Chapter 26, Problem 26.44P

Interpretation Introduction

(a)

Interpretation:

The synthesis of polyoxetane via anionic polymerization is to be proposed and the mechanism for the first two propagation steps is to be shown.

Concept introduction:

Oxetane is a four membered ether ring and is highly strained. Anionic polymerization is a type of chain growth polymerization in which polymerization is initiated by anions. In the propagation step, the reactive intermediates are anions and not free radicals. Termination occurs in anionic polymerization when an acid is added to the reaction mixture. The anion is neutralized in the resulting proton transfer, which ends the propagation cycle. The anionic polymerization mechanism is an ${\text{S}}_{\text{N}}\text{2}$ reaction that occurs under basic conditions. The relief of the ring strain compensates the poor leaving group ability of the alkoxy group.

Answer to Problem 26.44P

The oxetane is a four membered ether ring and is highly strained. It can undergo anionic ring-opening polymerization to relieve the strain. The anionic polymerization is initiated by an oxide ion which acts as a nucleophile. This is an ${\text{S}}_{\text{N}}\text{2}$ reaction that occurs under basic conditions. The relief of the ring strain compensates the poor leaving group ability of the alkoxy group. The mechanism for the first two propagation steps is shown below:

Explanation of Solution

The structure of oxetane is as below:

The oxetane is a four membered ether ring and is highly strained. It can undergo anionic ring-opening polymerization to relieve the strain. The anionic polymerization is initiated by an oxide ion which acts as a nucleophile. This is an ${\text{S}}_{\text{N}}\text{2}$ reaction which occurs under basic conditions. The relief of the ring strain compensates the poor leaving group ability of the alkoxy group.

The oxygen atom of oxetane gains a negative charge and thus acts as a nucleophile. This acts as a site of chain growth. The propagation step can be shown as below:

Conclusion

The anionic polymerization mechanism is an ${\text{S}}_{\text{N}}\text{2}$ reaction which occurs under basic conditions. The relief of the ring strain compensates the poor leaving group ability of alkoxy group.

Interpretation Introduction

(b)

Interpretation:

The condensed formula for polyoxetane is to be drawn.

Concept introduction:

In a condensed formula, each nonhydrogen atom is written explicitly, followed immediately by the number of hydrogen atoms that are bonded to it. Adjacent nonhydrogen atoms in the condensed formula are interpreted as being covalently bonded to each other. Parentheses in a condensed formula denote that the group is attached to the previous C. Lone pairs are not shown explicitly in the condensed formula.

Answer to Problem 26.44P

The condensed formula for polyoxetane is

Explanation of Solution

The structure of oxetane is as below:

The oxetane is a four membered ether ring and is highly strained. It can undergo anionic ring-opening polymerization to relieve the strain. The anionic polymerization is initiated by an oxide ion which acts as a nucleophile. This is an ${\text{S}}_{\text{N}}\text{2}$ reaction which occurs under basic conditions. The relief of the ring strain compensates the poor leaving group ability of alkoxy group.

In a condensed formula, each nonhydrogen atom is written explicitly, followed immediately by the number of hydrogen atoms that are bonded to it. Adjacent nonhydrogen atoms in the condensed formula are interpreted as being covalently bonded to each other. Parentheses in a condensed formula denote that the group is attached to the previous C. Lone pairs are not shown explicitly in the condensed formula. Thus, the condensed formula of oxetane is:

Conclusion

In a condensed formula, each nonhydrogen atom is written explicitly, followed immediately by the number of hydrogen atoms that are bonded to it.

Interpretation Introduction

(c)

Interpretation:

It is to be determined if polyoxetane is expected to be more or less soluble in water than poly(ethelene oxide).

Concept introduction:

The solubility of a polymer in any solvent depends on the strength of intermolecular forces present in that particular polymer. Water is a polar protic solvent, so as the strength of intermolecular forces increase, the solubility will increase. A polymer with a lesser number of carbon atoms will be more soluble. Hydrogen bonding is the most important intermolecular force that determines the extent of the solubility of a particular polymer in any solvent.

Answer to Problem 26.44P

Polyoxetane (PO) is expected to be less soluble in water than poly(ethelene oxide) (PEO), as the extent of hydrogen bonding is less along the chain of polyoxetane than in polyethelene oxide.

Explanation of Solution

The condensed structures of polyoxetane and poly(ethelene oxide) are shown below:

The repeating units in both are shown in the parenthesis. Poly(ethelene oxide) has two carbon atoms while polyoxetane has three carbon atoms in the repeating unit. As the number of carbon atoms increases the solubility of the polymer decreases. Both the repeating units have an oxygen atom in it. Due to which hydrogen bonding is possible. The number of carbon atoms in polyoxetane is more so, oxygen occurs less frequently in polymer chain in than in polyethelene oxide. As a result, there will be less hydrogen bonding with water along the chain of polyoxetane than in polyethelene oxide. Hence, it is expected that polyoxetane (PO) will be less soluble in water than poly(ethelene oxide) (PEO).

Conclusion

As the strength of intermolecular forces increases, the solubility of a polymer in a given solvent increase.

Interpretation Introduction

(d)

Interpretation:

It is to be determined if polyoxetane is expected to have a higher or lower melting point than poly(ethelene oxide).

Concept introduction:

The melting point of a polymer depends on the strength of intermolecular forces present in that particular polymer. Among two polymers, a more polar polymer has stronger intermolecular forces than a less polar polymer. The polarity depends on the asymmetric distribution of electrons and the presence of electronegative atoms. Hydrogen bonding is the strongest intermolecular force present in covalent compounds.

Answer to Problem 26.44P

Polyoxetane (PO) is expected to have a lower melting point than poly(ethelene oxide) (PEO), as the extent of hydrogen bonding is less along the chain of polyoxetane than in polyethelene oxide. Also, as the number of carbon atoms in poyoxetane is more, it is slightly less polar than polyethelene oxide.

Explanation of Solution

The condensed structures of polyoxetane and poly(ethelene oxide) are shown below:

The repeating units in both are shown in the parenthesis. Poly(ethelene oxide) has two carbon atoms while polyoxetane has three carbon atoms in the repeating unit. As the number of carbon atoms increases the polarity decreases. Polyoxetane is slightly less polar than polyethelene oxide.

Both the repeating units have an oxygen atom in it. Due to which hydrogen bonding is possible. The number of carbon atoms in polyoxetane is more so, oxygen occurs less frequently in polymer chain in than in polyethelene oxide. As a result, there will be less hydrogen bonding. Hence, polyoxetane (PO) is expected to have a lower melting point than poly(ethelene oxide) (PEO).

Conclusion

As the strength of intermolecular forces increases, the melting point increase.