of polymerization increases in the order of the counterions: Na+ < K+ < Cs+.
Explanation:
Strong electrostatic attraction between the alkali metal cation and the propagating macroanion slows down monomer insertion. The strength of this ion-pair interaction depends on the ionic radius of the cation; larger cations experience weaker electrostatic attraction. Because ionic radii increase in the order Na+ < K+ < Cs+, the polymerization rate increases in the same sequence.