Polymerization is the process by which polymers are made or “polymerized”. In polymerization, chemical reactions occur between small simple hydrocarbon monomers. Generally there are two main types of polymerization reactions. One is addition polymerization or chain growth and the other is condensation or step growth polymerization.

Addition (Chain Growth) Polymerization

Addition polymerization involves a rapid “chain reaction” of chemically activated monomers.
It occurs mainly in three stages which are initiation, propagation, and termination. For example, in the case of polyvinylchloride (PVC), the polymerization of vinyl chloride (monomer), initiation can come from a free radical generated on vinyl chloride monomer by initiator. Free radical can also act to initiate and terminate the reaction. This process generally produces linear structures but can produce network structures. A variety of initiators may be used, for example a peroxide or azide containing molecule can be activated by thermal or light. Atom-transfer radical-polymerization (ATRP), reversible addition  fragmentation chain transfer polymerization (RAFT), and group transfer polymerization GTP are examples of addition (chain growth) polymerization. Figure 3 shows a simple mechanism of an addition (chain growth) polymerization process from monomer to polymer.

Condensation (Step Growth or Stepwise) Polymerization

In condensation polymerization, individual chemical reactions between reactive functional groups of the monomer that occur one step at a time, are slower than addition polymerization. Whether the final product of the polymer will be linear or network depends on the number of functionality of the monomer (a functionality of two generally gives linear, whereas functionality of three gives network type/crosslink). Ring-opening polymerization (ROP) and polycondensation are examples of condensation (step growth or stepwise) polymerization processes. Figure 4 shows a simple process, in which phenol and formaldehyde form phenol formaldehyde resins (polymer) through a (step growth or stepwise) polymerization process.

There are many techniques available for polymerization, as given in Fig. 5. In addition, polymerization processes are also classified in bulk, suspension, solution, and emulsion.

Properties of Polymers

Structure of Polymer

A polymer structure can be classified into three possible molecular structures based on architecture which are linear polymer, branched polymer, and crosslinker polymer which is shown in Fig. 6.

(a) Linear polymer: A linear polymer consists of a long chain of monomers which are covalently bonded to each other. Some common examples for linear polymers are high density polyethylene (HDPE), PVC, nylon, polyester, and PAN etc.
(b) Branch polymer: A branched polymer has small branches covalently attached to the main chain. Some common examples are low density polythene (LDPE), glycogen, and starch.
(c) Crosslinker polymer: In cross-linked polymers, polymer chains are linked to each other through a covalent bond. Cross linking results in a giant macromolecule with a three-dimensional network structure. In elastomers, crosslink density is low or loosely bonded, while thermosets have high density crosslink networks, which make it hard, rigid, and brittle in nature. Bakelites and malamine formaldehyde resins are some example of Crosslinker polymer.

Another classification of polymers is based on the chemical type of the monomers used during polymerization process (Fig. 7). Homopolymers are made from the same monomers whereas copolymers are made of more than one different monomer repeating units. In addition, depending on the arrangement of the types of units in the polymer chain, they can also be classified as random, alternating, block, and graft polymers. In random copolymers two or more different monomer units are organized randomly in polymer chain, whereas in alternating copolymers repeating units of the differentmonomers are arranged in alternating sequences. In block copolymers a long series of the same monomer is followed by a long chain of another monomer. Graft copolymers consist of a polymer chain made from one type of monomer with branches which are made from another type or similar type of monomer.

Homopolymers can be classified based on tacticity into isotactic, syndiotactic, and atactic polymers (Figs. 8, 9, and 10).

Isotactic Polymer

In isotactic polymer all the substituents are located on the same side of the polymer backbone. Polypropylene synthesise using Ziegler-Natta catalysis is an isotactic polymer. Isotactic polymers are generally semicrystalline in nature, e.g., isotactic polystyrene.

Syndiotactic Polymer

In syndiotactic polymers the substituents are arranged in alternate positions along the polymer back bone. Polystyrene synthesized in the presence of metallocene catalysis during polymerization gives syndiotactic polystyrene which is crystalline in nature and has a melting point of about 161 C, e.g., syndiotactic polystyrene.

