Addition Polymerization

Addition polymerization requires a chain reaction in which one monomer molecule adds to a second, then a third and so on to form a macromolecule. Addition polymerization monomers are mainly low molecular-weight olefinic compounds (e.g., ethylene or styrene) or conjugated diolefins (e.g., hutadiene or isoprene). 

Condensation polymerization can occur hy reacting either two similar or two different monomers to form a long polymer. This reaction usually releases a small molecule like water, as in the case of the esterifrcation of a diol and a diacid. In condensation polymerization where ring opening occurs, no small molecule is released (see Condensation Polymerization later in this chapter). 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer  

Branching occurs especially when free radical initiators are used due to chain transfer reactions (see following section, Free Radical Polymerizations ). For a substituted olefin (such as vinyl chloride), the addition primarily produces the most stable intermediate (I). Intermediate (II) does not form to any appreciable extent  

L = a free radical P, cation L, or an anion I R = alkyl, phenyl. Cl, etc. 

The addition polymerization invariably proceeds by a chain-reaction mechanism involving three elementary steps, i.e. initiation, propagation and termination (Fig. 2). The preferred mode of monomer addition to the growing chain de- 

Polymer Structure Tensile strength (MPa) Elongation (%) Modulus i elasticity (GPa) 

Density (Mg/cm ) Izod Impact Joules/, Heat Deflection Temperature at 445 kPa Applications 

valves, fittings, floor tile, wire insulation, vinyl automobile roofs 

Packaging and insulation foams, lighting panels, appliance and furniture components, egg cartons 

Investigation of the polymerization of levoglucosan at various temperatures and pressures led to the conclusion that dimers, tetramers, hexamers, and octamers are formed. One preparation was shown to contain approximately 2% of a nondialyzable fraction.209 

In a series of papers commencing in 1959, Schuerch and coworkers have reported their extensive examination of the polymerization of levo-glucosan113 214 and several of its derivatives,215 and of 1,6-anhydro-/3-D-galactopyranose24 (o-galactosan) and its 2-methyl ether.24 Preliminary experiments on the polymerization of levoglucosan were conducted under diminished pressure over a temperature range of 100-130°, using several acidic catalysts—formic, acetic, monochloroacetic, phosphoric, and hydrochloric acids,214 and zinc chloride—in various mole ratios of catalyst to 

Polymer Catalyst CICH CO H, mmole/ mole of Time Yield by precipitation in EtOH (85%), g MJ 

The specific optical rotation of these polymers ([a]f + 91° =b 5°) led Schuerch and coworkers 1 to suggest that they contain a minor preponderance of a-D-glucoeidic linkages. Only pyranoid forms were assumed to be present. In view of the finding of furanoid residues in other synthetic polysaccharides,15 1 2 this supposition is no longer valid. Additional experimental data, such as the results of methylation studies, must be acquired before the chemical structure of these complicated polymers can be formulated. 

Abe and Prins214 suggested that the first step in the polymerization reaction of levoglucosan consists of a dimerization of all of the levoglucosan units. This involves the opening of all 1,6-anhydro rings, followed by the formation of a reactive, (1— 6)-linked intermediate (52), which polymerizes 

Addition or chain-growth polymerization is the most important industrial process for the production of polymers. Polyethylene, polypropylene and polystyrene are all formed 

The nature of the reactive centre defines the chemistry of the polymerization, the rate and conditions under which high polymer may form, and particular features of the polymer architecture (such as the tacticity see Section 1.1.2). The nature of the reactive centre and the monomer may also control the side reactions (such as branching) and defect groups that may be introduced, which may affect the subsequent performance of the polymer. In the following, we will consider the most common types of addition polymerization since this may define the properties of the polymer that then control the chemorheology. Certain of these reactions are more important than others in reactive processing, and the particular examples of reactions that occur in forming networks as well as modification of polymers will be considered in more detail. 

Some of the types of polymerization that come into this category include 

In some cases, the monomer may be polymerized by more than one method, and in ringopening polymerization the reaction may also occur through step-growth polymerization (Section 1.2.1) in addition to the initiated chain-growth polymerization. 

The chemistry of chain-growth polymerization may be readily distinguished from step-growth polymerization by the features in Table 1.4. 

