Question

Polymers are classified in a number of ways. On the basis of structures, they are divided into three types: linear polymers, branched chain polymers and cross linked polymers. Depending upon the nature of the repeating structural unit, polymers are classified as homopolymers and copolymers. Based on the magnitude of the intermolecular forces, they are classified into four types namely elastomers, fibres, thermoplastics and thermosetting plastics. Polymers are again classified as addition polymers and condensation polymers depending on the mode of polymerization. The advantage of polymerization using Ziegler-Natta catalyst is (a) It takes place by free radical polymerization. (b) It takes place by anion polymerization mechanism. (c) It gives linear polymer permitting stereochemical control. (d) It gives highly branched polymer.

Short answer

Answer: It gives linear polymer permitting stereochemical control.

Step-by-step solution

Understand what Ziegler-Natta catalyst is

Ziegler-Natta catalysts are coordination compounds used to catalyze the polymerization of unsaturated hydrocarbons (such as alkenes). They allow for the polymerization process to occur under mild conditions and provide better control over polymer properties like structure and stereochemistry.

Evaluate the given choices for the advantage of polymerization using Ziegler-Natta catalyst

Let's have a closer look at the given choices: (a) It takes place by free radical polymerization. Ziegler-Natta catalysts are not known for free radical polymerization. They work using coordination polymerization mechanism. (b) It takes place by anion polymerization mechanism. Ziegler-Natta catalysts are not known for anionic polymerization. They work by coordinating the unsaturated hydrocarbons to the metal center in the catalyst. (c) It gives linear polymer permitting stereochemical control. This is the correct answer. Ziegler-Natta catalysts produce linear polymers and permit better control over the stereochemistry of the polymer chain, resulting in the formation of isotactic, syndiotactic, or atactic polymers depending on the catalyst used. (d) It gives highly branched polymer. This is not correct, as Ziegler-Natta catalysts are known to produce linear polymers, not highly branched ones.

Choose the correct answer

The correct answer is (c) It gives linear polymer permitting stereochemical control.

Key concepts

Polymer Classification

Polymers, the macromolecules we encounter in various forms in our daily life, are classified based on several factors. The structure plays a pivotal role in determining the unique properties of each polymer.

Linear polymers consist of long, straight chains and are exemplified by materials like polyethylene. These can be somewhat entangled, much like strands of spaghetti in a bowl. Branched chain polymers have side chains attached to the main chain, which influence the density and strength of the material. Common examples include low-density polyethylene. Cross-linked polymers have chains that are interconnected at various points, providing a firm structure that is seen in materials like vulcanized rubber.

From the chemical composition perspective, homopolymers consist of a single type of repeating unit, while copolymers are formed from two or more different monomers. This distinction allows for a vast range of polymer properties tailored for specific uses. Furthermore, polymers are often categorized by their physical properties into elastomers, fibers, thermoplastics, and thermosetting plastics, each designated for a particular functionality based on their response to heat and traction.

Regarding the polymerization process, addition polymers result from the addition of monomer units without the loss of small molecules, whereas condensation polymers are formed by the joining of monomer units with the simultaneous elimination of smaller molecules such as water. Each classification leads to distinct polymer families, each with their unique characteristics and applications.

Polymerization Mechanisms

Polymerization is the chemical process that combines small molecules called monomers into a covalently bonded chain or network to form a polymer. There are two main types of polymerization mechanisms: addition polymerization and condensation polymerization.

In addition polymerization, monomers add to each other in such a way that the polymer contains all the atoms of the starting monomers. This type involves three stages: initiation, where an active center is created; propagation, where successive addition of monomers occurs; and termination, which results in the end of the polymer chain growth.

Condensation polymerization, on the other hand, involves the repeated condensation reaction between two different monomers with the loss of small molecules like water, creating a bond between the monomers. This process continues, building up a larger structure. These reactions often require a catalyst to proceed at a reasonable rate and to control molecular weight.

Ziegler-Natta Catalysts

The Ziegler-Natta catalyst is a crucial contributor to polymerization mechanisms, specifically during the polymerization of alkenes to form polyolefins. These catalysts are known to promote polymerization via coordination polymerization, a path distinct from free radical or anionic mechanisms. This process usually results in a stereoregular polymer, which means the configuration of substituent groups are ordered in a specific way along the chain, contributing to the desired physical properties of the final polymer.

Steric Control in Polymers

Steric control is an essential aspect of polymer chemistry, especially when discussing the characteristics and applications of polymers. The term 'steric' refers to the spatial arrangement of atoms in a molecule, and in the context of polymers, it signifies the arrangement of substituent groups around the main polymer chain.

Steric control in polymers can dramatically influence a polymer's physical properties, such as melting point, density, and crystallinity. For instance, in polypropylene, the orientation of its methyl groups can lead to different types of the polymer: isotactic, where all methyl groups are oriented the same way along the polymer chain; syndiotactic, with alternating orientations; and atactic, which has a random placement of methyl groups.

Ziegler-Natta catalysts have enabled scientists to exert this steric control in the manufacturing process, allowing for the production of these stereoregular polymers. Through this control, polymers can be designed to meet specific performance criteria essential in creating materials for a wide range of industrial applications, from plastic films and containers to high-strength fibers and auto parts. This level of control makes Ziegler-Natta catalysts invaluable in modern polymer production, facilitating the creation of materials with properties tailored to the precise needs of various sectors.