Atactic Polymer

When substituents are positioned randomly along the polymer chain they are known as atactic polymers. This kind of structure is generally formed by freeradical mechanisms. Polyvinylchloride and polystyrene are generally atactic in nature. Due to their random nature atactic polymers are usually amorphous.
Tacticity of the polymers is technically very important for application. For example polystyrene (PS) can exist in atactic or syndiotactic form and shows very different properties in the different structures. In atactic PS, polymer chains stack in an irregular fashion, cannot crystallize and form a glass, whereas syndiotactic PS is a semi crystalline material. This is a very general example for many polymers. Tacticity also affects other physical properties, such as melting temperature and solubility.
Besides tacticity another classification, based on head-tail configuration of vinyl polymers, should be taken into account when considering polymer defects (Fig. 11). In a head to tail configuration all monomers are normally linked in regular polymer units in which the substituent group on “β” position is separated by three carbon atoms whereas it is two and four carbon atoms for head to head and tail to tail configuration, respectively.

Generally nuclear magnetic resonance (NMR proton or C13 NMR), X-ray powder diffraction (XRD), secondary ion mass spectrometry (SIMS), and vibration spectroscopy (FTIR) characteristics techniques are used to measure the tacticity of the vinyl polymer.

Microscopic Structure

Properties of polymeric materials are very much affected by its microscopic arrangement of molecules or chains. Polymers can have an amorphous or partially crystalline/semicrystalline structure. Generally in amorphous polymers, molecules or chains are arranged in a random manner. In semi-crystalline polymer, molecular chains are partially aligned and chains fold together and form ordered regions known as lamellae which compose larger spheroidal structures named “spherulite”, an example of which is shown in Fig. 12. Formation of spherulite is controlled by many factors such as the number of nucleation sites, structure of the polymer molecules, cooling rate, etc. These factors control the diameter size of spherulite and may vary from a few micrometers to millimeters. Spherulites show higher density, hardness, and brittleness as compared to disordered polymers. The lamellae are connected by amorphous regions which provide certain elasticity and impact resistance. The degree of crystallinity is estimated by different analytical methods such as DSC, XRD, etc. Crystallinity of polymer typically ranges from 10 % to 80 %. That is why crystallized polymers are often called “semicrystalline”.
The properties of semicrystalline polymers are determined by the degree of crystallinity, size, and orientation of the molecular chains. Cooling rates, chain structure, and mer chemistry, side branching and chain regularity (isotactic or syndiotactic) are some of the factors which control the degree of crystallinity of the polymer. In general, crystallinity of the polymer increases with slow cooling rate, simple chain and mer structure, and less branching.
Polymer structure and intermolecular forces are two major reasons for high crystallinity or high amorpharsity of a polymer. For example if the polymer chain is regular and orderly, it will turn into crystals easily. If it is not, it will not. To better understand, let’s compare the structure of atactic in syndiotactic form of polystyrene when the R group is a phenyl ring (Fig. 13).

Figure 13 shows that the syndiotactic polystyrene is very orderly, with the phenyl groups falling on alternating sides of the chain. This means it can pack very easily into crystals. However, atactic styrene does not have such an order. The phenyl groups come on any side of the chain they want and the chains cannot fit for well packing which leads to highly amorphous character in the atactic polystyrene. Other atactic polymers like PMMA (poly(methyl methacrylate)) and PVC (poly (vinyl chloride)) are also amorphous. In general, stereo-regular polymers like isotactic PP (polypropylene) and polytetrafluoroethylene are highly crystalline.
PE is another good example and can be crystalline or amorphous. Linear PE is nearly 100 % crystalline where branched PE just cannot pack the way the linear PE can, so it is highly amorphous (Fig. 14).

