Chapter 2　Polymer Chemistry

2.1　Rubber Polymerization

This chapter is intended to provide a general introduction to the industrial techniques used in rubber synthesis， by focusing on some relatively well known polymerizations that occur by chain-growth processes.

For rubber monomers， the polymerization process is classified by the way in which polymerization is initiated and thus the nature of the propagating chain， namely emulsion polymerization， solution polymerization and coordination polymerization etc.

2.1.1　Solution Polymerization

Polymerization method which monomer and initiator dissolving in appropriate solvents are called solution polymerization. It mainly includes the free radicals and ion solution polymerization.

There are many advantages for solution polymerization compared with bulk polymerization，such as low viscosity， easy mixing and heat transfer， high temperature control， less gel effect and no local overheating.

The solvent is crucial for this process.The choice of solvent is based on two principles because solvents have an effect on polymerization activity， the dissolution performance of polymer and gel effect.

This is perhaps the most well-known method of rubber polymerization. In the common synthetic rubbers， many kinds of rubber are synthesized in this way， such as polybutadiene rubber， polyisoprene rubber， styrene-butadiene rubber， polyacrylate rubber.

2.1.2　Emulsion Polymerization

Emulsion polymerization is frequently used for rubber synthesis. A variety of synthetic rubbers， such as styrene-butadiene， nitrile rubber， chloroprene rubber adopt emulsion polymerization method.

Mechanism and kinetics polymerization in aqueous emulsions， which has been widely developed technologically， represents a special case of free radical chain polymerization in a heterogeneous system. Most emulsion polymerization systems comprise a water-insoluble monomer in water with a surfactant and a free radical initiator. Although it might be thought that polymerization of water insoluble monomers in an emulsified state simply involves the direct transformation of a dispersion of monomer into a dispersion of polymer， this is not really the case， as evidenced by the following features of a true emulsion polymerization.The polymer emulsion（or latex） has a much smaller particle size than the emulsified monomer， by several orders of magnitude. The polymerization rate is much faster than that of the undiluted monomer， by one or two orders of magnitude. The molecular weight of the emulsion polymer is much greater than that obtained from bulk polymerization， by one or two orders of magnitude.

The mechanism of emulsion polymerization can best be understood by examining the components of this system， as depicted in Figure 2.1， for a typical “water-insoluble” monomer such as styrene（solubility=0.07g/L）.Figure 2.1 shows the various loci in which monomer is found， and which compete with each other for the available free radicals. Thus， in the initial stages， the monomer is found in three loci： dissolved in aqueous solution， as emulsified droplets， and within the soap micelles. Both the dissolved monomer and the relatively large monomer droplets represent minor loci for reaction with the initiator radicals（except， of course， in the case of highly water-soluble monomers）. The large number of soap micelles containing imbibed monomer， however， represents a statistically important locus for initiation of polymerization. It is thus not surprising that most of the polymer chains are generated within the monomer-swollen soap micelles. The large number（about 1015/mL）of very small polymer particles thus formed which are stabilized by adsorbing monolayers of soap until depleting the available molecularly dissolved soap， thus destroying the soap micelles at an early stage of the polymerization（about 10% conversion in the usual recipe）. As all the available soap is distributed， and redistributed， over the surface of the growing particles， the amount of soap is the main factor controlling latex particle size. During the second stage of the emulsion polymerization， therefore， the loci for available monomer consist of the dissolved monomer， the free monomer droplets， and the monomer imbibed by the numerous polymer particles. As before， the first two of these loci make a minor contribution， whereas the polymer-monomer particles provide a major locus for reaction with the initiator radicals diffusing from the aqueous phase. The major portion of the polymerization reaction apparently occurs within this large number of latex particles which are isolated from each other by electrostatic repulsion and kept saturated with monomer diffusing from the monomer droplets. It is this aspect which leads to the unique characteristics of this system. Thus， once an initiator radical enters a polymer monomer particle and initiates a chain， the latter must continue to propagate with the available monomer until another radical enters the same particle. In this way， the rate of chain termination is actually controlled by the rate of entry of radicals into the particles， and this generally increases the lifetime of the growing chains， and hence the chain length. Furthermore， because the growing chains are all located in different particles， they are unable to terminate each other， leading to a higher concentration of growing chains and a hence faster rate.

A much faster rate and a much higher molecular weight are achieved by emulsion polymerization systems. It is obvious that the main difference lies in the fact that the emulsion system is capable of raising the steady-state concentration of growing chain radicals by two to three orders of magnitude. The situation described earlier， i.e.， where radicals entering individual latex particles successively initiate and terminate growing chains， is referred to as ideal emulsion polymerization， as defined by the Smith-Ewart Theory.

2.1.3　Cation Polymerization

The art of cationic polymerization， like that of many other types of polymerization， is at least a century old. However，cationic polymerization kinetics is more complex because the existence of coinitiator complicate the initiation reaction， and that trace amounts of initiator has a great influence on polymerization rate. The polymerization rate is extremely fast， and initiation and growth are almost synchronous to complete instantaneously. Process repeatability is poor. So only several rubbers are polymerized in this mode， such as butyl rubber.

