Addition Polymers from Unsaturated Monomers

Addition polymerizations of unsaturated monomers leading to the formation of products of high molecular weight invariable proceed by chain reaction mechanisms. Primary activation of a monomer M (or a pair of monomers) is followed by the addition of other monomers in rapid succession 

The synthesis of a given polymer molecule through condensation polymerization, on the other hand, is accomplished by a series of independent condensations, which ordinarily occur at intervals scattered 

Monomer Type of polymerization Structural unit Approximate melting (Tm) or softening (Tg) temperature in Properties 

Ethylene Free radical at pressures up to 2000 atmospheres at 100 to 200°C using traces of oxygen or aqueous persulfate as catalyst —CHr-CHj— T , = 110 to 130 Waxlike, highly crystalline, very strong when oriented. Insoluble in all solvents at temperatures much below Tm 

Vinyl chloride Free radical polymerization in bulk or emulsion rapid in presence of peroxides susceptible to photochemical polymerization —CH2—CH— 75 Largely amorphous, except when highly oriented by stretching. Hard. Soluble in ketones and esters 

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The molecular formula of the condensation polymer is not an integral multiple of the formula of the monomer molecule owing to the elimination of a by-product, which in this case is water. The most important class of addition polymers consists of those derived from unsaturated monomers, such as the vinyl compounds... 

Table II.—Representative Addition Polymers Formed from Unsaturated Monomers... 

The preparation of polymers from heterocyclic monomers that contain polymerizable functional groups undoubtedly constitutes the most common method of incorporating heterocycles into polymeric materials. Polymer-forming reactions are of two possible types addition reactions and condensation reactions. Addition monomers in general contain a site of unsaturation, i.e. a double or triple bond, through which polymerization occurs by successive single bond formation from one monomer to the next. With condensation monomers a bond is formed between two monomers with concomitant elimination of a... 

We shall consider in the present review only some typical addition homopolymers, obtained from unsaturated monomers by opening of >C=C< double bonds therefore, we shall exclude those polymers, such as polyglycols, that, even though may be considered addition polymers, being formed from alkylene oxides, can formally be included in condensation polymers. 

Polymers are extremely large molecules that are made of repeat units called monomers. Addition polymers are formed from unsaturated monomers that commonly link through free-radical reactions. Most condensation polymers are formed by linking two types of monomer through a dehydration-condensation reaction. 

Carothers, in 1929, classified synthetic polymers into two classes, according to the method of their preparation, i.e., condensation polymers and addition polymers. In polycondensation, or step-growth polymerization, polymers are obtained by reaction between two polyfunctional molecules and elimination of a small molecule, for example water. Typical condensation polymers are shown in Figure 2. Addition (or chain reaction) polymers are formed from unsaturated monomers in a chain reaction. Examples of addition polymers are shown in Figure 2. 

Addition polymers are formed from unsaturated monomers that commonly link through free-radical reactions. 

Synthetic polymers used to form fibers are often classified on the basis of their mechanism of polymerization--step growth (condensation) or chain growth (addition) polymerization. Step growth polymerization involves multifunctional monomers which undergo successive condensation with a second monomer or with itself to form a dimer, which in turn condenses with another dimer to form a tetramer, etc., usually with loss of a small molecule such as water. Chain growth involves the instantaneous growth of a long molecular chain from unsaturated monomer units, followed by initiation of a second chain, etc. The two methods are outl ined below schematically ... 

It is also possible to generate hyperbranched polymers from vinyl monomers that undergo chain polymerizations. In addition to a polymerizable unsaturation, these monomers also contain a site that can initiate polymerization after... 

As a second example, there is a wide variety of breakdown products and oligomeric products that may be formed from the reactive monomers that are the building blocks of plastics. For plastics, the general assumption has been that any side-reaction products and breakdown products are likely to be significantly less toxic than the monomers, and so restricting the migration of the monomer was accepted as an indirect way to limit any hazard from the oligomers also. Whilst this approach is probably acceptable for addition polymers, such as those made from the unsaturated monomers vinyl chloride, butadiene and acrylonitrile where the unsaturated monomer is far more noxious than their products, the validity of this means of indirect control is questionable for condensation polymers such as polyesters and for polyethers formed from epoxide monomers. 

The majority of radiation curable materials are derived from or consist of acrylic and methacrylic unsaturated monomers, oli-gonomers, and polymers (including unsaturated polyethers) which cure through free radical addition-propagation reactions. 

Other processes also contribute to chain growth termination under special conditions. In particularly crowded catalysts, fi-methyl transfer to the metal centre can occur instead of p-H transfer. When other reaction paths are blocked, a-bond metathesis, i.e. transfer of an H atom from a monomer to the metal-bound alkyl C atom can release a polymer with a saturated chain end with formation of a new unsaturated metal-bound chain start. Saturated chain ends will also result when H2 gas is added to a catalyst system thus leading to the production of shortened polymer chains. Such an H2 addition will often also cause an increase in overall catalyst activity, since H2 will predominantly react with species - such as occasional 2,1-inserted units - which are rather... 

Figure 5.9. Reactions involved in free-radical addition polymerization. Shown are (a) (i)-(iii) generation of free radicals from a variety of initiators, (b) initiation of polymer chain growth through the combination of a free radical and unsaturated monomer, (c) propagation of the polymer chain through the combination of growing radical chains, (d) chain-transfer of free radicals between the primary and neighboring chains, and (e) termination of the polymer growth through either combination (i) or disproportionation (ii) routes.

Polyacrylates as binders consist of copolymers of acrylate and methacrylate esters. Other unsaturated monomers (e.g., styrene and vinyltoluene) may also be incorporated, but usually to a lesser extent. Copolymers formed exclusively from acrylates and/or methacrylates are termed straight acrylics. The comonomers differ as regards the alcohol residues of the ester group, which also allow incorporation of additional functional groups. Choice of suitable monomers allows wide variation of the physical and chemical properties of the resulting polymer. Hydrophilicity, hydrophobic-ity, acid base properties as well as can be adjusted resins containing hydroxyl, amine, epoxy, or isocyanate groups can also be produced. 

First comes the unsaturated polyester resin itself. From Chapters 12 and 15 we should have a clear idea of what is meant by a polyester resin and, in particular, a saturated polyester resin. Although a drying-oil alkyd is, in a sense, an unsaturated polyester resin, the term has come to be applied solely to polyester resins based on components which introduce unsaturation directly into the polyester backbone. This unsaturation must be capable of direct addition copolymerization with vinyl monomers. To give a linear polymer, any of the dibasic acids or dihydric alcohols mentioned in Chapters 12 and 15 may be used, but the resin should include some unsaturated components. These are usually, though not necessarily, acids, e.g. ... 

These are polymers formed from a single monomer. An example is polyethene (polyethylene), which is made by polymerization of ethene (CH2 CH2). Typically such polymers are formed by addition reactions involving unsaturated molecules. Other similar examples are polypropene (polypropylene), polystyrene, and polytetrafluoroethene (PTFE). Homopolymers may also be made by condensation reactions (as in the case of polyurethane). 

Addition polymers are synthesized by the addition of unsaturated monomers to the growing chain. The synthesis of polyethylene from ethylene is formally represented by... 

In addition polymers, no loss of small molecules takes place. The most important group of addition polymers are synthesised from unsaturated vinyl monomers, see Reaction 6.2. 

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