Crystallinity and Intermolecular Forces

Intermolecular forces can play a major role in polymer crystallinity. For example, crystallinity of nylon is due to the internal force. It can be seen from Fig. 15, that the polar amide groups of nylon chains are strongly attracted to each other through strong hydrogen bonding. This strong binding results in crystalline behaviors of the nylon.
Another example of this is poly(ethylene terephthalate) also known as polyesters.
In this case polar ester groups of the poly(ethylene terephthalate) hold polyester into crystals. In addition pi-pi stacking of the phenyl ring is in orderly fashion, making the crystal even stronger (Fig. 16). Polypropylene, syndiotactic polystyrene, nylon, Kevlar, and Nomex are some examples of highly crystalline polymers whereas poly(methyl methacrylate), atactic polystyrene, and polycarbonate are some examples of highly amorphous polymers.
There is a way to find out how much of a polymer sample is amorphous or crystalline. Generally, differential scanning calorimetric (DSC) and X-ray diffraction (XRD) are the instruments used to determine the crystalline or amorphous property of the polymer. The crystallinity can affect the physical and thermal properties of polymers. For example, density, mechanical strength, heat resistance, and creep resistance increases with an increase in crystallinity.

Melting and Glass Transition Temperatures of Polymers

The glass transition is the reversible transition in polymeric materials from a hard and relatively brittle state into a molten or rubber-like state. The term melting temperature for polymers, suggests a transition from a crystalline or semicrystalline phase to a solid amorphous phase. Though the abbreviation of melting temperature is Tm, it should more precisely be called crystalline melting temperature. Crystalline melting is only discussed for crystalline or semi-crystalline polymers among synthetic polymers. Thermoset polymers are closely densely cross-linked in the form of a network, degrade upon heating, cannot be reused (e.g., crosslinked polyisoprene rubber); while thermoplastics, which do not contain cross-links, will melt upon heating, and can be recycled for reuse, e.g., polypropylene, polyethylene, PMMA, etc.
Glass transition is a phenomenon that occurs at a specific temperature known as glass transition temperature (Tg), when amorphous materials or amorphous regions within semi crystalline materials go from a hard and relatively brittle state into a rubbery like state or vice versa. This is a reversible phenomenon which very much depends on the nature of the polymer.
Tm and Tg usually characterize, respectively, the upper and lower temperature limits for applications of semi-crystalline polymers. Tg may also describe the upper use temperature for amorphous materials.
Tm and Tg, are much affected by molecular weight (MW) of polymers, presence of secondary bonding, chain flexibility/chain stiffness, density of branching, and thickness of the lamellae. The melting temperature of a polymer can be over a range of temperatures due to the variation of MW, and generally increases with increasing molecular weight (MW); whereas polar side groups, ether or amide linkages on the main chain, double bonds, aromatic groups, and crystallinity increase the melting temperature. Presence of the bulky, large size non polar groups, branching may lower Tm and Tg because it will decrease crystallinity thickness of the lamellae (crystallizing the solid at a low temperature or annealing just below Tm will do this) and increase the rate of heating.

Viscosity

Viscosity is an important property and one of the key issues of the polymer that needs to be considered during manufacturing materials, it is the measured resistance of the material which is being deformed by either shear stress or tensile stress. Viscosity very much depends on temperature. It is the proportionality constant between the shear stresses and the velocity gradient and can be represent as,

Shear stress(τ) = Viscosity(η) x Velocity gradient(dv/dy):

Figure 17 can state the viscosity (η) behavior of polymers in different various regions.

Mechanical Properties of Polymeric Materials

Viscoelasticity, as a property of materials, is a combination of viscous and elastic behavior. It is both time dependent and temperature dependent.
When a polymer is subjected to a step constant stress, polymeric materials experience a time-dependent increase in strain. This phenomenon is known as viscoelastic creep. It is temperature dependent and tests are conducted in the same manner as for metals.
Creep modulus is a parameter to quantify this behavior of polymeric materials.

