Polymerization is a fundamental process in polymer science that underpins the synthesis of materials used

across diverse industrial and technological sectors. This review provides a comprehensive examination of the

two principal polymerization mechanisms—step-growth and chain-growth polymerization—highlighting their

distinct reaction pathways, kinetics, and structural outcomes. In step-growth polymerization, monomeric units

bearing complementary functional groups combine gradually, often producing small by-products, and

requiring high monomer conversion to achieve high molecular weights. Conversely, chain-growth

practical applications of each method. Overall, this review aims to offer a consolidated understanding of

polymerization mechanisms, serving as a valuable reference for students, researchers, and professionals

involved in polymer chemistry and materials science.

Keywords: Polymerization, Step-Growth Polymerization, Chain-Growth Polymerization, Radical

Polymerization, Anionic Polymerization, Cationic Polymerization

Introduction: Polymerization, is a process of joining together of monomer units or molecules in a chemical

reaction to form long chains or three-dimensional networks called polymer [1, 2, 3].

In chemical compounds, polymerization occurs through a variety of reaction mechanisms depending upon

various factors like nature of different functional groups present in the reactants

between two styrene monomers [3].

Discussion: Some other alkenes polymerization is polyethylene and polyvinyl chloride (PVC) [3].

Polyethylene is formed by polymerization of several Ethylene monomers, joining together to form long chains

of ethylene molecules [3]. Polyethylene forms variety of products of daily life like useful poly bags, disposable

packets etc [3]. Polyvinyl chloride, similarly is formed by polymerization of monomeric units of vinyl

chloride, where each vinyl chloride joins together to form long chain of polyvinyl chloride [3]. PVC finds

varied application in manufacturing processes of useful commercial products, such as pipes, insulation and

packaging, footwear bags etc [1, 2, 3]. Example: Polyvinyl chloride, polyethylene, polystyrene, etc are referred

to as "homopolymers", consisting of long chains of same type of monomeric units, repeated manner [1, 2, 3,

4]. In contrast, polymers consisting different types of monomeric units are called copolymers [5]. Bakelite,

at a faster rate is known as auto-acceleration, may cause fires and even explosions occasionally [4, 5,6].

Step-growth and chain-growth are the two major classifications of polymerization reaction mechanisms [1, 2,

4, 6]. In step growth polymerization, control of stoichiometry, is a decisive factor and can be applied to most of

the monomers. While in chain growth polymerization, affords high molecular-weight polymers, and applies to

certain limited monomers [2, 3, 4, 5, 6].

Fig: Schematic diagram classification of polymerization.

In step-growth polymerization, monomeric units, joins together stepwise manner forming a long polymer

molecule [6, 7]. The average molar mass of polymer however increases slowly., as the addition of monomer

takes place at every step. Thus, the long chains forms in the reaction. [6, 7].

bifunctional monomersjoin together without loss of water or other volatile molecules. Such polymers are

referred to as addition polymers [6, 7, 11].

The molecular weight of Step-growth polymers increases at a very slow rate at lower conversions and reach

moderately high molecular weights at very high conversion (i.e., >95%). Solid state polymerization is in

polyamides (e.g., nylons) [8].

In chain-growth polymerization, the addition of a monomer to a growing chain with an active center like free

radical, cation or anion takes place [5, 6, 7]. At first the chain growth is initiated by formation of an active

center, followed by propagation of chain through repeated addition of a sequence of monomers, resulting in

Chain-growth polymerization also known as addition polymerization is formed through linking together of

unsaturated monomers, especially containing carbon-carbon double bonds [6, 7, 8]. The pi-bond slowly

disappears and the formation of a new sigma bond takes place, thereby joining two monomeric units together

[8]. Some of the commonly manufactured polymers through Chain-growth polymerization are

polyethylene, polypropylene, polyvinyl chloride (PVC), and acrylate [6, 7, 8]. In chain growth polymers, the

alkenes RCH=CH

are converted to high molecular weight alkanes (-RCHCH

-)

R = H, CH

, Cl, CO

Scheme - Polymerization of ethylene

The initiation, propagation, and termination stages of chain polymerization are controlled by various factors

like temperature control, or heat management [5, 6, 7, 8]. Chain polymerization reactions are usually highly

exothermic. For example, polyethylene is formed by release of 93.6 kJ of energy per mole of monomer [8].

Some highly evolved technology Methods like emulsion polymerization, solution polymerization, suspension

polymerization, and precipitation polymerization are involved in chain growth polymerization [9]. The

polymer dispersity and molecular weight may however be improved; these methods would require additional

processing in order to isolate the product from a solvent.[9].

Some polymerization reactions are initiated by photons [9]. These polymerization reactions are called photo

polymerization reaction [9]. They are mostly chain-growth polymerization reactions initiated by the absorption

transfer step however remains unchanged [6]. In step-growth photopolymerization, absorption of light triggers

an addition (or condensation) reaction between two monomeric units [10, 11]. A propagation cycle is not

initiated because each growth step requires the assistance of light [11].

Photopolymerization finds varied application in photographic or printing process [9, 10, 11]. In this process,

polymerization occurs only in regions which have been exposed to light or photons [10, 11]. Unreacted

monomer can be removed from unexposed regions, leaving a relief polymeric image [6, 9, 10, 11]. Several

forms of 3D printing—including layer-by-layer stereolithography and two-photon absorption 3D

photopolymerization — use photopolymerization [9, 10, 11, 12].

Multiphoton polymerization using single pulses have also been demonstrated for fabrication of complex

structures using a digital micromirror device.[13].

involving condensation reactions, progresses through successive interactions between bifunctional or

multifunctional monomers, requiring high conversion to attain significant molecular weight. In contrast, chain-

growth polymerization, initiated by reactive species such as free radicals or ions, allows rapid chain extension

and the early formation of high molecular weight polymers.

The discussion also extended to specialized approaches such as photopolymerization, which has opened new

avenues in precision manufacturing and responsive material design. By understanding the advantages and

In conclusion, a solid grasp of polymerization mechanisms remains essential not only for academic

understanding but also for advancing industrial processes and developing innovative materials tailored to

emerging needs. Ongoing research in controlled polymerization and environmentally conscious synthesis

strategies is expected to further expand the potential of polymer science in the years to come.

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