Which polymers are chain growth polymers

One of the more successful polymers that have been synthesized using the substituent CGC technique , is polybenzoxazole. The CGC method has also been employed to synthesize well-defined block copolymers and star-shaped polymers. The second phase is a monomer depleted liquid to which an initiator is added. Polymerization starts when monomers migrate through the phase boundary and react with the initiator.

If the polymer end groups are more reactive than the monomers, and the rate of diffusion through the boundary is adequate, monomers predominantly react with the polymer end groups, resulting in polymers with low polydispersity.

This method is not new, but as Yokozawa and Yokoyama 2 pointed out, traditional phase transfer polymerization 13,14 , has given little to no control over molecular weight and polydispersity of condensation polymers, whereas good control over MW has been achieve when utilizing the substituent effect.

Yokozawa, T. Asai, R. Sugi, S. Ishigooka, and S. Hiraoka, J. The observation that end groups of condensing polymers are more reactive than monomers had been reported by several other scientist earlier.

However, none of their methods yielded polymers with precisely controlled molecular weight and low polydispersity due to the insolubility of the growing polymers and due to contaminations. Yokozawa, D. Muroya, R. Sugi, A. Yokoyama, Macromol. The rate of polymerization strongly depends on the type of side chain.

For example, monomers with a tri ethylene glycol monomethyl ether TEG side chain instead of an alkyl side chain polymerize much slower. Polymer Properties Database. Chain-Growth versus Step-growth Polymerization Step-growth e. The mixture contains primarily monomers and polymers, and only small amounts of growing polymer chains; Step-growth versus Chain-growth Polymerization. Summary Step-Growth takes place by reaction between monomers, oligomers, and polymers.

Chain-Growth takes place by repeated additions of monomers to the growing polymer chain. Theromophysical Data Key data on over two hundred and fifty polymers. In principle, once started a radical polymerization might be expected to continue unchecked, producing a few extremely long chain polymers.

In practice, larger numbers of moderately sized chains are formed, indicating that chain-terminating reactions must be taking place. The most common termination processes are Radical Combination and Disproportionation. These reactions are illustrated by the following equations. The growing polymer chains are colored blue and red, and the hydrogen atom transferred in disproportionation is colored green. Note that in both types of termination two reactive radical sites are removed by simultaneous conversion to stable product s.

Since the concentration of radical species in a polymerization reaction is small relative to other reactants e. The relative importance of these terminations varies with the nature of the monomer undergoing polymerization.

For acrylonitrile and styrene combination is the major process. However, methyl methacrylate and vinyl acetate are terminated chiefly by disproportionation. Another reaction that diverts radical chain-growth polymerizations from producing linear macromolecules is called chain transfer. As the name implies, this reaction moves a carbon radical from one location to another by an intermolecular or intramolecular hydrogen atom transfer colored green.

These possibilities are demonstrated by the following equations. Chain transfer reactions are especially prevalent in the high pressure radical polymerization of ethylene, which is the method used to make LDPE low density polyethylene.

Further polymerization at the new radical site generates a side chain radical, and this may in turn lead to creation of other side chains by chain transfer reactions. As a result, the morphology of LDPE is an amorphous network of highly branched macromolecules. Polymers may also be classified as straight-chained or branched, leading to forms such as these:.

The monomers can be joined end-to-end, and they can also be cross-linked to provide a harder material:. If the cross-links are fairly long and flexible, adjacent chains can move with respect to each other, producing an elastic polymer or. Polymerization of isobutylene 2-methylpropene by traces of strong acids is an example of cationic polymerization. This process is similar to radical polymerization, as demonstrated by the following equations. Chain growth ceases when the terminal carbocation combines with a nucleophile or loses a proton, giving a terminal alkene as shown here.

Monomers bearing cation stabilizing groups, such as alkyl, phenyl or vinyl can be polymerized by cationic processes. These are normally initiated at low temperature in methylene chloride solution.

Strong acids, such as HClO4 , or Lewis acids containing traces of water as shown above serve as initiating reagents. At low temperatures, chain transfer reactions are rare in such polymerizations, so the resulting polymers are cleanly linear unbranched. Treatment of a cold THF solution of styrene with 0. Note: In lieu of an abstract, this is the article's first page. Cited By. This article is cited by publications. Michael V. Bautista, Anthony J. ACS Macro Letters , 9 9 , Varni, Andria Fortney, Matthew A.

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