Ec(t) = σo/ε(t),

where ε(t) is time dependent strain and σo is constant stress at a particular temperature.
Stress relaxation, which is also a result of viscoelasticity, describes how polymers relieve stress under constant strain, like viscoelastic creep, it is also time dependent. Relaxation modulus is a common parameter to quantify this behavior of polymeric materials and can be given as:

Er = σ(t)/εo,

where σ(t) = measured time dependent stress and εo is constant stress at a particular temperature.
Tensile modulus or elastic modulus or just “modulus” of polymer is identical to Young’s modulus for metals. The value of tensile modulus tends to be much lower for polymers compared to metals. In the case of semi-crystalline polymers the tensile modulus arises from the combination of the modulus of the crystalline and the amorphous regions. Similarly, tensile strength, impact strength, and fatigue strengths of polymers are defined in the same way as for metals. In general these values are much lower for polymers.
Ductility values are usually much higher for polymers than metals whereas fatigue curves are the same as for metals, and some polymers may or may not have fatigue limits. The values tend to be lower than for metals and very much affected by loading frequency. Tear strength which is related to the tensile strength, is the energy required to remove a cut specimen that has a standard geometry. Hardness of the polymer is determined by measuring the resistance to penetration, scratching, or marring the surface. Durometer and Barcol are common instruments used for hardness tests for polymers.
Polymer properties are very sensitive to temperature and generally with increasing temperature, tensile strength, elastic modulus decreases whereas ductility decreases. Besides temperature, the properties of polymer are very much affected by environment, e.g., moisture, oxygen, UV radiation, organic solvents, etc.

Deformation of Polymers

Elastic and plastic deformations of polymers are general properties that are experienced every day. Elastic deformations in thermoplastics, is similar to a metal spring which upon stretching shows uncoiling but returns to its original shape when the stretch force is released. In polymer, elastic deformation behavior comes from coil polymer chains which uncoil upon stretching, and the chains revert to their original conformations when forces are removed. This is a reversible process. During elastic deformation, primary bonds are being stretched but not broken.
Plastic deformations come from the chains moving past one another, secondary bonds are being broken and reformed. However, when enough force is applied, the primary covalent-bonds within the chains are also broken. It is not a reversible process like elastic deformation. A polymer chain containing a double bond or bulky group will restrict ability of the chain to rotate freely, making the material more rigid.
Typical stress-strain curves for the three different types of polymers are shown in Fig. 18.
Figure 18a, shows the properties of brittle polymer which fail during elastic deformation. Figure 18b shows rubber like elasticity property of polymer whereas Fig. 18c shows the typical stress-strain curves of plastic polymer.

Application of Polymers

Commodity Polymer

Initially polymers had major applications in manufacturing commodity goods (some of them are shown in Table 1) but with time they expanded to include engineering trough polymer matrix composites (PMCs), which will be discussed later.