Since the required activation energy for ionic polymerization is small， these reactions may occur at very low temperature.The carbocation， including the macrocarbocations， repel each other， hence， chain termination cannot take place by combination but is usually the result of reaction with impurities.

Both the initiation step and the propagation step are dependent on the stability of the carbocation. Isobutylene（the first monomer to be polymerized commercially by ionic initiators）， vinyl ethers， and styrene may be polymerized by this technique. The order of activity for olefins is（CH3）2CCH2＞CH3（CHCH2）＞CH2CH2， and for para-substituted styrene， the order for the substituents is OCH3＞CH3＞H＞Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophilicity of the gegenion. Rearrangements may occur in ionic polymerizations. The mechanism of cation polymerization focusing on isobutylene is shown in Figure 2.2.

The rate of initiation for typical reactions is proportional to the concentration of the monomer and the concentration of the catalyst-cocatalyst complex.

Propagation， or chain growth， takes place in a head-to-tail configuration as a result of carbocation（M+） addition to another monomer molecule. The rate constant（kp）is essentially the same for all propagation steps and is affected by the dielectric constant of the solvent. The rate is fastest in solvents with high dielectric constants， promoting separation of the carbocation-gegenion pairs.

The termination is simply the dissociation of the macrocarbocation-gegenion complex forming BF3and H2O and the new neutral polymer chain.

It has been known for some time that cationic polymerizations can produce polymers with stereoregular structures. While a number of vinyl monomers have been evaluated in this regard， much of the work has centered about vinyl ethers. Several general observations have been noted.

（1）The amount of stereoregularity is dependent on the nature of the initiator.

（2）Stereoregularity increases with a decrease in temperature.

（3）The amount and type of polymer（isotactic or syndiotactic） is dependent on the polarity of the solvent.

2.1.4　Anionic Polymerization

Anionic polymerization was used to produce synthetic elastomers from butadiene at the beginning of the twentieth century. Early investigators used alkali metals in liquid ammonia as initiators， but these were replaced in the 1940s by metal alkyls such asn-butyllithium. In contrast to vinyl monomers with electron-donating groups polymerized by cationic initiators， vinyl monomers with electron-withdrawing groups are more readily polymerized by anionic initiators. Accordingly， acrylonitrile is readily polymerized by anionic techniques， and the order of activity with an amide ion initiator is as follows： acrylonitril＞methyl methacrylate＞styrene＞butadiene. As might be expected， methyl groups on the carbon decrease the rate of anionic polymerization， and chlorine atoms on the carbon increase the activity.Cis-1，4-polyisoprene rubber and high trans-polybutadiene rubber are synthesized by this method.

As shown by the following chemical（Figure 2.3）， syntheticcis-1，4-polyisoprene is produced at an annual rate of about 76000 tons by the polymerization of isoprene in a low dielectric solvent， such as hexane， usingn-butyllithium as the initiator. It is assumed that an intermediate cisoid conformation assures the formation of a cis elastomer. The propagating species in anionic polymerization are carbanions instead of carbonium ions， but the initiation， propagation， and chain transfer termination steps in anionic polymerization are similar to those described for cationic polymerization.No formal termination step was shown in the previous equations， since in the absence of contaminants the product is a stable macroanion. So the term “living polymers” is used to describe these active species. Thus， these macroanions or macrocarbanions may be used to produce a block copolymer.

Living polymers are generally characterized by：an initiation rate that is much larger than the polymerization rate；polymer molecular weight is related to [monomer]/[initiator]；linear molecular weight conversion relationship；narrow molecular weight range；stabilization of the living end groups allowing the formation of telechelics， macromers， block copolymers， and star polymers.

2.1.5　Coordination Polymers

Coordination polymerization is a form of addition polymerization in which monomer adds to a growing macromolecule through an organometallic active center. The development of this polymerization technique started in the 1950s with heterogeneous Ziegler-Natta catalysts based on titanium tetrachloride and an aluminium co-catalyst such as methyl aluminoxane. Coordination polymerization has a great impact on the physical properties of vinyl polymers such as polyethylene and polypropylene compared to the same polymers prepared by other techniques such as free radical polymerization. The polymers tend to be linear and not branched and have much higher molar mass. Coordination type polymers are also stereoregular and can be isotactic or syndiotactic instead of just atactic. This tacticity introduces crystallinity in otherwise amorphous polymers.

Typical monomers are nonpolar ethylene and propylene. The development of coordination polymerization that enables copolymerization with polar monomers is more recent. In rubber field， ethylene propylene diene monomer（EPDM） is a typical representative of the anionic polymerization.

The polymerization mechanism is shown in Figure 2.4， taking propylene for example. Triethylaluminum reacts to produce ethyltitanium chloride as the active site for polymerization of a nonpolar vinyl monomer such as propylene. It is generally agreed that a monomer molecule（CH2CHCH3） is inserted between the titanium atom and the terminal carbon atom in the growing chain and that this propagation reaction takes place on the catalyst surface at sites activated by the ethyl groups of the cocatalyst. The process outlined for initiation is repeated for propagation， and stereo regularity is maintained. The monomer molecule is always the terminal group on the chain.