7. Radical Chain Polymerization

The most commonly used peroxide is benzoyl peroxide 1, which undergoes thermal homolysis to form benzoyloxy radicals (Eq. 7.9). The benzoyloxy radicals may undergo a variety of reactions besides adding to monomer, including recombination (reverse of Eq. 7.9), decomposition to phenyl radicals and carbon dioxide (Eq. 7.10), and radical combination (Eqs. 7.11 and 7.12). These secondary reactions occur because of the confining effect of solvent molecules (cage effect), and as a result, the concentration of initiator radicals is depleted. Another “wastage” of initiator reaction is induced decomposition (Eq. 7.13).

Two other common peroxide initiators are diacetyl peroxide 2 and di-t-butyl peroxide 3.

Hydroperoxides such as cumyl hydroperoxide 4 decompose to form alkoxy and hydroxyl radicals (Eq. 7.14). Because hydroperoxides contain an active hydrogen atom, induced decomposition occurs readily, for example, by a chain-end radical (Eq. 7.15). Peroxy radicals may also combine with subsequent formation of oxygen (Eq. 7.16).

The extent of side reactions depends on the structure of peroxide, the stability of the initially formed radicals, and the reactivity of the monomers.

α, α′-Azobis(isobutyronitrile) 5, is the most widely used azo compound which decomposes at relatively low temperatures. The driving force for decomposition is the formation of nitrogen and the resonance-stabilized cyanopropyl radical (Eq. 7.17). The initially formed radicals can also combine in the solvent cage to deplete initiator concentration as with the peroxide decomposition. The combination of the radicals leads to both tetramethylsuccinonitrile 6 (Eq. 7.18) and the ketenimine 7 (Eq. 7.19).

The stability of thermal initiator is expressed by their half-life at different temperature as discussed below.

The thermal, homolytic dissociation of initiators is the most widely used mode of generating radicals to initiate polymerization for both commercial polymerization and theoretical studies. Polymerizations initiated in this manner are often referred to as thermal initiated or thermal catalyzed polymerizations. Thermal initiators are usually having dissociation energies in the range 100–170 kJ mol⁻¹. Compounds with higher or lower dissociation energies will dissociate too slowly or too rapidly. Only a few classes of compounds—including those with O–O, S–S, or N=N bonds—possess the desired range of dissociation energies. The peroxides are most extensively used as radical sources. Several common peroxy compounds are tabulated in Table 7.5.

The differences in the decomposition rates of various initiators are conveniently expressed in terms of the initiator half-life (\( t_{1/2} \)) defined as the time for the concentration of I to decrease to one half its original value. The rate of initiator disappearance iswith integration yieldsorwhere \( [I]_{\rm{0}} \) is the initiator concentration at the start of polymerization. \( t_{1/2} \) is obtained by setting \( [I] = \left[ I \right]_{\rm{0}} /2 \), then

Table 7.6 lists the initiator half-life for several common initiators at various temperatures. The peroxides are rather unstable. They should store in the refrigerator to prolong their shelf life. Azobis (isobutyronitrile), benzoyl peroxide, and acetyl peroxide are decomposed at relatively low temperature, so they usually add into monomer right before the polymerization to avoid any premature decomposition and polymerization. t-Butyl peroxide and t-butyl hydroperoxide are decomposed at high temperature. They can be mixed with monomer and stored in room temperature without premature reaction for more than 3 months.

Decomposition of peroxides can be induced at lower temperatures by the addition of promoters. For example, addition of N, N-dimethylaniline to benzoyl peroxide causes the latter to decompose rapidly at room temperature. Kinetics studies indicate that the decomposition involves formation of an unstable ionic intermediate (Eq. 7.23) that reacts further to give benzoyloxy radical and a radical cation (Eq. 7.24).

The redox initiators are useful in initiation of low temperature polymerization and emulsion polymerization. Reaction rates are easy to control by varying the concentration of metal ion or peroxide. For nonaqueous polymerization, metal ions: Co²⁺, Mn²⁺, Cu²⁺, and Fe²⁺ are generally introduced as the naphthenates shown in below. Cobalt naphthenate is commonly used as unsaturated polyester (Alkyd resin) drying agents for the autoxidative crosslinking of the double bond.

