Photopolymerization
Definition:
Photo-polymerization is a process in which small monomer units are chemically bonded to form a polymer chain through a photochemical reaction, initiated by the absorption of light or luminous energy.
Examples:
Examples of materials produced via photopolymerization include plastics such as those used in Lego® components, nylon fibers utilized in stockings, and Teflon, commonly employed in non-stick cookware applications.
Applications:
Photopolymerization chemistry finds widespread application across various fields, including coatings, adhesives, sealants, photoresists, printing plates, and photolithography, etc.
Principle of Photopolymerization:
The fundamental principle of photopolymerization involves the light-induced generation of a reactive species, which then initiates thermal reactions of low molecular weight compounds, leading to the formation of a polymer or network structure. In general, these reactions are characterized by low activation energies (~ 60 kJ mol–1 for radical chain polymerization). As a result, these processes can occur rapidly even at room temperature. he photo-initiator in the system generates the reactive species X in high yields (Φ ≈ 0.5) upon exposure to irradiation. The reactive species X can be either a radical or a cation, such as a Brønsted or Lewis acid. A Lewis acid is an electron-pair acceptor (e.g., H+, BF3) while a Brønsted-Lowry acid is a proton donor (e.g., HCl, CH3COOH, NH4+). Most commercially viable photopolymers, however, are produced through radical processes.
a) Free radical polymerization:
Free radical polymerization involves three key stages: initiation, propagation, and termination. The initiation step involves the formation of free radicals from catalysts such as benzoyl peroxide, acetyl peroxide, or tert-butyl peroxide, which then react with monomers. During propagation, monomers are rapidly and progressively added to the growing polymer chain without altering the active center. The termination step occurs when the growth of the polymer chain or the active center in the polymer chain is disrupted. A wide range of alkenes, dienes, and their derivatives can be polymerized in the presence of an initiator catalyst, such as peroxides.
Example: The polymerization of ethylene to polyethylene is initiated by heating or exposing a mixture of ethylene and a small amount of benzoyl peroxide to light.
Mechanism: The reaction involves the following steps
i) Initiation:
A free radical is required to initiate the chain reaction. These free radicals are typically generated by the decomposition of peroxides such as benzoyl peroxide, acetyl peroxide, or tert-butyl peroxide. When exposed to heat or light, these peroxides decompose to produce a pair of free radicals. The process begins when one of these free radicals adds to the ethene double bond, forming a larger free radical. This step is known as the chain initiation step. For example, benzoyl peroxide decomposes upon photolysis to yield phenyl free radicals.
The phenyl radical adds to a monomer such as ethylene, resulting in the formation of a new free radical.
ii) Propagation:
This new free radical reacts with another ethene molecule, forming a larger free radical. The continuous repetition of this process, with the formation of progressively larger radicals, drives the reaction forward. This step is known as the chain propagation step.
iii) Termination:
The product radical formed in the propagation step reacts with another radical to create the polymerized product. This step is known as the chain termination step.
The properties of a polymer are influenced by the length of its chain. The length of the polymer molecule can be controlled by telomers. Thiols (R-SH) are commonly used as telomers, they provide a hydrogen radical that terminates the polymer chain.
The RS radical is not reactive enough to initiate a new monomer chain. The chain reaction can be terminated by using inhibitors such as quinones or iodine.
b). Cationic polymerization:
Initiation:
In cationic polymerization, the initiator acts as an electrophile, adding to the double bond of ethylene to form an intermediate carbocation. This carbocation then reacts with another ethylene molecule, leading to the initiation of a chain reaction.
Propagation:
In this step, an ethylene molecule is continuously added to the newly formed carbocation with each addition, resulting in the growth of a long, continuous polymer chain.
Termination:
The termination step involves the loss of a proton, leading to the formation of a double bond at the end of the polymer chain.
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