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 formed from base-catalyzed polymerization of (S)-(+)-lactide is represented as: -[--(-CO-CH(CH\(_3\)-OH)-)\(_n\)--] The polymer will be optically active, as it has chiral carbons in its backbone.

Step-by-step solution

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

Lactide is a cyclic ester that is derived from two molecules of lactic acid. To draw \((S)-(+)\) lactide, we connect the carboxylic group of one \((S)-(+)\)-lactic acid molecule to the hydroxyl group of the second \((S)-(+)\)-lactic acid molecule, and vice versa, forming a cyclic ester structure, which should have the \(bold(S)\) chiral configuration.

Understand the base-catalyzed polymerization process

Base-catalyzed polymerization, often referred to as "ring-opening polymerization," involves breaking the existing ester bond of the lactide molecules, enabled by adding a base catalyst. This initiates the polymerization process, in which the nucleophilic attacks the electrophilic sites and generates new ester linkages, forming a long-chain molecule called a polymer.

Draw the structural formula of the resulting polymer

Upon undergoing base-catalyzed ring-opening polymerization, the lactide molecules open up (the cyclic ester breaks), and ester bonds are formed between the opened molecules, generating the polylactide (PLA) polymer. The structural formula of the resulting polymer can be represented as follows: -[--(-CO-CH(CH\(_3\)-OH)-)\(_n\)--] Here, \(n\) denotes the number of repeat units in the polymer.

Determine if the polymer is optically active

The resulting polymer will be optically active if it has chiral carbons in its backbone. Since the original \((S)-(+)\) lactide is itself chiral, the resulting polylactide (PLA) polymer will also be chiral. As a result, we can conclude that the polymer is optically active.

Key concepts

Ring-Opening Polymerization

Ring-opening polymerization (ROP) is a unique and efficient method of forming polymers with a wide range of properties. It involves the opening of a ring-shaped molecule, known as a cyclic monomer, to form a long, linear chain polymer. The process typically requires an initiator or a catalyst, which in the case of base-catalyzed polymerization, is a substance that provides an electron-rich species, or a base, to start the reaction.

The mechanism of ROP begins with the breaking of the bonds holding the ring structure together. This can be initiated by a nucleophile (a compound with an unshared pair of electrons) that attacks an electrophilic center of the monomer, such as the carbon within an ester group, leading to the ring opening. The newly created end of the chain remains active, allowing additional monomer units to join and create longer polymer chains. This form of polymerization is less prone to side reactions, which makes the resulting polymers more regular and predictable in structure.

Advantages of ROP

• It allows the creation of polymers with defined structures and high molecular weights.
• The process is typically highly specific, resulting in well-defined polymers.
• By changing the monomer or the conditions, it's possible to synthesize a variety of polymers with different properties.

Polylactide (PLA)

Polylactide, commonly known as PLA, is a biodegradable and bioactive polyester derived from renewable resources like corn starch or sugar cane. It has gained popularity due to its eco-friendly profile and its ability to decompose into lactic acid, which is not harmful to the environment. PLA is produced by the ring-opening polymerization of lactide, which is a cyclic ester derived from lactic acid.

PLA has a wide range of applications, from medical implants to packaging materials. In the medical field, PLA is used for sutures, stents, and drug delivery systems because it naturally degrades in the body over time. In packaging, PLA offers an environmentally sustainable alternative to petroleum-based plastics.

Key Qualities of PLA

• Biocompatible and biodegradable.
• Derived from renewable resources.
• Can be processed using conventional plastic processing equipment.
• Possesses high strength and stiffness relative to other bioplastics.

Optical Activity

Optical activity is a fascinating phenomenon exhibited by substances that can rotate the plane of polarized light. A substance is considered optically active if it contains chiral centers—carbon atoms bonded to four different groups—and lacks a plane of symmetry. When such a substance is dissolved in a solution or melted, it interacts with polarized light by rotating its plane either to the right (dextrorotatory) or to the left (levorotatory).

The degree of this rotation is measured by a polarimeter, and the specific rotation is a characteristic property of the chiral substance. The presence of optical activity in polymers like PLA is directly related to the configuration of the original monomer. If the monomer is optically active, like the (S)-(+)-lactic acid used to create lactide, the resulting polymer will also be optically active. This optical activity can lead to different material properties and biological interactions, making it an important trait in polymers, especially those used in medical applications and devices.

Importance of Optical Activity

• Indicates the presence of chirality in a molecule.
• Essential for understanding the interaction of molecules with light, which has applications in identifying substances and purifying enantiomers.
• Influences the biological activity of compounds, especially in pharmaceuticals where different enantiomers can have different effects on the body.