In general, addition polymerization reactions can be considered as an aggregation process characterized by the following three types of equilibria  

Because of these different types of equilibria, the polymerizability of a monomeric species is largely determined by various thermodynamic factors such as temperature, pressure, and monomer concentrations. Additionally the nature of the monomer species, such as amorphous or crystalline, substituents, and structure are found to have a significant influence on the polymerizability and selectivity of a 

The representative metals constituting the addition polymerization catalyst include mostly Ni and Pd (34). 

Addition copolymers of cycloolefin compounds with a polar substituents in the side chain exhibit excellent heat resistance and transparency. They are also capable of crosslinking to improve the adhesion properties, the dimensional stability and the chemical resistance (35). 

A Ni based catalyst for addition polymerization can be prepared from nickel(2-ethylhexanoate), methylaluminoxane, and triethyl-borane (34). The polymerization is carried out in toluene as a solvent under pressure. 

Instead of methylaluminoxane, fluorinated aromatic compounds, such as a, a, n-trifluorotoluene can be used (36). The polymerization yield may be deteriorated when the mole ratio is out of the range, i.e., the fluorine-containing aromatic hydrocarbon compound is used in an amount of less than one mole with respect to one mole of the nickel salt compound. Otherwise, when the fluorine-containing aromatic hydrocarbon compound is used in an amount of more than 100 moles with respect to one mole of the nickel salt compound, there occurs a discoloration of the product with a deterioration of efficiency in the aspect of economy. 

The introduction of functional groups is suitable to control the chemical and physical properties of the polymer. However, the introduction of functional groups may cause a reaction of the unshared electron pairs of the functional groups with the active catalytic sites. Thus, the active sites of the catalyst are destroyed. In order to overcome this problem, a procedure has been developed, where the functionalized monomers, such as maleic acid, nadic acid or their anhydrides are grafted after the polymerization reaction (4,37). Grafting takes place as a radical reaction, using e.g., dicumyl peroxide. Other attempts use excessive amounts of catalysts. 

Coordination Polymerization. A third general polymerization type is coordination polymerization. Like addition polymerization, it occurs by addition of monomer units, one by one. Coordination polymerization is a form of addition polymerization, but differs in that the addition of the monomer involves a third molecular species besides the monomer and the growing polymer chain. In coordination polymerization, the addition step takes place with the monomer and polymer coordinated to the third species, which functions to promote the formation of the new bond. Usually, this third species is a metal complex. 

The most important chain-addition polymerizations are processes in which the reacting end of the polymer is a free radical. This type of polymerization is commonly called radical-chain polymerization. The basic mechanistic concepts are those introduced in Part A, Chapter 12, where free-radical chemistry was considered. 

Polymerization is normally dependent on an initiator, which begins the chain reactions in the polymerization mixture. Typical initiators are molecules that have rather low thermal stability and generate radicals on decomposition. Other initiator systems depend on photolytic decomposition or on the generation of radical intermediates as the result of redox reactions. The rate constants for the individual propagation steps that follow initiation are usually very large, but polymerizations are normally carried out in such a way that the concentration of the reacting chains is very low ( 10 M). As a result, the overall rate of polymerization is moderate. 

Kinetic analysis of this type of system is possible. The basic equations for the kinetics of radical-chain polymerization are derived by assuming a low steady-state concentration of reacting radicals. Under these conditions, the rate of termination equals that of initiation  

The term kt may contain several individual rate constants corresponding to each of several possible termination processes. This kinetic equation is closely analogous to the expressions in Part A, Chapter 12, for the kinetics of free-radical addition reactions. 

With this backgrotmd, let us explain the mechanism of chain (addition) polymerization. If we write the electronic stmcture of the main group, vinyl  

In configuration (A) one pair of electrons in each monomer unit is impaired (in the ir-orbital), which enables a single electron to react with an external single electron and end up as a free radical. This is the key to the most conventional mode of polymerization, via free radicals. Configuration (B) leads to an excess of electrons on one side (anion) and a shortage on the other side (cation). This leads to ionic polymerization (cationic or anionic). Hence there are choices of various mechanisms for polymerization, where the chemical nature of the monomer (characteristics of the substituent groups) dictates the preferred mechanism. This is shown in Table 2-3. 

We begin with radical initiation because it is most commonly used. In order to activate the monomers, materials that release free radicals are called on, known as the free radical initiators. (It should be stressed at the outset that the term initiator differs from the term catalyst, because the former becomes part of the generated molecule, as will be described later. A catalyst does not participate directly in the chemical reaction, but speeds it up.) In order to xmderstand the mechanism of polymerization via free radical initiation, we write the relevant chemical equations. 