Amphiphilic Block Copolymer

In addition to the above application, it has many other applications with a special kind of polymer known as amphiphilic block which is exploited in various fields. Amphiphilic block copolymers consist of distinct hydrophilic and hydrophobic segments which are able to form micelles in appropriate solvents. The formation of micelle depends on the nature of block, solvent, and concentration (Fig. 19). The size and shape of micelles depend on the chemical structure of block, molecular weight of each block, number of aggregation, and nature of solvent (Hadjichristidis et al. 2002; Raez et al. 2003; Yan et al. 2001a; Cao et al. 2002). Recent progresses in the synthetic techniques have led to the successful synthesis of a wide range of block copolymers containing different types of core and corona blocks with desired properties. Micelles of different shapes and sizes can be obtained. The aggregates can be in the form of rods, spheres or vesicles depending on various factors including the type of solvent and aggregation number (Antonietti et al. 1995; Zhang and Eisenberg 1995; Shen et al. 1999; Zhang et al. 2000; Alexandridis and Lindman 2000; Discher and Eisenberg 2002; Jain and Bates 2003; Wang et al. 2003; Liu et al. 2003).
Amphiphilic block copolymers are multipurpose useful materials. In recent years, block copolymers have found wide application in many areas. These are also used as a vehicle for controlling as well as targeting the release of vector agents, for both hydrophobic and hydrophilic (Gref et al. 1994; Allen et al. 1999). Thus these are exploited for applications in drug delivery (Qiu et al. 2009; Lavasanifar et al. 2002; Jones et al. 2003; Riley et al. 2003; Tang et al. 2003), tissue engineering, cosmetic, water treatment, and industrial waste treatment (Hadjichristidis et al. 2002). Biocompatible block copolymers such as PEO (Nojiri et al. 1990), polycaprolactone-b-poly(ethylene oxide), are particularly promising in the field of drug and gene delivery.
The synthesis of metal or metal oxide nanoparticles in block copolymer micelles has brought substantial interest as a result of their unique properties. Block copolymer micelles filled with nanoparticles have shown special catalytic (Seregina et al. 1997; Bronstein et al. 2005; Klingelho¨fer et al. 1997; Mayer and Mark 1997; Jaramillo et al. 2003), magnetic, electrical (Platonova et al. 1997; Rutnakornpituk et al. 2002), and optical (Diana et al. 2003) properties.
For block copolymers, the selectivity toward the core-forming block is important because of a physico-chemical interaction between the polymer core and the metal precursor that accelerates the metal incorporation into the micelle. A polymer block containing particular functional groups forming the miceller core can be loaded with some specific metal compounds. The micelles can often serve as a nanoreactor for nanoparticles formation. It may also play a role in stabilizing the nanoparticles as the core forming polymer block that can be considered to exist in a quasi-solid state (as the core forming block is insoluble in the selective solvent) (Bronstein et al. 2005).
Block copolymers are used as colloidal stabilizers to synthesize metallonanoparticle with controlled shape and size. They can provide an environment that can be used to prevent corrosion and leakage of heavy metals. They can also protect catalytic nanoparticles from deactivation (Antonietti et al. 1995; Mayer and Mark 1997; Fo¨rster and Antonietti 1998).
Using this concept, block copolymer micelles containing a polystyrene core and a functional corona have been used to produce gold nanoparticles (Mo¨ller et al. 1996). Gold (Au)-labeled micelles have been incorporated using di-blocks of poly(2-vinylpyridine) (P2VP) (Bronstein et al. 2005)/poly(4-vinylpyridine) (P4VP) (Sidorov et al. 2004) and poly(ethylene oxide) (PEO) in water. The size of gold nano-particles obtained is dependent on the nature and number of unit of the two blocks. For example the size of gold nanoparticle is 1–4 nm for P2VP135-b-PEO350 whereas gold nanoparticles formed from P4VP28-b-PEO45 ranged in size from 5 to 10 nm. The advantage of gold-labeled micelles with poly(ethylene oxide) PEO corona is that they allow for their preparation in water, an environmentally friendly medium. Zubarev et al. reported a stimulating approach for controlling the interfacial assembly of nanoparticles (e.g., gold nanoparticles) in self-assembled nanostructures of block copolymers, starting from gold nanoparticles covalently attached to V-shaped heteroarm chains (Zubarev et al. 2006). The supramolecular self-assembly of heteroarm star polymers leads to the precise location of gold nanoparticles at the core-shell interfaces of rod-like micelles or vesicles (Zubarev et al. 2006). Park and co-workers investigated the assembly of CdS or CdSe/ZnS quantum dots in vesicles or nanorods of PAA-based block copolymers (Sanchez-Gaytan et al. 2007). The application of block polymer is also reported for nanolithography for the development of biological performance of mineral oil, in biological and pharmaceutical applications (Spatz et al. 1999a; Loginova et al. 2004; Jeong et al. 1997; Otsuka et al. 2003). The synthesis of metal or semiconductor nanoparticles in the cores of amphiphilic block copolymer micelles in organic solvents was reported by many research groups (Antonietti et al. 1995; Moffitt et al. 1995; Saito et al. 1993). Using block copolymer micelle cores as nano reactors allows the synthesis of mono and bimetallic nanoparticles. The size of the nanoparticles, among other factors, depends on the particular reducing conditions as depicted in Fig. 20.

