Linear addition polymers

In addition polymerization, monomers are added one at a time to a growing dinin hy propagation of a free radical through a liquid of monomers  

In the case of addition polymerization without termination, the number fraction distribution function (the probability that a given chain has degree of polymerization N) is given by the Poisson distribution function  

The weight fraction distribution function for addition polymerization 

The polydispersity index for the Poisson distribution is quite narrow, since there is no termination (see Problem 1.37)  

Many addition polymerization reactions with very low concentrations of impurities have propagation rates much faster than initiation rates and have essentially no termination. Such reactions produce narrow molar mass distributions that can be approximated by the Poisson distribution. Comparison of the polydispersity index of anionically polymerized butadiene with Eq. (1.69) is shown in Fig. 1.20. 

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Its thermolysis is evidently strictly first order and no complications arise because the induced decompositions so common with peroxidic initiators do not take place with this compound. This permits closer control of the formation of linear addition polymers. 

Composition and microstructure determination of polybutadiene (BR) and natural rubber (NR) can be done by infrared spectra. Three different base units are possible for linear addition polymers of 1,3 butadiene units with cis or trans internal double bands from 1,4 addition and units with side vinyl groups from 1,2 addition (see Scheme 3.1a). 

Another widely used rubber is NR, which can be present in four base units in linear addition polymers of isoprene (shown in Scheme 3.1b). Infrared analysis of polyisoprene is difficult. The out of plane hydrogen deformations are satisfactory for determining... 

Polyethylene, the prototype or model chain for linear addition polymers, is composed of ethylene monomer units linked by covalent bonds to form long chains (Figure 1.1a.) But, this linear chain was only produced in the mid 50 s by the Ziegler-Natta catalysts. Before this, the Fawcett process that required high pressure and temperature produced a polyethylene chain that contained many small side chains or branches attached by covalent bonds to the main chain (Figure 1.1b). Nuclear... 

During the synthesis of linear addition polymers, molar masses are controlled by the free radical lifetime and the rate constant for propagation. Since there is a distribution of radical lifetimes the resulting polymer will have a distribution of molar masses. Methods of measuring molar masses of polymers are not discussed here, but molar masses themselves are important in that most polymer properties depend upon them. 

In practice, the hnear polymer we might expect for alkenes is not the major product of the free radical process. (Cationic polymerization is generally used to prepare linear addition polymers of alkenes.) The product chains have many alkyl branches, which most often are the four-carbon-atom butyl groups produced by short chain branching. These products are the result of intramolecular hydrogen abstraction by way of a six-membered transition state that generates a secondary radical from a primary radical. 

Linear, addition polymer formed at constant rate of initiation, constant monomer concentration, transfer to solvent but not to monomer and termination by disproportionation. 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

As already mentioned, aromatie polymers are thermally stable but aliphatic portions of them are not as thermally stable. Typical maleimide resins have aliphatic units. This is inevitable because the Michael addition was used to prepare the maleimide-based oligomers. On the other hand, if an adhesive consists of a linear thermoplastic polymer, it is not usable at temperatures above its softening temperature. Introdueing chemical crosslinking is one way to prevent thermal weakening of a material. 

A polymer is a large molecule built up by the repetition of small, simple chemical units. In some cases the repetition is linear while in other cases the chains are branched or interconnected to form three-dimensional networks. The polymer can be formed not only through linear addition, but also through condensation of similar units as well. 

Gaylord, N. G., and Mark, H. F., Linear and Stereoregular Addition Polymer. Wiley (Interscience), New York, 1959. 

As previously indicated, both condensation and addition polymers may be prepared from monomers of functionality exceeding two, with resulting formation of nonlinear polymers. Hence the distinction between linear and nonlinear polymers subdivides both the condensation and the addition polymers, and four types of polymers are at once differentiable linear condensation, nonlinear condensation, linear addition, and nonlinear addition. The distinction between linear and nonlinear polymers is clearly warranted not only by the marked differences in their structural patterns but also by the sharp divergence of their properties. 

The extraction time has been observed to vary linearly with polymer density and decreases with smaller particle size [78,79]. The extraction time varies considerably for different solvents and additives. Small particle sizes are often essential to complete the extraction in reasonable times, and the solvents must be carefully selected to swell the polymer to dissolve the additives quantitatively. By powdering PP to 50 mesh size, 98 % extraction of BHT can be achieved by shaking at room temperature for 30 min with carbon disulfide. With isooctane the same recovery requires 125 min Santonox is extractable quantitatively with iso-octane only after 2000mm. The choice of solvent significantly influences the duration of the extraction. For example talc filled PP can be extracted in 72 h with chloroform, but needs only 24 h with THF [80]. pH plays a role in extracting weakly acidic and basic organic solutes, but is rarely addressed explicitly as a parameter. 

In conclusion, the lesson learned from the research carried out to date on the subject of polycarbosilanes is that the general rule that linear, noncrosslinked polymers are not suitable preceramic polymers applies here as well. Crosslinked network-type polymers are needed. Such structures can be generated in more than one way, but in the case of the polycarbosilanes they have, to date, been obtained mainly by thermolytic routes thermal treatment (with or without other chemical additives) in the case of the Yajima polycarbosilanes and the thermolysis of tetramethylsilane in the case of the Bayer process-derived polycarbosilane. 

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. 

Fig. 1.11. The molecular weight of linear condensation polymers is generally controlled by the addition of a small percentage of monofunctional molecules.

The heat of fusion AHf (obtained from the area under the DSC melting curve) and percentage crystallinity calculated from AHf is found to be linearly dependent on butadiene content, and independent of the polymer architecture. This is shown in Figure 3. Also, the density of the block copolymers was found to be linearly dependent on butadiene content (see Figure 4). The linear additivity of density (specific volume) has been observed by other workers for incompatible block copolymers of styrene and butadiene indicating that very little change in density from that of pure components has occurred on forming the block copolymers.(32) While the above statement is somewhat plausible, these workers have utilized the small positive deviation from the linear additivity law to estimate the thickness of the boundary in SB block copolymers.(32)... 

The quantitative relationship of flammability of a polymer with respect to the concentration of flame retardant is usually not linear, and there is no logical reason to expect combinations of different flame retardants to show a linearly additive result either (43). The actual result is often found to be "synergistic" or "antagonistic", or in regression analysis terminology, the interaction term is often found to be statistically significant. 

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