Some typical examples of redox initiators are given in Eqs. 7.25–7.27. They have been used in the emulsion copolymerization of styrene and butadiene to form styrene—butadiene rubber in low temperature. Hydrogen peroxide or persulfate systems, Eqs. 7.26 and 7.27, are used in the emulsion polymerization. They can produce radicals in an aqueous phase.

Peroxides and azo compounds dissociate photolytically as well as thermally. The major advantage of photoinitiation is that the reaction is essentially independent of temperature. Furthermore, better control of the polymerization reaction is generally possible because narrow wavelength bands may be used to initiate decomposition (less side reactions), and the reaction can be stopped simply by removing the light source. The reaction is fast (s vs. min) as compared to the thermal initiation. A wide variety of photolabile compounds are available, including disulfides (Eq. 7.28), benzoin 8 (Eq. 7.29), and benzyl 9 (Eq. 7.30). Table 7.7 lists some commercial available photoinitiators and their decomposition wavelength. It is interesting to note that most of the initiators contain benzoyl group which can form stable benzoyl radical through the resonance stabilization of phenyl ring.

Electrolysis of a solution containing both monomer and electrolyte can be used to initiate polymerization. At the cathode an electron may be transferred to a monomer molecule to form a radical anion (Eq. 7.31), and at the anode a monomer molecule may give up an electron to form a radical cation (Eq. 7.32). The radical ions in turn initiate free radical or ionic polymerization or both, depending on electrolysis conditions.

Like suspension, solution polymerization allows efficient heat transfer. Solvent must be chosen carefully to avoid chain transfer reactions that may limit the growth of molecular weight of polymer. Apart from the environmental concerns associated with organic solvents, a major problem in solution polymerization is that it is often difficult to remove solvent. As a result, supercritical carbon dioxide has been used to do the polymerization as a solvent. However, it is only suitable for polar monomers such as acrylates; there is need for more development work for other monomers.

Emulsion polymerization and suspension polymerization are two water-based heterogeneous polymerizations that are used extensively to control the thermal, viscosity, and environmental problems of polymer industry. However, they are quite different in the components and behaviors in the polymerization. The comparison between the two polymerization processes is summarized in Table 7.9. The major difference between the two polymerizations is the reaction site, micelle for emulsion polymerization, and monomer for suspension polymerization.

When the concentration of a surfactant exceeds its critical micelle concentration (CMC), the excess surfactant molecules aggregate together to form small colloidal clusters called micelles. Typical micelles have dimensions of 2–10 nm, with each micelle containing 50–150 surfactant molecules. The largest portion of the monomer (>95 %) is dispersed as monomer droplets whose size depends on the stirring rate. Monomer droplets have diameters in the range 1–100 μm. Thus, in a typical emulsion polymerization system, the monomer droplets are much larger than the monomer-containing micelles. Consequently, while the concentration of micelles is 10¹⁹–10²¹ L⁻¹, the concentration of monomer droplets is at most 10¹²–10¹⁴ L⁻¹. A further difference between micelles and monomer droplets is that the total surface area of the micelles is larger than that of the droplets by more than two orders of magnitude.

Polymerization takes place exclusively in micelle. Monomer droplets do not compete effectively with micelles in capturing radicals produced in solution because of the much smaller total surface area of the droplets. The micelles act as a meeting place for the organic (oil soluble) monomer and the water-soluble initiator. The micelles are favored as the reaction site because of their high monomer concentration compared to the monomer in solution. As polymerization proceeds, the micelles grow by the addition of monomer from the aqueous solution whose concentration is replenished by dissolution of monomer from the monomer droplets. A simplified schematic representation of an emulsion polymerization system is shown in Fig. 7.1. The system consists of three types of particles: monomer droplets, inactive micelles in which polymerization is not occurring, and active micelles in which polymerization are occurring. The latter are no longer considered as micelles but are referred to as polymer particles. An emulsifier molecule is shown as ○— to indicate one end (○) is polar or ionic and the other end (—) is nonpolar.

The rate of polymerization equation can be modified to include the initiator efficiency (f) as shown in Eq. 7.53. The equation indicates the polymerization rate is proportional to the square root of initiator concentration and to the first power of monomer concentration. Thus, doubling the initiator concentration causes the rate to increase by a factor of about 1.4. This relationship has been confirmed experimentally for a variety of free radical polymerizations. Propagation and termination rate constants, for several commercially important monomers are given in Table 7.10. The tetrafluoroethylene has an extremely low \( k_{t} \) which results in higher molecular weight polymer as compared with the polymerization of ethylene.