A common initiator is benzoyl peroxide BPO (C6H5COO)2, which decomposes at 80-95 C to form free radicals, as follows  

Another useful initiator is azobiisobutyronitrile (AZBN) which decomposes at 50-70 C, as follows  

Polymer synthesis or formation occurs by two major mechanisms. The mechanisms are known as addition polymerization and condensation polymerization. 

In the mechanism for addition polymerization, the monomers react to form a polymer without loss of any atom. The total number of atoms in the formed polymer is simple multiples of the number of monomeric units used to form the polymer. 

Monomer name Formula Polymer formula Common name 

Vinyl chloride CHC1=CH2 -(CHCI-CH2) - Pol3mnyl chloride 

Most of the common synthetic polymers are formed by the mechanism of free radical mediated addition polymerization where the monomer units are activated by free radical formation and 

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Polymerization processes yielding polymers, whose mers are constitutionally identical to the reacting monomers are now classified as addition polymerizations. Thus styrene can be converted, by addition polymerization, to polystyrene ... 

Condensation polymerization differs from addition polymerization in that the polymer is formed by reaction of monomers, each step in the process resulting in the elimination of some easily removed molecule (often water). E.g. the polyester polyethylene terephthalate (Terylene) is formed by the condensation polymerization (polycondensation) of ethylene glycol with terephthalic acid ... 

The mechanism of these reactions places addition polymerizations in the kinetic category of chain reactions, with either free radicals or ionic groups responsible for propagating the chain reaction. 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

In Chaps. 5 and 6 we shall examine the distribution of molecular weights for condensation and addition polymerizations in some detail. For the present, our only concern is how such a distribution of molecular weights is described. The standard parameters used for this purpose are the mean and standard deviation of the distribution. Although these are well-known quantities, many students are familiar with them only as results provided by a calculator. Since statistical considerations play an important role in several aspects of polymer chemistry, it is appropriate to digress into a brief examination of the statistical way of describing a distribution. 

Haward et al.t have reported some research in which a copolymer of styrene and hydroxyethylmethacrylate was cross-linked by hexamethylene diisocyanate. Draw the structural formula for a portion of this cross-linked polymer and indicate what part of the molecule is the result of a condensation reaction and what part results from addition polymerization. These authors indicate that the crosslinking reaction is carried out in sufficiently dilute solutions of copolymer that the crosslinking is primarily intramolecular rather than intermolecular. Explain the distinction between these two terms and why concentration affects the relative amounts of each. 

The active centers that characterize addition polymerization are of two types free radicals and ions. Throughout most of this chapter we shall focus attention on the free-radical species, since these lend themselves most readily to generalization. Ionic polymerizations not only proceed through different kinds of intermediates but, as a consequence, yield quite different polymers. Depending on the charge of the intermediate, ionic polymerizations are classified as anionic or cationic. These two types of polymerization are discussed in Secs. 6.10 and 6.11, respectively. 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

In the next three sections we consider initiation, termination, and propagation steps in the free-radical mechanism for addition polymerization. One should bear in mind that two additional steps, inhibition and chain transfer, are being ignored at this point. We shall take up these latter topics in Sec. 6.8. 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

Several polymerization techniques are in widespread usage. Our discussion is biased in favor of methods that reveal additional aspects of addition polymerization and not on the relative importance of the methods in industrial practice. We shall discuss four polymerization techniques bulk, solution, suspension, and emulsion polymerization. 

Addition polymerization through anionic active species. This is discussed in the next section. 

These are addition polymerizations in which chain growth is propagated through an active center. The latter could be a free radical or an ion we shall see that coordinate intermediates is the more usual case. 

The reaction rate of fumarate polyester polymers with styrene is 20 times that of similar maleate polymers. Commercial phthaHc and isophthaHc resins usually have fumarate levels in excess of 95% and demonstrate full hardness and property development when catalyzed and cured. The addition polymerization reaction between the fumarate polyester polymer and styrene monomer is initiated by free-radical catalysts, commercially usually benzoyl peroxide (BPO) and methyl ethyl ketone peroxide (MEKP), which can be dissociated by heat or redox metal activators into peroxy and hydroperoxy free radicals. 

Because no molecule is spHt out, the molecular weight of the repeating unit is identical to that of the monomer. Vinyl monomers, H2C=CHR (Table 2) undergo addition polymerization to form many important and familiar polymers. Diene (two double bonds) monomers also undergo addition polymerization. Normally, one double bond remains, leaving an unsaturated polymer, with one double bond per repeating unit. These double bonds provide sites for subsequent reaction, eg, vulcanization. 

