Proposed polymerization mechanism

The proposed polymerization mechanism is shown in Scheme 9.12. Thermal decomposition of the hexasubstituted ethane derivative yields hindered tertiary radicals that can initiate polymerization or combine with propagating species (primary radical termination) to form an oligomeric macroinitiator. The addition of the diphenylalkyl radicals to monomer is slow (e.g. k[ for 34 is reported as KT M"1 s l at 80 °C84) and the polymerization is characterized by an inhibition period during which the initiator is consumed and an oligomeric macroinitiator is formed. The bond to the Cl I formed by addition to monomer is comparatively thermally stable. 

The synthesis and proposed cure mechanisms of this resin are described in reference 2. While the cure mechanism of the BCB terminated resin is not yet known, it is speculated that it reacts via one of two different routes. Initially the strained four member ring of the benzocyclobutene undergoes a thermally Induced ring opening. The opened rings then react with one another by a linear type addition to form a network type of structure or by cycloaddition to form linear polymer chains. An Illustration of the proposed polymerization mechanism of benzocyclobutene (BCB) terminated resins is shown below. 

X-ray powder diffraction and FTIR measurements of the reaction products indicated anti-parallel -sheets formation, in agreement with our proposed polymerization mechanism, where the homochiral oligopeptide products should self-assemble into alternating poly-L- and poly-D-chains. 

The proposed polymerization mechanism suggests that a tt complex is formed from the spirane cyclopropane and the carbo cation active site (Equation 10). 

FIGURE 1.13 Proposed polymerization mechanism for the synthesis of PHA. Reproduced with permission from Ref. [55]. 2013 Elsevier. 

Scheme 1. Proposed Polymerization Mechanism of TMSB by Platinum Catalyst.

Scheme 11 Proposed polymerization mechanism of chain-growth condensation polymerization of 14...

Scheme 38 Proposed polymerization mechanism of catalyst-transfer condensation polymerization of 35a...

Recently, Lambrinos et al. [101] observed some deviation from the proposed polymerization mechanism in iniferter systems. These authors pointed out the bimolecular termination leading to the deactivation of the iniferter site in the polymerization of n-butyl acrylate initiated by / -xylene bis(iV,iS -die-thyl-dithiocarbamate and claimed that the polymerization was not strictly living. Doi et al. [102] proposed a new two-cx)mponent iniferter system to prevent the deactivation of the iniferter site. In this system, BDC and TD act as an iniferter (or an initiator) and a chain transfer agent and/or a primary radical terminator, respectively. In the polymerization of methylacrylate (MA) with BDC bimolecular termination leading to the deactivation of the iniferter site occurred in preference to chain transfer to BDC and the dithio-carbamayl radical that produce the iniferter site, resulting in a deviation from... 

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