Another important parameter related to polymerization rate is the average kinetic chain length, \( \overline{\nu } \), which is defined as the average number of monomer units polymerized per chain initiated, which is equal to the rate of polymerization per rate of initiation. Since \( R_{i} = R_{t} \) under steady-state conditions,Substituting for \( R_{p} \) and \( R_{t} \) into Eq. 7.54:Substituting the expression for \( [M \cdot ] \) from Eq. 7.51,

Kinetic chain length is related to a variety of rate and concentration parameters. It will decrease as both of initiator concentration and initiator efficiency increase. This is reasonable because increasing the number of growing chains increases the probability of termination. Thus, varying initiator concentration can control molecular weight. In the absence of any side reactions, kinetic chain length is related directly to degree of polymerization depending on the mode of termination. If termination occurs exclusively by disproportionation, \( \overline{DP} = \overline{\nu } \); if it occurs by coupling, \( \overline{DP} = 2\overline{\nu } \).

Autoacceleration, a marked increase in polymerization rate, can occur at very viscous medium even though the chain mobility is reduced and the termination is reduced. The small monomer molecules can still diffuse to the active chain ends to be polymerized. Autoacceleration may cause processing difficulties, particularly in bulk polymerizations, because the increase in rate is usually accompanied by an increase in reaction exotherm and gelation of polymer.

Chain transfer reactions are the process transferring the growing polymer chain to another species which terminates the chain, but at the same time generates a new radical. The chain transfer reactions result in low molecular weight polymer and broad molecular weight distributions. Chain transfer can occur among polymer, monomer, initiator, solvent in the reaction mixture. Equations 7.57 and 7.58 show the polymer inter- and intrachain transfer reactions, respectively. A chain-end radical may abstract a hydrogen atom from a chain, leading to a reactive site for chain branching (Eq. 7.57).

Hydrogen abstraction may also occur intramolecularly, a process referred to as backbiting. Polyethylene, produced by high-pressure free radical polymerization of ethylene, is highly branched through backbiting involving five- or six-membered cyclic transition states (Eq. 7.58). The properties of branched polymers are expected to differ remarkably from those of the corresponding linear polymers as we discussed in Chaps.​ 3 and 4.

Chain transfer may also occur with initiator (Eq. 7.59) or monomer (Eq. 7.60), or it may take place with solvent. Polystyrene prepared in carbon tetrachloride, for example, contains chlorine at the chain end of polymer due to chlorine transfer (Eq. 7.61) and initiation by the resultant ·CCl₃ radicals (Eq. 7.62). Transfer to monomer is particularly important with monomers containing allylic hydrogen, such as propylene, because the formation of resonance-stabilized allylic radicals (Eq. 7.63) is highly favorable. Thus, high-molecular-weight polypropylene cannot be prepared using conventional free radical polymerization described here. The coordination polymerization is used to obtain high molecular weight polypropylene (detailed in Chap.​ 9).

From the molecular weight control point of view, the chain transfer agent plays the most effective way. For instance, a thiol compound has been used widely as chain transfer agent due to its high affinity for hydrogen transfer as

When a chain transfer agent is used in polymerization, it is necessary to redefine the kinetic chain length as being the ratio of propagation rate to the combined rates of termination and transfer, as

Since transfer reactions are second order, withwhere \( T \) is the transfer agent, one can rewrite the expression for \( \overline{\nu }_{tr} \), taking into account all possible transfer reactions, as

It is known that

One can write the reciprocal of Eq. 7.67 for \( \overline{\nu }_{tr} \) as

The ratio of the transfer rate constant to that of propagation is commonly defined as the chain transfer constant, \( C_{T} \), for a particular monomer:

Substituting Eq. 7.69 into Eq. 7.68, one obtains

As the rate of transfer and concentration of the transfer agent increase, the kinetic chain length becomes progressively smaller. Chain transfer constants for a number of compounds and monomers are available in the polymer literature, several of which are given in Table 7.11.

