Intermolecular chain transfer to polymer

The intermolecular chain transfer to polymer (scrambling), however, is detrimental for preparation of functional polymers, leading to disproportionation of monofunctional macromolecules, as shown schematically below ... 

Formation of block copolymers in the sequential polymerization may be affected by chain transfer to polymer. As already discussed, in several systems the intramolecular chain transfer to polymer leads to formation of cyclic fraction. Cyclic macromolecules, being neutral, do not participate in further reaction and constitute the homopolymer fraction in resulting copolymer. Intermolecular chain transfer to polymer may lead to disproportionation, i.e., formation of fraction of macromolecules which do not carry active species ... 

Intermolecular chain transfer to polymer leads also to the exchange of segments between macromolecules (scrambling). This may effectively preclude the isolation of block copolymers. This phenomenon is especially pronounced in the polymerization of cyclic acetals. 

The MCRs can also be formed by intermolecular chain transfer to polymer (leading to long-chain branches), but its contribution is small. 

Intermolecular chain transfer to polymer results in long-chain branches and proceeds via abstraction of either a backbone tertiary hydrogen atom (in repeat units from vinyl monomers) or an atom from a substituent group. An example... 

Thus, intermolecular chain transfer to polymer leads to premature termination of the growth of one propagating chain and the reactivation of a dead chain which then grows a long-chain branch. As a consequence, the molar mass distribution of the polymer broadens. The changes in skeletal structure and molar mass distribution inevitably have major effects upon bulk polymer properties. 

Scheme 3.7 LCB formation by (a) intermolecular chain transfer to polymer and (b) addition to a terminally unsaturated polymer chain.

Non-linear polymers are frequently characterized by their solubility in a given solvent. The insoluble part, which corresponds to high molecular weight heavily branched polymer and polymer networks, is called gel. In the polymerization of monofunctional monomers that form gel by intermolecular chain transfer to polymer followed by termination by combination (e.g., butyl acrylate), the addition of CTA may reduce the gel content to nil, whereas the sol MWD remains essentially unaffected [89]. 

Intermolecular chain transfer to polymer is well documented in the cationic polymerization of cyclic acetals. In the polymerization of TOX, as will be discussed in Section 4.10.3, chain transfer to polymer is essential for the preparation of thermally stable polyacetal. Intermolecular chain transfer to polymer is detrimental to the synthesis of monofunctional polymers such as macromonomers because segment exchange (scrambling) leads to disproportionation and formation of products having two, one, and none of the functional groups (Scheme 21). " Intermolecular chain transfer to polymer prohibits also the synthesis of block copolymers by sequential polymerization of two cyclic acetals. Addition of DXP to a solution of living polyDXL resulted in further polymerization but the copolymer formed had a nearly statistical distribution of units." ... 

Scheme 21 Disproportionation of end-groups by intermolecular chain transfer to polymer.

Scheme 24 Intermolecular chain transfer to polymer leading to formation of cyciic fraction in the cationic poiymerization of cyclic acetals.

Intramolecular and intermolecular chain transfer to polymer are significant. The former leads to the formation of rings... 

Chain transfer reactions are bimolecular or unimolecular (spontaneous). Typical bimolecular chain transfer reactions are transfer to monomer, initiator, and external chain transfer agents (especially impurities), and intermolecular chain transfer to polymer typical unimolecular chain transfer reactions are transfer to counterion in ionic polymerizations, intramolecular chain transfer to polymer, and transfer to solvent (pseudo xmimolecular). 

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