Chloe Zhang
Plastics are polymers, which are large molecules formed of many smaller molecules covalently bonded in a repeating pattern. The small molecules which make up the polymer are called monomers. Polymers are formed by two main ways: addition polymerization and condensation polymerization. In addition polymerization, monomers with double or triple bonds undergo a chain reaction to form polymers. This process takes place in three steps: initiation, propagation, and termination. Some of the plastics made by addition polymerization include polyethylene, polyvinyl chloride, polystyrene, and polyoxymethylene (acetal).
| Characteristics | Values |
|---|---|
| Process | Addition polymerization, also called chain-growth polymerization, is a chain reaction that adds new monomer units to the growing polymer molecule one at a time through double or triple bonds in the monomer. |
| Polymerization Steps | 1. Chain initiation: Usually by means of an initiator which starts the polymerization process. The reactive initiation molecule can be a radical (free radical polymerization), cation (cationic polymerization), anion (anionic polymerization), and/or organometallic complex (coordination polymerization). 2. Chain propagation: A monomer adds onto the chain, and each new monomer unit creates an active site for the next attachment. 3. Chain termination: The radical, cation, or anion is "neutralized" to stop the chain propagation. |
| Examples of Plastics | Polyethylene, polyvinyl chloride, acrylics, polystyrene, polyoxymethylene (acetal), polypropylene, Teflon, Buna rubbers, polyacrylates, PCTFE, Saran wrap, polyurethane, and nylon. |
| Monomers | Unsaturated monomers with carbon-to-carbon double bonds, such as alkenes, ethene, propene, styrene, vinyl chloride, and ethylene oxide rings. |
| Polymer Properties | High molecular weight chains with rapid growth, retaining much monomer. |
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Polyethylene
In the context of polyethylene, the monomer used is ethene, which undergoes addition polymerization to create the polymer. This process was first discovered by accident in 1933 when ICI chemists Reginald Gibson and Eric Fawcett reacted ethene with benzaldehyde under high temperatures and pressures. They obtained a small amount of a white waxy solid, which was later identified as polyethylene.
The production of polyethylene involves different processes, including the use of peroxide or Ziegler-Natta catalysts, operating at varying temperatures and pressures. The Ziegler-Natta catalyst method, for example, involves mixing hydrogen with ethene to control the chain length of the polymer. This process occurs at relatively low pressures (10-80 atm) and temperatures ranging from 350-420 K.
The addition polymerization process that forms polyethylene can be categorized as chain-growth polymerization, where new monomer units are added one at a time through double or triple bonds. This process occurs in three steps: chain initiation, chain propagation, and chain termination. Overall, the formation of polyethylene through addition polymerization plays a significant role in the production of various plastic products used in our daily lives.
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Polyvinyl chloride
The preparation of PVC involves several stages. Firstly, ethene is converted into 1,2-dichloroethane. This compound is then cracked to produce chloroethene (vinyl chloride). Finally, polymerisation of vinyl chloride occurs, forming PVC. Ethene is obtained from the cracking of ethane, propane, naphtha, and gas oil. Chloroethene can also be produced by reacting ethylene with hydrochloric acid and oxygen over a copper catalyst.
During the polymerisation process, vinyl chloride is treated with peracid under pressure to form PVC. This polymerisation can be performed through suspension polymerisation, where an initiator is required to start the reaction. Organic peroxide is often used as an initiator as it is soluble in chloroethene. Emulsion polymerisation is another method, where ammonium peroxodisulfate is used as a catalyst due to its solubility in water.
PVC is a versatile plastic with a wide range of applications. It can be formulated with different additives and molecular masses, resulting in products ranging from rigid to pliable. PVC is commonly used in building and construction, such as for window and door profiles, pipes, and cable insulation. It is also used in packaging, including food film and bottles. Furthermore, PVC can be modified through chlorination to increase its chlorine content, resulting in chlorinated polyvinyl chloride (CPVC).
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Polystyrene
In the case of polystyrene, the chain initiation step involves the use of a radical initiator, such as azoisobutyronitrile (AIBN), which starts the polymerization process. The monomer units are then added to the growing polymer chain through the formation of new bonds, resulting in chain propagation. Finally, the chain termination step occurs when the radical is "neutralized", stopping the chain propagation.
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Polyoxymethylene
POM is characterized by its high strength, hardness, and rigidity, even at low temperatures. It has a density of 1.410–1.420 g/cm3 and is intrinsically opaque white due to its high crystalline composition, although it can be produced in various colours. Injection-molded POM is commonly used in high-performance engineering components, such as small gear wheels, eyeglass frames, ball bearings, ski bindings, fasteners, and gun parts.
The process of creating polyoxymethylene involves the polymerization of formaldehyde. To make the homopolymer, anhydrous formaldehyde is generated by reacting aqueous formaldehyde with an alcohol to create a hemiformal. This hemiformal/water mixture is then dehydrated, and formaldehyde is released by heating the hemiformal. The polymerization of formaldehyde occurs through anionic catalysis, and the resulting polymer is stabilized by reacting with acetic anhydride.
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Polyurethane
The formation of polyurethane involves the reaction of polyol and isocyanate monomers, specifically a diol and a diisocyanate. These monomers combine to form a urethane dimer with an alcohol group on one end and an isocyanate group on the other. This dimer can then react with other dimers or higher oligomers, leading to the creation of a high molecular weight polyurethane.
The polymerization of polyurethane can be influenced by catalysts such as diazobicyclo [2.2.2] octane (DABCO). This catalyst facilitates the polymerization process by providing a pair of unshared electrons that can attach to a positively charged nucleus. The choice of catalysts can impact both the rate of reaction and the degree of polymerization length, which are crucial factors in engineering urethane.
Overall, the formation of polyurethane through addition polymerization involves the reaction of polyol and isocyanate monomers, influenced by catalysts, resulting in a high molecular weight plastic with unique characteristics.
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Frequently asked questions
Addition polymerization, also known as chain-growth polymerization, is a process where new monomer units are added to a growing polymer molecule through double or triple bonds in the monomer.
Some plastics formed through addition polymerization include polyethylene, polyvinyl chloride (PVC), polystyrene, polypropylene, and Teflon.
In addition polymerization, monomers react to form a single product, while in condensation polymerization, a small molecule, usually water, is eliminated or lost during the reaction.
Polyethylene, formed through addition polymerization, is used in plastic bags, bottles, toys, and electrical insulation. It can have different properties depending on the length of the polymer chains and how efficiently they pack together.
Addition polymerization occurs in three steps: chain initiation, chain propagation, and chain termination. An initiator or catalyst reacts with a starting monomer, creating an unsatisfied bond that can react with another monomer, adding to the chain. This process repeats until the chain is terminated.
