Self-branching

CONVERGENT (SELF-BRANCHING) ANIONIC POLYMERIZATION METHOD... 

A procedure used in the self-branching polymerization reaction for the preparation of dendritic polystyrenes is outlined in Scheme 7. Oligomeric polystyryl-lithium chains are reacted with a coupling agent such as 4-(chlorodimethyl-silyl)styrene (CDMSS), which contains both a polymerizable double bond as well... 

Self-branching in the polymerization of vinyl acetate. Proc. Roy. 

Another type of branching occurs in some free-radical polymerizations of monomers like ethylene, vinyl chloride, and vinyl acetate in which the macroradicals are very reactive. So-called self-branching can occur in such polymerizations because of atom transfer reactions between such radicals and polymer molecules. These reactions, which are inherent in the particular polymerization process, are described in Chapter 6. 

Reactive radicals like that of polyethylene can also undergo self-branching by a backbiting reaction in which a radical at the end of the polymer chain obstructs a hydrogen from a methylene unit in the same chain ... 

If the monomer and polymer are not mutually soluble, the bulk reaction mixture will be heterogeneous. The high pressure free radical process for the manufacture of low density polyethylene is an example of such reactions. This polyethylene is branched because of self-branching processes illustrated in reaction (6-89). Branches longer than methyls cannot fit into the polyethylene crystal lattice, and the solid polymer is therefore less crystalline and rigid than higher density (0.935-0.96 g cm ) species that are made by coordination polymerization (Section 9.5). 

The main features of degradation for individual chains due to high temperature then are chain scission, depolymerization, branching, self branching, self cyclization, removal of a substituent, anJ formation of double bonds. 

While long branches are formed by the normal chain transfer to polymer, as shown above [Eq. (6.124)], reactive radicals like those of polyethylene can also undergo self-branching by a backbiting intramolecular transfer reaction (see Fig. 6.9) in which the chain-end radical abstracts a hydrogen atom from a methylene unit of the same chain resulting in the formation of short branches (as many as 30-50 branches per 1000 carbon atoms in the main chain) that outnumber the long branches by a factor of 20-50. 

Since polymer concentration increases with conversion, self-branching also increases with conversion. Branching can not only be caused by polymer free radical transfer, but also by initiator free radical transfer to polymer molecules. The following is obtained for the polymerization of styrene with AIBN, for example ... 

Strand, SP Issa, MM Christensen, BE Varum, KM Artursson, P. Tailoring of Chltosans for Gene Delivery Novel Self- Branched Glycosylated Chitosan Oligomers with Improved Functional Properties. Biomacromolecules, 9 (11), 2008, 3268-3276. 

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