Question

Draw a structural formula of the polymer resulting from base-catalyzed polymerization of each compound. Would you expect the polymers to be optically active? \((S)-(+)\)-lactide is the dilactone formed from two molecules of \((S)\) - \((+)\)-lactic acid.

Short answer

Answer: The structural formula of the polymer resulting from base-catalyzed polymerization of (S)-(+)-lactide is polylactic acid (PLA) with the following repeating structural unit: ``` -[C(O)C]**n** Where: n is the number of repeating units in the polymer. ``` And yes, the resulting polymers would be optically active because the chiral carbon atoms in the repeating units are preserved during the polymerization process.

Step-by-step solution

Understand the structure of \((S)-(+)\)-lactide

\((S)-(+)\)-lactide is a dilactone formed from two molecules of \((S)\)-\((+)\)-lactic acid, which is the chiral form of lactic acid with a specific rotation. The structure of \((S)\)-\((+)\)-lactic acid is as follows: ``` H \ C -- OH / \ HO C \ O ``` Since \((S)-(+)\)-lactide is formed from two molecules of \((S)\)-\((+)\)-lactic acid, it would have the following cyclic structure: ``` O || C -- O / \ O C \ / \ C -- O / \ O C \ OH ```

Base-catalyzed polymerization of \((S)-(+)\)-lactide

In the base-catalyzed polymerization of \((S)-(+)\)-lactide, the ester linkage between the two lactic acid molecules in the lactide is broken and reactive sites are generated. These reactive sites on the lactide molecules combine with each other, leading to the formation of the polymer. The resulting polymer, polylactic acid (PLA), has the following repeating structural unit: ``` -[C(O)C]**n** Where: n is the number of repeating units in the polymer. ``` To better visualize the polymer structure, let's draw it: ``` O || C -- O -- C -- O -- C -- O -- C -- O -- ... -- C -- O -- C / \ ^ / \ ^ / \ ^ / \ ... / \ ^ O C O C O C O O C \ / \ / \ / \ C -- O -- C -- O -- C -- O -- ... -- C -- O / \ O C \ OH ```

Determine the optical activity of the resulting polymer

Optical activity refers to the ability of a compound to rotate the plane of plane-polarized light. Chiral compounds exhibit optical activity. In the case of \((S)-(+)\)-lactide, the starting material is optically active as it has a chiral center (the carbon atom bonded to the hydroxyl group). When this lactide undergoes base-catalyzed polymerization, the resulting polymer (PLA) also has the chiral carbon atoms in its repeating units. Meanwhile, the stereochemistry of the chiral centers is preserved throughout the polymerization process. Thus, the resulting polylactic acid (PLA) polymer would also be optically active.

Key concepts

Optical Activity

Optical activity is a fascinating property of certain compounds used to determine their chiral nature. This property allows a compound to rotate the plane of plane-polarized light. The direction and degree to which the light is rotated reveal much about the compound's structure.
The extent of this rotation is measured using a polarimeter and expressed as "specific rotation," commonly denoted in degrees. Optically active compounds possess one or more chiral centers. These are specific atoms within a molecule that contribute to its ability to rotate light.
Reasons for optical activity include:

• Presence of at least one chiral center
• Molecular asymmetry causing an unequal distribution of electrons
• In some cases, the entire molecular structure can introduce optical activity

In our focus on polylactic acid (PLA), the specific optical activity comes from the chiral nature of its building blocks, which remain intact throughout the polymerization process.

Chiral Compounds

Chiral compounds hold a special place in chemistry due to their ability to exist as enantiomers. These are non-superimposable mirror images of each other, much like a person's left and right hands. Each enantiomer can have markedly different effects, especially in biological systems.
A key aspect of chirality is the existence of a chiral center, typically a carbon atom bonded to four different atoms or groups.

• Each combination of bonded groups leads to unique stereochemistry.
• Enantiomers of a chiral compound will rotate plane-polarized light to the same degree, but in opposite directions.

For polylactic acid (PLA), chirality comes from the lactic acid units that comprise it. In our exercise, PLA is derived from the polymerization of (S)-(+)-lactide, itself formed from (S)-(+)-lactic acid, maintaining the chirality within the polymer structure.

Polylactic Acid

Polylactic acid (PLA) is a biodegradable and bioactive thermoplastic derived from renewable resources, making it an eco-friendly alternative to petrochemical-based plastics. The synthesis of PLA can be achieved through the polymerization of lactic acid, which can be sourced from the fermentation of carbohydrate-rich materials.
PLA is noted for its mechanical properties similar to conventional plastics but with the ability to decompose into natural substances over time.
Key characteristics of polylactic acid include:

• Biodegradability, breaking down in industrial composting environments
• High strength and plasticity, useful in packaging, textiles, and biomedical applications
• Optical activity, retained by the repeating units of chiral lactic acid in the polymer chain

The polymerization process, particularly when initiated through base-catalysis, preserves the chirality from the monomer units, contributing to PLA's optical activity. This makes PLA a unique material beyond its environmental benefits, offering additional scientific and industrial applications associated with its chiral nature.