Topics in Current Chemistry (2018) 376:44
1 3
1 Introduction
Polymerization is widely involved in chemistry, chemical engineering, and materials science, through which monomer molecules are transformed to various polymers with specific molecular weights, structures, and shapes for the fabrication
of high-performance materials [1–3]. Polymerization occurs via a variety of reaction mechanisms that vary in complexity due to functional groups presenting in
reactive compounds and their inherent steric effects. It can be categorized into
two kinds of polymerization, including step-growth polymerization and chaingrowth polymerization [4]. Accordingly, step-growth polymers are referred to as
polymers formed by the stepwise reaction between functional groups of monomers that usually contain heteroatoms such as nitrogen or oxygen. Chain-growth
polymerization typically involves the linking together of molecules incorporating double or triple carbon–carbon bonds to form repeating chains, and it is usually divided into free radical polymerization, cationic polymerization, and anionic polymerization. In particular, the free radical mechanism is frequently used
for the clarification of chain-growth polymerization, which can be divided into
three stages, including chain initiation, chain propagation, and chain termination
[5]. Free radicals with unpaired electrons are very reactive atoms or molecules,
and precise control over the initiation, propagation, and termination rates during
polymerization is required in order to produce high-molecular-weight and uniform polymer products. Polymerization processes usually belong to exothermic
reactions [1, 6]. Various methods are applied to remove excess concentrated heat
during these exothermic reactions to prohibit explosive polymerization. These
operational methods mainly include solution polymerization, emulsion polymerization, suspension polymerization, and precipitation polymerization, which
are classified into heterogeneous polymerization. Photopolymerization is a special kind of polymerization that is initiated by the absorption of visible or ultraviolet light [7–9]. Most photopolymerization processes belong to chain-growth
polymerization, in which the light may be absorbed either directly by the reactant
monomer (direct photopolymerization), or else by a photosensitizer that absorbs
the light and then transfers energy to the monomer. Apparently, the process control over photopolymerization is more complex compared with ordinary thermal
polymerization since the photon transport and light intensity distribution are
additional factors in a photopolymerization process.
Round-bottom flasks are frequently applied as batch reactors for various kinds
of polymerization processes on the laboratorial scale. However, many polymerization processes are kinetically fast and highly exothermic, in which polymer
chain growth can complete at a level of seconds or even shorter periods [10]. For
these polymerization processes, the heat and mass transfer rates in batch reactors are usually insufficient, leading to obvious gradients of temperature and concentration inside the reactors. Therefore, drawbacks of batch reactors regarding
transport properties might result in local overheating of reactors and occurrence
of explosive polymerization, giving rise to broad molecular weight distributions
or even runaway of polymerization processes.
148
Reprinted from the journal
1 3
1 Introduction
Polymerization is widely involved in chemistry, chemical engineering, and materials science, through which monomer molecules are transformed to various polymers with specific molecular weights, structures, and shapes for the fabrication
of high-performance materials [1–3]. Polymerization occurs via a variety of reaction mechanisms that vary in complexity due to functional groups presenting in
reactive compounds and their inherent steric effects. It can be categorized into
two kinds of polymerization, including step-growth polymerization and chaingrowth polymerization [4]. Accordingly, step-growth polymers are referred to as
polymers formed by the stepwise reaction between functional groups of monomers that usually contain heteroatoms such as nitrogen or oxygen. Chain-growth
polymerization typically involves the linking together of molecules incorporating double or triple carbon–carbon bonds to form repeating chains, and it is usually divided into free radical polymerization, cationic polymerization, and anionic polymerization. In particular, the free radical mechanism is frequently used
for the clarification of chain-growth polymerization, which can be divided into
three stages, including chain initiation, chain propagation, and chain termination
[5]. Free radicals with unpaired electrons are very reactive atoms or molecules,
and precise control over the initiation, propagation, and termination rates during
polymerization is required in order to produce high-molecular-weight and uniform polymer products. Polymerization processes usually belong to exothermic
reactions [1, 6]. Various methods are applied to remove excess concentrated heat
during these exothermic reactions to prohibit explosive polymerization. These
operational methods mainly include solution polymerization, emulsion polymerization, suspension polymerization, and precipitation polymerization, which
are classified into heterogeneous polymerization. Photopolymerization is a special kind of polymerization that is initiated by the absorption of visible or ultraviolet light [7–9]. Most photopolymerization processes belong to chain-growth
polymerization, in which the light may be absorbed either directly by the reactant
monomer (direct photopolymerization), or else by a photosensitizer that absorbs
the light and then transfers energy to the monomer. Apparently, the process control over photopolymerization is more complex compared with ordinary thermal
polymerization since the photon transport and light intensity distribution are
additional factors in a photopolymerization process.
Round-bottom flasks are frequently applied as batch reactors for various kinds
of polymerization processes on the laboratorial scale. However, many polymerization processes are kinetically fast and highly exothermic, in which polymer
chain growth can complete at a level of seconds or even shorter periods [10]. For
these polymerization processes, the heat and mass transfer rates in batch reactors are usually insufficient, leading to obvious gradients of temperature and concentration inside the reactors. Therefore, drawbacks of batch reactors regarding
transport properties might result in local overheating of reactors and occurrence
of explosive polymerization, giving rise to broad molecular weight distributions
or even runaway of polymerization processes.
148
Reprinted from the journal