Reaction, chain, copolymer

For flexible chain copolymers based on acrylic and methacrylic acids (AA and MA) crosslinked with a polyvinyl component, the inhomogeneity of the structures formed depends on the nature of the crosslinking agent, its content in the reaction mixture and the thermodynamic quality of the solvent [13,14],... 

Another consequence of the absence of sponataneous transfer and termination reactions is that the polymer chains formed remain living 3), i.e. they carry at the chain end a metal-organic site able to give further reactions. Block copolymer synthesis is probably the major application 12 14), but the preparation of co-functional polymers, some chain extension processes, and the grafting onto reactions arise also directly from the long life time of the active sites. 

Although the mechanism of copolymerization is similar to that discussed for the polymerization of one reactant (homopolymerization), the reactivities of monomers may differ when more than one is present in the feed, i.e., reaction mixture. Copolymers may be produced by step-reaction or by chain reaction polymerization. It is important to note that if the reactant species are Mi and M2, then the composition of the copolymer is not a physical mixture or blend, though the topic of blends will be dealt with in this chapter. 

The most widely used chain reaction block copolymers are those prepared by the addition of a new monomer to a macroanion. AB and ABA block copolymers called Soprene and Kraton, respectively, are produced by the addition of butadiene to styryl macroanions or macrocarbanions (Equation 7.32). This copolymer is normally hydrogenated (Equation 7.33). 

Polymers containing azo groups as part of their backbone chain can be used for the synthesis of block copolymers. The azo-containing prepolymers can, for example, be synthesised by condensing small molecule azo compounds with functionalized polymers, by partial decomposition of polymeric azo compounds in the presence of a monomer or via polymer analogue reactions. Block copolymers are obtained when those prepolymers are decomposed in the presence of another monomer. 

It is important to appreciate that polymer produced by an anionic chain-growth mechanism can have drastically different properties from one made by a normal free radical reaction. Block copolymers can be synthesized in which each block has different properties. We mentioned in Chapter 4 that Michael Szwdrc of Syracuse University developed this chemistry in the 1950s. Since that time, block copolymers produced by anionic polymerization have been commercialized, such as styrene-isoprene-styrene and styrene-butadiene-styrene triblock copolymers (e.g., Kraton from Shell Chemical Company). They find use as thermoplastic elastomers (TPE), polymers that act as elastomers at normal temperatures but which can be molded like thermoplastics when heated. We will discuss TPEs further in Chapter 7. 

Copolymerization is a variant of polymerization where macromolecules composed of two or more kinds of monomer are formed. According to the frequency of entry of various monomers into the chains, copolymers (and even the reactions by which they are formed) are sometimes more closely specified by special attributes. Thus copolymers may be unspecified, statistical, random, periodic, alternating, block or graft. 

The simple copolymer model, with two reactivity ratios for a binary comonomer reaction, explains copolymer composition data for many systems. It appears to be inadequate, however, for prediction of copolymerization rates. (The details of various models that have been advanced for this purpose are omitted here, in view of their limited success.) Copolymerization rates have been rationalized as a function of feed composition by invoking more complicated models in which the reactivity of a macroradical is assumed to depend not Just on the terminal monmomer unit but on the two last monomers in the radical-ended chain. This is the penultimate model, which is mentioned in the next Section. 

The relationship between sensitivity and component ratio in copolymer negative resists was studied theoretically on the basis of Charlesby s gel formation theory. The formulas for sensitivity as a function of component weight ratio are derived for a nonchain reaction and for a chain reaction, respectively. Copolymer sensitivities for any component ratio can be estimated numerically using the derived formulas, from the data on sensitivities for homopolymers composed of individual constituent monomers in the copolymer. The calculated results are in good agreement with the experimental data reported. 

Chain transfer to a second polymer can be exploited as a possible avenue for block copolymer synthesis. The homopolymerization of a given monomer A in the presence of a preformed polymer B or the Interaction between two homopolymers in the presence of a cationic initiator (J, 153. 154) produce in the first step of the reaction block copolymers. Synthesis of other block copolymers was... 

Sodium polyacrylate is produced by the reaction between acrylic acid (H2C=CHC00H) and its sodium salt (H2C=CHC00Na). The product of this reaction is a long-chain copolymer consisting of alternate units of acrylic acid and sodium acrylate. A copolymer is a polymer made of two different monomers, in this case, acrylic acid and sodium acrylate. What makes this polymer different from most other... 

A monomer is a starting material used in the preparation of polymers, long-chain molecules made up of many repeating units. Addition polymers are long-chain molecules prepared from alkene monomers through numerous addition reactions. A copolymer is prepared from two monomer starting materials. 

Copolymerization of substituted and unsubstituted monomers was also investigated [74]. The density of side-chains along the PPP backbone was slowly decreased by varying the ratio of substituted to non-substituted monomers in the reaction mixture. Copolymers exhibit statistically distributed repeating units no evidence for alternation or for block-sequences was found. Their was a marked effect on the UV spectra, indicative of a modification in the extent of conjugation between adjacent benzene rings induced by side-chains. This effect is linked to steric hindrance and it shows that a disubstituted benzene unit acts as an electronic insulator between unsubstituted blocks of PPP. DPs of 50 are reported for these copolymers. 

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