Download Free-radical polymerization

Photolytic Polymerization
Review of radical, ionic polymerization
Free-radical polymerization (chain reaction)
- Initiators (direct method, Azo compounds,
peroxides, or redox reactions)
Ionic polymerization
- Initiators (alkali metal suspensions and alkyl or
aryllithium reagents)
Photo polymerization can also be induced by supplying
the initiation energy through irradiation with visible or
ultraviolet light, high-energy or ionizing radiation, or by the
passage of an electric current.
1. Photochemical polymerization
1.1 Advantages and Uses
Avoidance of chemical contamination by initiator residues
Convenience of photochemical reactions
A wide range of reaction temperatures
Utilization of photo polymerization and photo cross-linking for
practical photographic process
1.2 Photopolymerization Process
Light
Monometer or un-cross-linked polymer
Negative
Base
Solvent to remove unpolymerized or uncross-linked material
Cross-linked
polymer
Relief image
Figure_ Photopolymerization as photographic process. Illumination of a
monomer or a cross-linkable polymer forms an insoluble relief image.
1.3 Monomers that undergo photopolymerization
Any monomer that will undergo chain reaction polymerization is
susceptible to photopolymerization or photosensitized polymerization.
The absorption steps and the termination reactions are generally not
affected.
The advantage of photopolymerization and photosensitized
polymerization is that the initiation process may take place over a wide
range of temperatures and with a greater specificity than is found in
chemically initiated systems.
Other monomers, such as styrene or methyl methacrylate, are
susceptible to direct photopolymerization when exposed to 300 nm or
shorter wavelength light.
1.4 Experimental Technique: A Photosensitized
Polymerization of Styrene
1.5 Mechanism of Initiation by Direct Photolysis of
the Monomer
Two different types of direct photoinitiation can be recognized.
First, light absorption yields an electronically excited monomer
molecule which subsequently decomposes to give radical fragments.
Rate of initiation
The rate of initiation of the chains and the dependence of this rate
on the monomer concentration and temperature is quite different in
photochemical initiator from the situation found in thermal initiation.
Rate of initiation for
thermal initiation
Rate of initiation for photoinitiation
The rate of initiating radical formation in a photoinitiation process is
almost independent of the temperature but is proportional to the
monomer concentration.
Second, initiation mechanism is exemplified by the
photopolymerization of styrene or methyl methacrylate.
Fluorescence
Diradical
1.6 Photosensitized polymerizations
Photosensitizers are substances that produce free radicals when
they absorb ultraviolet or visible light.
The same substances that are used for thermal initiation are often used
for photosensitization. Azo compounds and peroxides are
photosensitizers, and the photoinitiaion reaction is the same as is the
thermal initiation process.
For example, azoisopropane does not dissociate sufficiently rapidly
below 180 oC to be a useful thermal initiator. However, it photodissociates
even at low temperatures when irradiated with near-ultraviolet light:
1.7 Photosensitizers for photopolymerization
Photolithography
A process used in microfabrication to pattern parts of a thin film or the
bulk of a substrate
Transfer process of a geometric pattern from a photomaks to a lightsensitive chemical photoresist on the substrate
What is lithography?
석판화 기술, 인쇄 기술
Litho (돌) + graphy (그림, 글자)
Recently
Semiconductor manufacturing:
Microlithography = transfer the pattern of circuits from a photomask to a
wafer (a thin slice of silicon or other semiconductor material on which
chips are made
Lithography Analogy
Camera
Exposure
instruments
“Taking pattern on silicon wafer”
사진film
Mask
Photo paper (인화지)
Wafer
감광제
Photo-resist감광제
햇빛, 조명
Laser light source
Photo-resist
A photoresist is a light-sensitive material used in several industrial
processes, such as photolithography.
Phenolic
resins
Epoxy-based
polymer
Process of photolithography