When the concentration of transfer agent is high and \( k_{tr} \) is much greater than \( k_{p} \), very low molecular weight polymers called telomers are obtained. The process is called telomerization. Chain transfer reactions can also be used to prevent free radical polymerizations. One type of compound, added as a stabilizer to vinyl monomers, is an alkylated phenol 10, which can transfer its phenolic hydrogen to form a new radical (Eq. 7.71) that undergoes coupling reaction (Eq. 7.72) rather than initiating polymerization. Such compounds, called inhibitors, are commonly added to monomers to prevent premature polymerization during shipment or storage.

The living radical polymerization is also called controlled radical polymerization. The formal IUPAC name of this type of polymerization is called reversible deactivation radical polymerization (RDRP) [6]. The lifetime in the conventional radical polymerization is very short (in second) because of the presence of bimolecular termination. The living radical polymerization can be achieved by minimizing normal bimolecular termination and prolonging the lifetime of living polymers. Special mode of reaction for the propagating radicals by either reversible termination or reversible transfer has been developed [3]. Living radical polymerization (LRP) with reversible termination generally proceeds as follows:

The initiator RZ undergoes homolytic bond breakage to produce one reactive and one stable free radical (Eq. 7.74). The reactive radicals quickly initiate polymerization (Eq. 7.75), but the stable radicals are too stable to initiate polymerization. However, the stable radicals preserve the propagation chain being living without any termination (Eq. 7.76). For living polymerization, it is important to have all the initiator decomposes at once so that all propagating radicals grow almost at the same time. Fast initiation is important, but it is the fast equilibrium between the propagating radical and dormant species with an appropriate equilibrium constant that determines the living characteristics of polymerization. The equilibrium constant must be low but not too low; that is, the concentration of propagating radical must be sufficient to achieve a reasonable propagation rate but not so high that normal bimolecular termination becomes important.

At the beginning of the reaction, the concentrations of propagating and stable radicals are equal, but they change rapidly with the progress of reactions. The concentrations of stable radicals are increasing while the concentrations of propagating radicals are decreasing because the reactions are shifted toward more stable species as shown in Eqs. 7.74 and 7.76. Overall, the concentration of stable radicals is about four orders higher than that of propagating radicals. The concentration of propagating radicals is about the same or lower than the conventional radical polymerization. The stable radicals function as controlling agent to form reversible dormant species with propagating radicals. The equilibrium favors the dormant species by several orders of magnitude as compared with the propagating radicals. Thus, the concentration of dormant species is about six orders higher than that of propagating radical. In short summary, the introduction of the dormant species in the radical polymerization suppresses the bimolecular termination reaction to have a living polymer. The average life time of the living polymer has been increased by at least four orders of magnitude. Thus, the living free radical polymerization becomes possible due to the formation of stable radicals and reversible dormant species. By sequential living polymerization of one kind of monomer to another kind of monomer, well-defined block copolymers can be obtained through living polymerization. The second type of monomers usually has to add quickly after the first kind monomers are consumed to avoid any bimolecular reactions among propagating radicals. Currently, reversible termination reactions and reversible chain transfer reactions are employed in the living free radical polymerizations. They are discussed below.

The reactions of atom transfer radical polymerization (ATRP), involves an atom transfer, usually a halogen atom, to form propagating radicals and dormant species from the complex of atom transfer agent and copper (I) catalyst [6]. For instance, the polymerization of styrene using 1-chloro-1-phenylethane (11, or the bromo analog) as initiator in the presence of a copper (I) bipyridyl (bpy) complex. Initiation occurs when a halogen atom is transferred from 11 to the complex (Eq. 7.77). The resultant 1-phenylethyl radical in turn adds to a styrene molecule (Eq. 7.78). The halogen atom is then reversibly transferred to the styryl radical (Eq. 7.79), thus preventing radical termination reactions from occurring while allowing the propagation reaction to proceed.

Stable nitroxide radical such as TEMPO 12 can mediate polymerization as shown in Eq. 7.80. The TEMPO is too stable to initiate polymerization, but it does promote decomposition of benzoyl peroxide to benzoyloxy radicals which initiate chain growth. Then TEMPO combines reversibly with the growing polymer chain ends which protect the chain end radicals from termination. Dissociation in turn frees the chain end radical to add to other styrene molecules.