Different morphology of the nanoparticles significantly changes the catalytic properties of such systems, even though the size may be similar. The crosslinking of block copolymer micelles provides an additional degree of stabilization to nanoparticles and allows the modification of micelle morphology (Lu et al. 2001; Underhill and Liu 2000; Yan et al. 2001b). The possibility of using nanospheres as nanoreactors for inorganic nanoparticles was demonstrated by the formation of iron oxide magnetic particles (Yan et al. 2001b) and catalytic Pd nanoparticles (Lu et al. 2001). The spatial distribution of nanoparticles in block copolymers was also investigated by many researchers (Hadjichristidis et al. 2002; Antonietti et al. 1995; Jinnai et al. 2006). The selective separation of nanoparticles among blocks was achieved as a result of the presence of functional groups in a selected block (Bronstein et al. 2005; Sidorov et al. 2004). A large number of amphiphilic block copolymers forming micelles with a functionalized core in an organic medium are available but when aqueous solutions are favored, the choice of block copolymers is very limited. Here, nanoparticle formation is normally more complicated as the pH of the medium should be taken into consideration. A few examples of such block copolymers include poly(ethylene oxide)-blockpoly(2-vinylpyridine) (PEO-b-P2VP), polybutadiene-block-poly(ethyeleneoxide) (PB-b-PEO), polystyrene-block-poly(2-vinylpyridine)-block-poly(ethyelene oxide) (PS-b-P2VP-b-PEO), and poly-[methoxyhexa(ethylene glycol) methacrylate]-block-[2-(diethylamino)ethyl methacrylate] (PHEGMA-b-PDEAEMA). P2VP and PDEAEMA are examples of pH sensitive block as their core forming capability depends on pH (Spatz et al. 1999a). For example, at pH below 5, PEO-b-P2VP forms a molecular solution in water, whereas with a further decrease of pH, the PEO-b-P2VP forms a micellar solution. No micellar decomposition took place during incorporation of metal compounds or metal nanoparticle into micelle solution due to interaction with metal species (Bronstein et al. 1999). Morfit et al. reported the formation of spherical assemblies of CdS containing block copolymer using reverse micelles in aqueous solution (Moffitt et al. 1998). These stable assemblies were created by the slow addition of water into mixtures of the reverse micelles formed by polystyrene-b-poly(acrylic acid) diblock chains. This resulted in micelles containing quantum-confined CdS nanoparticles. This method allows the relocation of the CdS nanoparticles formed in the micelle cores in organic medium to aqueous medium without the loss of stability or nanoparticle aggregation (Moffitt et al. 1998).
Organization of nanocrystals has been demonstrated by taking advantage of block copolymer self-assembly (Chaurasia et al. 2011). The poly(styrene)-b-poly (2-vinyl-pyridine) (PS-b-P2VP) diblock copolymer was widely utilized as a template for synthesis of hexagonally arranged gold nanoparticles of controlled size (Mela et al. 2007; Spatz et al. 1999b). Non-spherical gold (Antonietti et al. 1996) and cobalt (Platonova et al. 1997) nanoparticles are prepared using poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer. However, synthesis of nanoparticles of various metal oxides, e.g., TiO2 (Li et al. 2005), SiO2 (Kim et al. 2004), and Fe2O3 (Bennett et al. 2005) with the help of various diblock copolymers have been reported. An attempt has been made to prepare organized ZnO nanoparticles using a Zn(CH2CH3)2 precursor in a PS-b-P2VP block copolymer. However, the procedures are difficult partly due to the highly reactive and moisture sensitive nature of the precursor, which was traditionally used for metal organic chemical vapor deposition (MOCVD) (Braun et al. 2010). Yoo et al. reported synthesis of ZnO nano-arrays using oxygen plasma treatment of PS-b-P4VP/zinc chloride film cast from toluene. ZnO nanoparticles with considerably larger particle size of about 16 nm diameter were obtained. Alok et al. reported a facile method for synthesis and organization of ZnO quantum dots (QDs) in various morphology using zinc acetate as precursor and PS-P4VP as a template (Chaurasia 2012).