Anionic Polymerization. Addition polymerization may also be initiated and propagated by anions (23—26), eg, in the polymerization of styrene with -butyUithium. The LL gegen ion, held electrostatically in... 

Polymerization to Polyether Polyols. The addition polymerization of propylene oxide to form polyether polyols is very important commercially. Polyols are made by addition of epoxides to initiators, ie, compounds that contain an active hydrogen, such as alcohols or amines. The polymerization occurs with either anionic (base) or cationic (acidic) catalysis. The base catalysis is preferred commercially (25,27). 

Polylactic acid, also known as polylactide, is prepared from the cycHc diester of lactic acid (lactide) by ring-opening addition polymerization, as shown below ... 

Nylon-6 [25038-54-4] (9) is made by the bulk addition polymerization of caprolactam. Monofilament Nylon-6 sutures are avadable undyed (clear), or in post-dyed black (with logwood extract), blue (ED C Blue No. 2), or green (D C Green No. 5). Monofilament nylon-6 sutures are sold under the trade names Ethilon and Monosof monofilament nylon-6,6 sutures, under the trade names Dermalon and Ophthalon and monofilament polyethylene terephthalate sutures, under the trade name Surgidac. 

The addition polymerization of diisocyanates with macroglycols to produce urethane polymers was pioneered in 1937 (1). The rapid formation of high molecular weight urethane polymers from Hquid monomers, which occurs even at ambient temperature, is a unique feature of the polyaddition process, yielding products that range from cross-linked networks to linear fibers and elastomers. The enormous versatility of the polyaddition process allowed the manufacture of a myriad of products for a wide variety of appHcations. 

Proliferous Polymerization. Eady attempts to polymerize VP anionicaHy resulted in proliferous or "popcorn" polymerization (48). This was found to be a special form of free-radical addition polymerization, and not an example of anionic polymerization, as originally thought. VP contains a relatively acidic proton alpha to the pyrroHdinone carbonyl. In the presence of strong base such as sodium hydroxide, VP forms cross-linkers in situ probably by the following mechanism ... 

As recently as 1986 almost all addition polymers were excluded from the ranks of engineering plastics. However, progress since then has been made in the development of addition polymeric resins such as polymethylpentene and polycyclopentadiene and its copolymers (see Cyclopentadiene AND DICYCLOPENTAD IENE). 

Polymerization of olefins such as styrene is promoted by acid or base or sodium catalysts, and polyethylene is made with homogeneous peroxides. Condensation polymerization is catalyzed by acid-type catalysts such as metal oxides and sulfonic acids. Addition polymerization is used mainly for olefins, diolefins, and some carbonyl compounds. For these processes, initiators are coordination compounds such as Ziegler-type catalysts, of which halides of transition metals Ti, V, Mo, and W are important examples. 

Group of plastics composed of resins formulated by addition polymerization of monomers containing allyl groups (e.g., diallyl phthalate). 

The molecular chains of plastics are formed by condensation or addition polymerization,. V condensation polymer forms by stepwise reacting molecules with each other and eliminating small molecules such as water. Addition polymer forms chains by the linking without elimin.ating small molecules,... 

Free radical polymerization is a key method used by the polymer industry to produce a wide range of polymers [37]. It is used for the addition polymerization of vinyl monomers including styrene, vinyl acetate, tetrafluoroethylene, methacrylates, acrylates, (meth)acrylonitrile, (meth)acrylamides, etc. in bulk, solution, and aqueous processes. The chemistry is easy to exploit and is tolerant to many functional groups and impurities. 

The first use of ionic liquids in free radical addition polymerization was as an extension to the doping of polymers with simple electrolytes for the preparation of ion-conducting polymers. Several groups have prepared polymers suitable for doping with ambient-temperature ionic liquids, with the aim of producing polymer electrolytes of high ionic conductance. Many of the prepared polymers are related to the ionic liquids employed for example, poly(l-butyl-4-vinylpyridinium bromide) and poly(l-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide [38 1]. 

When polymerizing dienes for synthetic rubber production, coordination catalysts are used to direct the reaction to yield predominantly 1,4-addition polymers. Chapter 11 discusses addition polymerization. The following reviews some of the physical and chemical properties of butadiene and isoprene. 

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