ATRP and NMP control chain growth by reversible termination. RAFT living polymerizations control chain growth through reversible chain transfer. A chain-transfer agent R’CSSR, such as cumyl dithiobenzoate 13 reversibly transfer a labile end group (a dithioester end group) to a propagating chain (Eq. 7.81). The polymerization is carried out with a conventional initiator such as a peroxide or AIBN in the presence of the chain-transfer agent. The key that makes RAFT a living polymerization is the choice of the RAFT transfer agent. Living polymerization occurs with dithioesters because the transferred end group in the polymeric dithioester is as labile as the dithioester group in R’CSSR. This results in an equilibrium between dormant polymer chains and propagating radicals (Eq. 7.82 with K = 1), which controls the living polymerization.

RAFT works with a wider range of monomers than TEMPO and ATRP. RAFT does not produce polymers with metal catalysts but produce polymers with dithioester group with odors and colors. The design and synthesis of RAFT agent has been thoroughly reviewed and outlined. The methods are extended to functional RAFT agent and macro-RAFT agent [7].

The limitations of living radical polymerization are that irreversible bimolecular termination of propagating radicals will occur at high monomer conversion, polyfunctional initiators, high initiator concentration, and high target molecular weight (>100,000).

The quantitative effect of temperature on rate of polymerization (\( R_{p} \)) and degree of polymerization (\( \overline{X}_{n} ) \) is very complex since they depend on a combination of three rate constants of initiation (\( k_{d} ) \), propagation (\( k_{p} ) \) and termination (\( k_{t} ) \). Each of the rate constants can be expressed by an Arrhenius equation orwhere A is the collision frequency factor, E the Arrhenius activation energy, and T the kelvin temperature. A plot of ln k versus 1/T allows the determination of both E and A from the slope and intercept, respectively. Table 7.13 shows the values of the frequency factor and activation energy of propagation (\( E_{p} \)) and termination (\( E_{t} \)) of several monomers.

The variations in the values of the frequency factor of propagation (\( A_{p} \)) are greater than those in \( E_{p} \) which indicates that steric effects are probably the more important factor to determine the absolute value of \( k_{p} \). Thus, the more hindered monomers such as methyl methacrylate have lower \( k_{p} \) and \( A_{p} \) values than the less hindered ones such as methyl acrylate. The \( A_{p} \) values in general are lower than the usual value (10¹¹–10¹³) of the frequency factor of a bimolecular reaction. The result is probably due to a large decrease in entropy on polymerization. The variations in the values of the frequency factor of termination (\( A_{t} \)), generally follow the trend of \( A_{p} \) values, but with larger values.

For a polymerization initiated by the thermal decomposition of an initiator, the polymerization rate depends on the ratio of three rate constants \( k_{p} (k_{d} /k_{t} )^{1/2} \) according to Eq. 7.53. The temperature dependence of this ratio can be obtained by combining three separate Arrhenius equations as below

The overall activation energy of rate of polymerization \( E_{R} \) is equal to \( E_{p} + \left( {E_{d} /2} \right) - \left( {E_{t} /2} \right) \). From Eq. 7.53, one can rewrite the above equation as

By plotting ln \( R_{p} \) versus 1/T, one can obtain \( E_{R} \) and \( A_{p} \left( {A_{d} /A_{t} } \right)^{1/2} \) from the slope and intercept, respectively.

Table 7.14 shows the thermal properties of commonly used initiators. The activation energy of initiator decomposition is in the range of 120–150 kJ mole⁻¹ in general. The \( E_{p} \) and \( E_{t} \) value of most monomers are in the ranges of 20–40 kJ mole⁻¹ and 8–20 kJ mole⁻¹ respectively (Table 7.13). Thus the \( E_{R} \) of most polymerization initiated by thermal initiator decomposition is about 80–90 kJ mole⁻¹. This corresponds to a two to three time rate increase for every 10°C temperature increase. The situation is different for other mode of initiation. The redox initiation (e.g., Fe²⁺ with thiosulfate or cumene hydroperoxide) usually takes place at lower temperature as compared to the thermal polymerizations because the former has lower activation energy. The \( E_{d} \) value of redox initiation is only about 40–60 kJ mole⁻¹ or about 80 kJ mole⁻¹ less than that of thermal initiation. This leads to an \( E_{R} \) redox polymerization about 40 kJ mole⁻¹ or about half of the nonredox initiators.

