Chain Polymerization, Addition Polymers

Graft copolymers are commonly produced by building reactive sites into a iinear polymer. Then in a subsequent reaction, polymerization by the comonomer is carried out at these reactive sites. For example, the incorporation of random vinyi bromide units in poiystyrene provides sites for subsequent production of a graft poiystyrene-poiymethyi methacrylate copolymer. 

Copolymers can also be a combination of types. For exampie, acryionitriie/ buta-diene/styrene (ABS) is a two-phase poiymer system that combines a random copolymer of styrene and acrylonitrile (SAN) and a dispersed graft copoiymer made of butadiene rubber grafted onto the SAN backbone (Fig. 3.10). 

In 1929, Wallace H. Carothers classified poiymers as addition polymers or condensation polymers based on their structure. Addition poiymers are those that are formed by adding the whoie monomer into the chain, resulting in a polymer in which the constitutionai unit is the same as the monomeric unit (or one in which the monomer is a muitipie of the constitutional unit, such as in the case of poly-ethyiene). Condensation polymers, on the other hand, are produced by a condensation reaction in which, usually, a small byproduct molecule is formed as each unit is added into the growing chain. Therefore the polymer s constitutional unit is different from the monomer. 

in 1953, P.J. Flory divided the polymers by their reaction mechanism into chain-reaction and step-reaction, rather than by comparing the polymer s constitutional unit and the monomer. The addition polymers are generally produced by a chain reaction mechanism, and the condensation polymers produced by a step-reaction mechanism. Currently it is customary, though not scientifically correct, to refer to addition or chain-reaction polymerization and to condensation or step-reaction polymerization. Some have suggested that the classification of polymers 

We will first discuss addition or chain-reaction polymerization and then discuss condensation or step-reaction polymers in Section 3.8. Addition polymers used in packaging include, among others, polyethylene, polypropylene, polyvinyl chloride, and polystyrene. Polyesters, nylons, and polycarbonate are condensation polymers. 

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Chain polymerization (addition reactions) polyoxymethylene, polymethyl methacrylate (PMMA), acrylic polymers, polystyrene and styrene copolymers, water-soluble polyamide... 

In addition to backbone modifications, the side chains of addition polymers can also be modified to generate polymer structures that would not be possible by polymerization techniques. The classic example is the preparation of polyvinyl alcohol by the deprotection of polyvinylacetate (Figure 20). In this transformation, the product polymer is the structure that would result from the polymerization of vinyl alcohol. However, vinyl alcohol is an unstable monomer that predominantly exists in its tautomeric form of acetaldehyde. Thus the polymerization of vinyl alcohol is not practical. Vinyl acetate, on the other hand, is readily polymerized and thus serves as an attractive precursor to polyvinyl alcohol. 

The following table contains the abbreviations and acronyms of names of polymeric materials whose base polymers were obtained by chain polymerization (addition polymerization), copolymerization, polycondensation (condensation polymerization), and polyaddition. Note that these abbreviations and acronyms do not apply in industry to polymers per se but to polymeric materials, i.e. polymers with or without additives, tillers, plasticizers, etc. 

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,... 

Different samples of aqueous solution containing radionuclides of Co and Eu were prepared at different copper sulphate concentrations and constant polymer concentrations (pAM) of 15 mg/1. The addition of salt to the system was done to reduce both the repulsion forces between the radionuclides and the interaction between the polymeric chains [7]. The polymer efficiency for the prepared samples was determined, results are shown in Fig. 15. It is clear that the polymer efficiency for Eu " is higher than for Co. This can be explained by the difference in the tightly bound structured water associated with different cationic species [14,107]. On this basis, we expect that Co is more hydrated than Eu. This is due to the difference in the ionic size. The hydra-... 

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

Complexes of tetravalent zirconium with organic acids, such as citric, tartaric, malic, and lactic acids, and a complex of aluminum and citric acid have been claimed to be active as dispersants. The dispersant is especially useful in dispersing bentonite suspensions [288]. Polymers with amine sulfide terminal moieties are synthesized by using aminethiols as chain transfer agents in aqueous addition polymerizations. The polymers are useful as mineral dispersants [1182]. 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

The monomers used to make an addition polymer need not be identical. When two or more different monomers are polymerized into the same chain, the product is a copolymer. For instance, we routinely copolymerize ethylene with small percentages of other monomers such as a-olefins (e.g., 1-butene and 1-hexene) and vinyl acetate. We call the products of these reactions linear low density polyethylenes and ethylene-vinyl acetate copolymer, respectively. We encounter these copolymers in such diverse applications as cling film, food storage containers, natural gas distribution pipes, and shoe insoles. 

Chain growth polymers, which are often referred to as addition polymers, form via chain addition reactions. Figure 2.2 presents a generic chain addition mechanism. Chain addition occurs when the active site of a monomer or polymer chain reacts with an adjacent monomer molecule, which is added to the end of the chain and generates a new active site. The active site is the reactive end of a monomer or polymer that participates in the polymerization reaction. 

The choice of one polymerization method over another is defined by the type of monomer and the desired properties of the polymer. Table 2.1 lists advantages and disadvantages of the different chain growth mechanisms. Table 2.2 summarizes some well known addition polymers and the methods by which they can be polymerized. 

Since the depolymerization process is the opposite of the polymerization process, the kinetic treatment of the degradation process is, in general, the opposite of that for polymerization. Additional considerations result from the way in which radicals interact with a polymer chain. In addition to the previously described initiation, propagation, branching and termination steps, and their associated rate constants, the kinetic treatment requires that chain transfer processes be included. To do this, a term is added to the mathematical rate function. This term describes the probability of a transfer event as a function of how likely initiation is. Also, since a polymer s chain length will affect the kinetics of its degradation, a kinetic chain length is also included in the model. 

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