Question

High-pressure polyethylene differs from polyethylene, made with the aid of Ziegler catalysts, in having a lower density and lower ${\mathrm{T}}_{\mathrm{m}}.$ It has been suggested that this is due to branches in the chains of the high-pressure material. Explain how such branches might arise in the polymerization process and how they would affect the density and ${\mathrm{T}}_{\mathrm{m}}$.

Short answer

The branches in high-pressure polyethylene, or low-density polyethylene (LDPE), arise during the free radical polymerization process when free radicals in the chain react with hydrogen atoms of existing chains, creating side chains. These branches lead to a lower density and lower melting temperature (Tm) compared to linear polyethylene made with Ziegler catalysts by causing a less compact structure and lower crystallinity. Due to the disrupted alignment and irregular structure caused by the side chains, the polymer chains cannot pack closely together, resulting in decreased density and Tm.

Step-by-step solution

1. Understanding Branching in Polymerization Process

High-pressure polyethylene, also known as low-density polyethylene (LDPE), is mainly produced through the process of free radical polymerization at high pressure and high temperature. The branching can occur when the free radical in the chain reacts with one of the hydrogen atoms of an existing chain instead of the ethylene monomer. This process creates a side chain, resulting in a branched structure.

2. Effect of Branching on Density

The presence of branches in high-pressure polyethylene leads to a lower density compared to linear polyethylene (like high-density polyethylene, HDPE). This is because branches prevent the polymer chains from packing closely together. The branched chains occupy more space and create a less compact structure, leading to a decrease in density.

3. Effect of Branching on Melting Temperature (Tm)

The melting temperature (Tm) is affected by the degree of crystallinity in a polymer. Branched polymers, such as high-pressure polyethylene, have less crystallinity compared to linear polymers due to the irregular structure caused by the side chains. The branches disrupt the regular alignment and reduce the ability of the chains to form an ordered, crystalline structure. As a result, the melting temperature of high-pressure polyethylene is lower than that of linear polyethylene. In conclusion, the branched structure formed during the polymerization process of high-pressure polyethylene results in a lower density and melting temperature compared to linear polyethylene made with Ziegler catalysts. The branches arise when free radicals in the chain react with hydrogen atoms of existing chains, leading to a less compact structure and lower crystallinity, which are responsible for the observed lower density and Tm.

Key concepts

High-Pressure Polyethylene

High-pressure polyethylene, also known as low-density polyethylene (LDPE), is a versatile material produced under specific conditions of high pressure and high temperature. This process distinguishes it from its counterpart, high-density polyethylene (HDPE), typically made with the aid of Ziegler catalysts. High pressure is essential as it facilitates the creation of free radicals necessary for polymerization.

In high-pressure conditions, the polyethylene chains become highly branched because the free radicals can react with the polymer chains themselves. This results in short and long branches emerging from the main chain structure. The unique processing conditions lead to a material that is more flexible, with a unique set of properties that makes it suitable for applications like packaging films and containers.

Free Radical Polymerization

Free radical polymerization is a common method for synthesizing polymers like LDPE. It involves the use of initiators that decompose to form radicals. These radicals are highly reactive species that can attack the double bonds of monomers, such as ethylene, leading to the formation of new polymer chains.

• The process starts with the initiation where the radical reacts with a monomer.
• Propagation follows, where the chain grows as the radical site moves along the chain reacting with additional monomers.
• The process concludes with termination, where the radicals combine to form a stable molecule.

This chain reaction enables the production of polymers with various molecular weights. It's the randomness of the radical formation and chain scission that can lead to branching, influencing the final properties of the polymer.

Branching in Polymers

Branching occurs when side chains sprout from the main backbone of a polymer. In the case of LDPE, branching happens when the free radical initiator causes a hydrogen atom from the growing or another chain to be replaced, creating a new radical site. This results in the development of a side chain, which may continue to grow.

Branching has a significant impact on the polymer’s physical properties. It results in:

• Reduced packing efficiency, as the side branches prevent chains from aligning neatly.
• Lower density, due to the increased volume occupied by the molecules.
• Greater flexibility, as the polymer chains can move more freely past each other.

These characteristics provide the polymer with distinct advantages, especially where flexibility and toughness are required.

Polymer Density

Polymer density is a crucial aspect of a polymer's physical properties and is influenced by its structure. In polymers like LDPE, density is lower due to the extensive branching that occurs in the polymer chains.

When polymers are densely packed, such as in HDPE, the chains are aligned closely with minimal branching, leading to a compact, efficient packing and higher density. In contrast, the branches in LDPE create spaces within the structure, reducing how tightly the chains pack and thus their final density. This decreased density translates to unique material properties like increased ductility and impact resistance, making it ideal for products where these attributes are meaningful.

Melting Temperature of Polymers

The melting temperature ($T_m$) of a polymer is the temperature at which it changes from a solid to a liquid state. This temperature is influenced by how crystalline the polymer is, or how well-ordered its structure is.

• In polymers with high crystallinity, such as HDPE, chains align in an orderly fashion, resulting in a high melting temperature because more energy is needed to disrupt the regular, tightly packed structure.
• Branched polymers like LDPE, however, have irregular, non-crystalline structures, which require less energy to disrupt. Therefore, they have lower melting temperatures.

Understanding the melting temperature is vital, particularly for applications that involve thermal processing or require certain flexibility levels at lower temperatures, ensuring the polymer remains fit for purpose in varying conditions.