The initiation step of photo polymerization is temperature independent (\( E_{d} = 0 \)), because the decomposition of initiator is induced by light rather than by heat. The overall activation energy of photo polymerization is then only about 20 kJ mole⁻¹ which indicates this polymerization is relatively insensitive to temperature. However, most photo initiators can also be decomposed thermally; the initiators may undergo appreciable thermal decomposition in addition to photo decomposition at higher temperatures. In such cases, one must take into account both the thermal and photo initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene, its \( {E}_{d} \) is 121 kJ mole⁻¹ and \( {E}_{R} \) is 86 kJ mole⁻¹. However, pure thermal polymerization proceeds at very slow rates because of very low frequency factor (10⁴–10⁶) resulting in very low probability of initiation process.

The effect of temperature on the molecular weight of the polymer produced by thermally initiated polymerization will be determined by the ratio of \( k_{p} /\left( {k_{d} /k_{t} } \right)^{1/2} \) according to Eq. 7.56 of the degree of polymerization, if transfer reactions are negligible. The variation of the ratio with temperature can be expressed by

The overall activation energy of degree of polymerization (\( E_{{\overline{X}_{n} }} ) \) is equal to \( \left[ {E_{p} - \left( {E_{d} /2} \right) - \left( {E_{t} /2} \right)} \right] \). Then the temperature effect on the degree of polymerization can be expressed by

\( E_{{\overline{X}_{n} }} \) has a value of about −60 kJ mole⁻¹ and \( \overline{X}_{n} \) decreases rapidly with increasing temperature. \( E_{{\overline{X}_{n} }} \) is about the same for a purely thermal, self-initiated polymerization (Fig. 7.2). For a pure photo polymerization, \( E_{{\overline{X}_{n} }} \) is positive by approximately 20 kJ mole⁻¹, since \( E_{d} \) is zero, and \( \overline{X}_{n} \) increases moderately with temperature. For all other cases, \( \overline{X}_{n} \) decreases with temperature.

When chain transfer occurs in the polymerization, the temperature effect on the \( \overline{X}_{n} \) depends on the relative importance of the chain transfer reaction. For the case where chain transfer to chain transfer agent T, one can obtain the following equation according to Eq. 7.70.

Now, \( E_{{\overline{X}_{n} }} \) is equal to (\( E_{p} - E_{tr} \)) and can be obtained by plotting the left side of equation versus 1/T. The (\( E_{p} - E_{tr} \)) is usually in the range of −20 to −65 kJ more⁻¹ and the molecular weight decreases with increasing temperature. Table 7.15 shows the activation parameters for chain transfer in styrene polymerization using different chain transfer agents. The frequency factors for transfer reactions are usually greater than those of propagations.

Although the polymerization reaction is thermodynamically favored, the kinetic feasibility of polymerization varies considerably from one monomer to another depending on the stability of monomer radical and polymer radical [3].

As shown in Table 7.16, the ΔH values for ethylene, propene, and 1-butene are very close to the difference (82–90 kJ/mole) between the bond energies of the π-bond in an alkene and the σ-bond in an alkane. The ΔH values for the other monomers vary considerably. The variations in ΔH for differently substituted ethylenes arise from any of the following effects: (1) differences in the resonance stabilization of monomer and polymer due to differences in conjugation or hyperconjugation, (2) steric strain differences in the monomer and polymer arising from bond angle deformation, bond stretching, or interactions between nonbonded atoms, and (3) differences in hydrogen bonding or dipole interactions in the monomer and polymer. The following monomers (I) can be stabilized through resonance that decreases ∆H of polymerization. However, the monomer (II) is poorly conjugated and its ∆H is similar to that of ethylene.

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus, hyperconjugation of alkyl groups with the C=C lowers ΔH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and α-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the ΔH is close to the value for ethylene.

The effect of 1,1-disubstitution manifests itself by decreased ΔH values [3]. This is a consequence of steric strain in the polymer due to interactions between substituents on alternating carbon atoms of the polymer chain. In 25, the polymer chain is drawn in the plane of the text with the H and Y substituents placed above and below the plane of the text. The dotted and triangular lines indicate substituents below and above this plane, respectively. Such interactions are referred to as 1,3-interactions and are responsible for the decreased ΔH values in monomers such as isobutylene, α-methyl styrene, methyl methacrylate, and vinyl chloride. The effect in α-methyl styrene is especially significant. The ΔH value of −35 kJ/mole is the smallest heat of polymerization of any monomer. To summarize, the stability of the polymer radical is more important than the stability of the monomer toward addition of a free radical in the propagation reactions.

