Chain-Growth Polymerisation with Termination

3 Chain-Growth Polymerisation with Termination A major exponent of this class of polymerisations are free radical polymerisations in the presence of a radical initiator. A classic example is the crosslinking (co)polymerisation of unsaturated polyester resins with styrene, initiated by the decomposition of a peroxide initiator. Some important reaction steps involved in free radical polymerisations are sketched in Reaction scheme 4. 

While for step-growth polymerisations (section 2.2.1) and chain-growth polymerisations without termination (section 2.2.2) an overall distribution of reacting species or one type of reacting species (the monomer) is mechanistically characterising the observed reaction rate, the balance between two distinct and mechanistically different species (the monomer and a macro-molecular radical) is determining the observed rate of chain-growth polymerisations with termination. 

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Before vitrification, a heat capacity change as a result of chemical reaction, ACp,react, is noticcd. For the anhydride-cured epoxy and the polyester-styrene resin a minor, but reproducible, and almost linear decrease of Cp with conversion is observed. The former system is supposed to be an anionic chain-growth living polymerisation (without termination), the latter is a chain-growth copolymerisation with termination. 

In our particular type of step-addition polymerisation, monomers, dimers, trimers, oligomers and polymers are the reactive species which participate in the chain growth. Initially, the monomers react with monomers and give dimers, dimers react with monomers and dimers and give trimers and tetramers, respectively. The high MW polymer is formed only in the last stages of the poly addition reaction, at high conversion rates. Chain transfer and termination reactions are absent. 

Apart from labelled ethene,[ ° l FT synthesis was carried out in the presence of other labelled compounds such as ethanol,larger alkenesi and higher alcohols.Most results indicate that more than one mechanism is responsible for the distribution of radioactivity in the products. Figure illustrates that polymerisation of labelled ethene on Co produced more label in even C-number alkanes up to Cio- Almost constant radioactivities were observed in the >Ce products with added propanol. Smaller products showed more incorporated and the monomethyl-alkanes contained more radioactivity. The authors concluded that these alkenes participate both in chain initiation and chain propagation. Alcohols, in turn, initiated chain growth but did not participate in chain propagation on an industrial Fe catalyst.Neither ethene nor ethanol (or ethene formed by its dehydration) participated in the chain termination step. ... 

The cyclopolymerisation of 1,5-hexadiene leads to polymers of substantially higher molecular weights than the polymerisation of 1-hexene with the same catalysts [498], undoubtedly owing to some steric hindrance of the atom transfer that usually terminates the growth of a polymer chain [30],... 

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