Free Radical and Condensation Polymerizations

In recent years, the uses of synthetic polymers have expanded from making simple objects to much more complex applications such as targeted drug delivery systems and flexible solar cells. In any case, the application for the polymer is driven by its physical and chemical properties, notably bulk properties such as tensile strength, elasticity, and clarity. The structure of the monomer largely determines the chemical properties of the polymer, as well as other important measurable quantities, such as the glass transition temperature, crystallinity, and solubility. While some impor- 

Monitoring Polymerization Reactions From Fundamentals to Applications, First Edition. Edited by Wayne F. Reed and Alina M. Alb. 2014 John Wiley Sons, Inc. Published 2014 by John Wiley Sons, Inc. 

A syndiotactic chain is the one where the stereochemical configuration between adjacent stereocenters alternates. 

An atactic chain lacks any stereochemical order along the chain, which leads to completely amorphous polymers. 

Altering the glass transition temperature is by no means the only reason to include comonomers in a polymerization. In designing copolymers with specialized applications, comonomers can be included for specific functions, or as sites for further functionalization or initiation of a secondary polymerization (e.g., to make graft copolymers in a graft-from approach). In more broadly used commercial polymers, comonomers can be included to alter different properties. 

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FIGURE 13.5 Representative profiles of viscosity versus conversion for free-radical and condensation polymerizations. The endpoint is 99.9% conversion with a number average chain length of 1000. 

Materials Synthesis and Characterization. In addition to the requirements of etching resistance, sensitivity, solubility and high glass transition temperature (Tg), one of the criteria used in designing both a negative and positive electron-beam resist system was synthetic simplicity. The trimethylsilylmethyl appendage allows the incorporation of silicon into polymeric resists without adverse synthetic complications. Standard free radical or condensation polymerization techniques can be employed with appropriately substituted monomers that are readily available. 

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

Polymerization (Section 6.21) Process by which a polymer is prepared. The principal processes include free-radical, cationic, coordination, and condensation polymerization. 

Even within the small numbers of studies conducted to date, we are already seeing potentially dramatic effects. Free radical polymerization proceeds at a much faster rate and there is already evidence that both the rate of propagation and the rate of termination are effected. Whole polymerization types - such as ring-opening polymerization to esters and amides, and condensation polymerization of any type (polyamides, polyesters, for example) - have yet to be attempted in ionic liquids. This field is in its infancy and we look forward to the coming years with great anticipation. 

Very highly branched polymers, like polyethylene made by free-radical, high-pressure processes, will have Mw/Mn ratios of 20 and more. Most polymers made by free-radical or coordination polymerization of vinyl monomers have ratios of from 2 to about 10. The M /M ratios of condensation polymers like nylons and thermoplastic polyesters tend to be about 2, and this is generally about the narrowest distribution found in commercial thermoplastics. 

In addition polymerization, a free radical transfers from one vinyl monomer to another, leaving behind a trail of chemical bonds (Fig. 6.6). The distinction of addition polymerization, compared with condensation polymerization, is the high correlation of the formed bonds along the path of a free radical. Certain monomers (with two double bonds, such as divinylbenzene) can be visited twice by free radicals and become crosslinks (black circles in Fig. 6.6). As the neighbouring trails of formed bonds begin to overlap, the system approaches its gel point. The model describing... 

Numerous examples exist of combining CRP methods with other polymerization techniques for preparation of block copolymers. Non-living polymerization methods like condensation, free-radical, and redox processes can easily be combined with CRP to produce novel materials. Transformation chemistry may be the only route to incorporate polymers like polysulfones (as described above), polyesters, or polyamides that are prepared solely through condensation processes into subsequent CRP to form block copolymers with vinyl monomers. The same can be said of polymers prepared through coupling techniques, like po-ly(phenylenevinylene) and poly(methylphenylsilylene), which can maintain their conductive or photoluminescence properties, but become easier to process... 

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