The reversibility of vinyl polymerization reactions has been observed which depends on the monomer reactivity and the stability of polymer radical as shown in the following:where \( k_{dp} \) is the depropagation rate constant. Just as radical stability retards propagation, it enhances depropagation. As the polymerization temperature is raised, the depropagation rate increases until a point is reached where the forward and back reactions are equal. The temperature at which this occurs is called the ceiling temperature (T _c ), and at that temperature ΔG of polymerization is zero, or

It follows that whatever affects ΔH or ΔS for a given monomer (e.g., monomer concentration, pressure) will also affect T _c . Table 7.17 lists ceiling temperatures for several pure liquid monomers.

While the ΔH values vary over a wide range for different monomers, the ∆S values are less sensitive to monomer structure, being relatively constant within the range of ~100–120 J/K-mole. The T∆S contribution to the ΔG of polymerization will be small and will vary only within a narrow range. Thus, the variation in the T∆S term at 50°C for all monomers is in the narrow range of 30–40 kJ/mole. The ∆S of polymerization arises primarily from the loss of the translation entropy of the monomer. Losses in the rotational and vibration entropies of the monomers are essentially balanced by gains in the rotation and vibration entropies of the polymer. Thus, ΔS for polymerization is essentially the translational entropy of the monomer, which is relatively insensitive to the structure of the monomer.

Radical chain polymerization of ethylene to polyethylene is carried out at high pressures of 120–300 MPa and at temperature above the \( T_{m} \) of polyethylene. Batch processes are not useful since the long residence time gives relatively poor control of product properties. Long-chain branching due to intermolecular chain transfer becomes excessive with deleterious effects on the physical properties. Continuous processes allow better control of the polymerization.

The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts. The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene . Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40–60 % vs. 70–90 %) and density (0.91–0.93 g cm⁻³ vs. 0.94–0.96 g cm⁻³).

Low-density polyethylene (LDPE) has a wide range and combination of desirable properties. Its very low \( T_{\text{g}} \) of about −120°C and moderately crystallinity and \( T_{\text{m}} \) of 105–110°C, give it flexibility and utility over a wide temperature range. Commercial low-density polyethylenes have number-average molecular weights in the range of 20,000–100,000, with \( \overline{X}_{w} /\overline{X}_{n} \) in the range of 3–20.

Film application accounts for over 60 % of polyethylene consumption. Injection molding of toys, housewares, paint-can lids, and containers accounts for another 10–15 %. About 5 % of the LDPE produced is used as electrical wire and cable insulation. Extrusion coating of paper to produce milk, juice, and other food cartons and multiwall bags accounts for another 10 %. Trade names for polyethylene include marlex, nipolon, etc.

Continuous solution polymerization is the most important method for the commercial production of polystyrene although suspension polymerization is also used. Emulsion polymerization is important for the synthesis of copolymer of acrylonitrile butadiene styrene(ABS). Commercial polystyrene (PS) has number-average molecular weights in the range of 50,000–150,000 with \( \overline{X}_{w} /\overline{X}_{n} \) values of 2–4. Although completely amorphous (\( T_{\text{g}} \) = 85°C), its bulky rigid chains (due to phenyl–phenyl interactions) impart good strength with high-dimensional stability (only 1–3 % elongation); PS is a typical rigid plastic. Expandable polystyrene, either crystal polystyrene or styrene copolymers impregnated with a blowing agent, e.g., pentane, is used to produce various foam products such as disposable drinking cap, egg cartons, etc.

Most poly(vinyl chloride) (PVC) is commercially produced by suspension polymerization. A typical polymerization includes 180 parts water, and 100 parts vinyl chloride monomer, chain transfer agent (trichloro ethylene). The reactants are then heated in the closed system to about 50°C, and the pressure rises to about 0.5 MPa. Typical number-average molecular weights for commercial PVC are in the range of 30,000–80,000. Tough and rigid PVC became flexible by using plasticizers (e.g., di-i-octylphthalate, tritolyl phosphate, epoxidized oils). Rigid pipe is used for home and construction, vinyl siding, window frames, rain-gutter, packaging, and gloves.

