Anionic addition polymers

It may be shown that M (Jcw x ). The averages are equ only for a monodisperse (all molecules the same size) polymer. The ratio M /M — x /x. is known as the polydispersity index PI, and is a measure of the breadth of the molecular weight tribution. Values range from about 1.02 for carefully fractionated or anionic addition polymers, to over 50 for some conomercial polymers. 

The early, independent work of Starks, Markosa and Brandstrom from ca. 1965-1969, brought into focus with Starks classical paper in 1971, showed PTC to be potent and versatile synthetic tool.[1-4] Since that time, the well-documented investigations of PTC have been massive and vigorous.[5-7] In polymer chemistry PTC was effectively exploited first in anionic addition polymerization and more recently has been extended to condensation polymerization.[5,8-9] However, until a very few years ago, the use of solid-liquid PTC systems in polycondensation has for the most part escaped this intensive scrutiny. Consequently, some time ago we began a rather broad study into the use of solid-liquid PTC to effect polycondensations.[10]... 

Preparation of addition polymers having the oxolene (dihydrofuran) functionality can be envisioned to occur in two possible ways (Scheme 13). Both, in fact, have been observed (77MI11102). Whereas furan (53) or its derivatives do not homopolymerize under free radical conditions, 1 1 alternating copolymers possessing the 1,4-structure are produced with maleic anhydride (50). Intermediate formation of a CT complex between monomers (50) and (53) is believed to be necessary before polymerization can occur. On the other hand, cationic polymerization is quite facile. The outcome is straightforward with benzo[f>]furan derivatives, producing 1,2-polymers. Optically active poly(benzofurans) are formed when the cationic polymerizations are conducted in the presence of a chiral anion. 

Thiamine-catalyzed transformations are reversible, thus TV,/V-dialkyl hydrazones were selected as alternative acyl anion equivalents that were reported to react with electrophiles without acidic activation.41 One especially reactive example, formaldehyde hydrazone resin 13, was constructed from polymer-supported hydrazines and was employed in the first polymer-supported, uncatalyzed acyl anion additions (Fig. 8).38 As test substrates, nitroalkenes (as Michael acceptors) and activated aldehydes were selected. Reactivity of these acyl anion equivalents depended critically not only on the nature of the starting hydrazine, but also on the protocol for hydrazine formation. 

Cumbal and Sengupta (2005) produced an effective arsenic removal system by dispersing high-surface area iron (oxy)(hydr)oxide nanoparticles within a base (anion exchange) polymer for permeable matrix support (Table 7.1). The polymer consisted of a quaternary ammonium functional group within a styrene-divinylbenzene matrix. The addition of the nanoparticles improved arsenic removal when compared with the polymer alone. 

A more sophisticated class of optical sensors with high selectivity towards ions are the ion-selective optodes (ISOs) [21], where the matrix (hydrophobic polymer such as PVC) contains a selective lipophilic ionophore (optically silent), a chromoionophore, a plasticizer and an anionic additive. The measurement principle is based on a thermodynamic equilibrium that controls the ion exchange (for sensing cations) or ion coextraction (for sensing anions) with the sample. The source of optode selectivity is a preferential interaction between the target ion and an ionophore. For a dye to act as a chromoionophore, it must... 

Chain-growth, or addition, polymers are made by adding one monomer unit at a time to the growing polymer chain. The reaction requires initiation to produce some sort of reactive intermediate, which may be a free radical, a cation, or an anion. The intermediate adds to the monomer, giving a new intermediate, and the process continues until the chain is terminated in some way. Polystyrene is a typical free-radical chain-growth polymer. 

As well as mechanisms involving radical intermediates, addition polymers can be made by mechanisms involving cationic or anionic intermediates. For example, polyisobutylene is made by treating isobutylene with a small amount of boron trifluoride and water. 

Most addition polymers are prepared from vinyl monomers. However, another type of addition polymer can be formed by ring-opening reactions. For example, the polymerization of ethylene oxide can be accomplished by treatment with a small amount of a nucleophile, such as methoxide ion. The product, a polyether, is formed by a mechanism involving anionic intermediates ... 

However, the well-known ability of organolithium compounds to form associated species or to form complexes with electron donor compounds (240—242) provides strong support for mechanisms involving cationic attack by the lithium cation on the monomer prior to an anionic addition. With three orbitals available for coordination, a monomeric lithium alkyl should be able to complex both double bonds of a diolefin to provide the orientation for making cis-1,4 polymer and still have an orbital available for forming associated species in hydrocarbon solvents. The lithium orbitals are presumed to be directed tetrahedrally. Looking at the top of a tetrahedron with the fourth lithium oibital above and normal to the plane of the paper, the complex could have structure A below. In the transition state B for the addition step, the structure... 

The process of forming an addition polymer by chain-growth polymerization involving an anion at the end of the growing chain, (p. 373)... 

A polymer that results from the rapid addition of one monomer at a time to a growing polymer chain, usually with a reactive intermediate (cation, radical, or anion) at the growing end of the chain. Most chain-growth polymers are addition polymers of alkenes and dienes, (p. 370)... 

Figure 5.14. Reactions involved in anionic addition polymerization. Shown are (a) generation of a carbanion from a Lewis basic initiator, (b) propagation of the polymer chain through the combination of the carbanionic polymer chain and additional monomers, and (c) termination of the polymer growth through the addition of a Lewis base. UnUke the other addition polymerization schemes, termination does not occur in situ, but must be initiated deUberately.

A polymer is a giant molecule composed of a repeating structural unit called a monomer. Addition polymers result from the addition of alkene molecules to one another. The polymerization occurs by cationic, free-radical, and anionic reaction mechanisms. Examples of addition polymers include polyethylene, polystyrene, PVC, and Teflon. 

Addition polymers can be formed by the reaction of an alkene with a radical, cation or anion initiator. 

Conjugated dienes are polymerized by n-BuCal (in admixture with n-Bu2Ca) by a typical anionic addition mechanism at 20°-50°C. Initiation by addition is slow compared to the subsequent polymerization, and the microstructure of the polymer is solvent dependent. Polybutadiene with predominantly trans-1,4 links (48-72%) is obtained in hydrocarbons or Et20, but 1,2-polymerization is promoted in THE or by addition of hexamethylphosphoramide (HMPA). Polymerization of butadiene by PhjCMX (M = Ca, Sr, Ba X = Cl, Br) at — 10°C in THE yields polymer with increasing... 

Synthetic polymers can be divided into two major classes, depending on their method of preparation. Chain-growth polymers, also known as addition polymers, are made by chain reactions—the addition of monomers to the end of a growing chain. The end of the chain is reactive because it is a radical, a cation, or an anion. Polystyrene—used for disposable food containers, insulation, and toothbrush handles, among other things—is an example of a chain-growth polymer. Polystyrene is pumped full of air to produce the material known as Styrofoam . 

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