Polyvinyl acetate (PVAC) [–CH₂–C(H)(O–CO–CH₃)–]_n is synthesized from vinyl acetate monomer via emulsion or solution polymerization. The \( T_{\text{g}} \) of PVAC is about 28°C which is not very useful, but widely used in water based paints, adhesives for papers, textiles, and wood. PVAC is also used for producing polyvinylalcohol (PVA) by hydrolysis that is used as thickening agents, coatings, and adhesives.

Polyvinylidene chloride (–CH₂–CCl₂–)_n and its copolymer with vinyl chloride, acrylonitrile, and acrylates, usually prepared by suspension or emulsion polymerization. They are useful as oil, fat, oxygen and moisture-resistant packaging films (Saran wrap), container, coatings, tank liners, and monofilaments in drapery fabrics and industrial filter cloths.

Poly(methyl methacrylate) (PMMA) can be polymerized by bulk, solution, suspension, and emulsion processes. PMMA is completely amorphous but has high strength and excellent dimensional stability due to the rigid polymer chains (\( T_{\text{g}} \) = 105°C). Polyacrylates differ considerably from the polymethacrylates. They are softer since the polymer chains are not nearly as rigid [e.g., poly(ethyl acrylate) (PEA), \( T_{\text{g}} \) = −24°C], due to the absence of the methyl groups on alternating carbons of the polymer chains.

Various rigid PMMA products, such as sheet, rod, and tube, are produced by bulk polymerization in a casting process. Polymerization is carried out in stages to allow easier dissipation of heat and control of product dimensions since there is a very large (21 %) volume contraction on polymerization. Partially polymerized monomer like syrup (about 20 % conversion) is produced by heating for about 10 min at 90°C with a peroxide. The syrup is cooled to ambient temperature and poured into a mold, and then the assembly is heated in a water or air bath to progressively higher temperatures. The maximum temperature used is 90°C, since higher temperatures can lead to bubble formation in the product as the boiling point of methyl methacrylate is 100.5°C. Trade name for acrylate and methacrylate polymer products includes Acrylite, Plexiglas, Lucite, etc.

In commercial production, polyacrylonitrile ([–C–C(CN)–]_n) (PAN) is synthesized by either solution or suspension polymerization. Polyacrylonitrile contains polar group to have high secondary force, so it exhibits good fiber property. PAN is a source of carbon fiber which is produced by cyclization of polymer at 2,000°C or above.

From the viewpoint of sales volume, all other members of the acrylic family constitute a small fraction of the total. However, many of them are useful specialty products. Polyacrylamide, poly(acrylic acid), and poly(methacrylic acid) and some of their copolymers are used in various applications that take advantage of their solubility in water. Poly(acrylic acid) and poly(methacrylic acid) are used as thickening agents, adhesives, dispersants for inorganic pigments in paints, flocculants, and crosslinked ion-exchange resins.

The fluoropolymers are obtained mainly by suspension polymerization; emulsion polymerization is also practiced. The most important members of this family are poly(tetrafluoro ethylene) (PTFE), poly(chloro trifluoro ethylene) (PCTFE), poly (vinyl fluoride) (PVF), and poly(vinylidene fluoride) (PVDF). High molecular weight PTFE is usually obtained due to lack of chain-transfer reactions and precipitation of growing radicals (leading to greatly decreased termination). Fluoropolymers can withstand a wide variety of chemical environments and are useful at temperatures as low as −200°C and as high as 260°C. The various copolymers of tetrafluoro ethylene and the other fluoropolymers, possessing lower T _m and crystallinity, were developed to overcome the lack of melt processability of PTFE. A true melt processable PTFE has been developed by blending two different molecular weight polymers [8]. They are DuPont Zonyl MP 1000 and Zonyl MP 1,500 J at a specific mixing window of weight ratio of 40/60 and 20/80, respectively. The PVDF exhibits novel piezoelectric properties which are useful for the applications of actuators, artificial arms/legs, and so on.

The cost of common polymers depends on the ease of synthesis and raw material cost. Fluoropolymer is usually a high cost polymer compared to other polymers. The cost comparison is in the increasing order of PE~PS < phenolic alkyds < PMMA < Nylon < PTFE. The raw materials used for polymer synthesis are derived from petroleum, with the increase in petroleum price and possible depletion someday. Raw materials obtained from renewable source such as plants or biomolecules are currently under intensive development. For instance, polylactic acid can be obtained from bacterial fermentation of corn starch. It is biodegradable and has been used in disposable cups and plates.