Homoaggregation

In Figure 11.2.1A the process of dispersion polymerization is shown focusing on the formation process of particles. Dispersion polymerization starts from a homogeneous solution, and when oligomeric radicals and polymer, formed in the monomer solution, do not have affinity for the medium, they become insoluble and precipitate. The precipitate is unstable and homoaggregates to become primary particles. Primary particles homoaggregate further until they become stable secondary particles. The mechanism to keep the particles stable depends on what type of stabilizer is used. The propagation processes from nuclei to primaiy particles and from primary to secondary ones does not have to be considered as discontinuous steps. However, it... 

Here, how the number of particles are determined quantitatively by Paine is followed. In his model, two unstable nuclei or primary particles homoaggregate with a reaction rate constant of k2 and the number of particles decrease with conversion by — 1 power. The surface area (SFA), which is decided by the rates of formation of unstable nuclei or primary particles and of their homoaggregation, could be written as a function of conversion X as follows (the details are in Ref. 21) ... 

It is not known whether lithium enolates exist in solution as homoaggregates or as mixed aggregates, nor is it known whether lithium enolates react as aggregates or via other species that might be present in low concentration. But it is certain that... 

Onouchi H, Miyagawa T, Morino K et al (2006) Assisted formation of chiral porphyrin homoaggregates by an induced helical poly(phenylacetylene) template and their chiral memory. Angew Chem Int Ed Engl 45 2381-2384... 

With the lithium amide of the diamine (,S )-l-isopropylamino-1-phenyl-2-pyrrol idinyl-ethane (12), an analogue of the ether amide shown previously, almost no mixed complexes were formed with n-BuLi (Li-12/n-BuLi), i.e. the equilibrium was almost completely shifted toward the homoaggregates. The apparent equilibrium constant, K, of Scheme 5 was determined to be 0.14 M. 

Figure 12. Dispersed emission spectra of jet-cooled monomer, dimer and larger homoaggregates upon 0,0 excitation top, BIN bottom, ABN. Reproduced with permission from Ref. [92a].

DLCA and RLCA processes represent universality classes for homoaggregation (aggregation involving similar particles), i.e. the aggregates display some characteristic features for which the kinetics of particle coagulation and the fractal structure are independent of the details of the system. One line of evidence that supports this hypothesis is the observation that the fractal dimensions of aggregates formed by RLCA are remarkably constant, as are those formed by DLCA, for many different particle types [39,40]. 

Usually one assumes absolute microscopic stability of the binary system when the zeta-potentials of the two particle components are equal in sign and large in absolute value. He-teroaggregation is expected when both components are oppositely charged, while homoaggregation occurs when the attractive interaction forces between similar particles dominate due to low surface potentials (Ra a et al. 2004). 

This example illustrates how covalent and noncovalent forces can be used for the assembly of novel dye aggregates with interesting optical properties. The stepwise synthesis process could be used not only for preparing homoaggregates, as in the case with methyl red, but also for making heteroaggregates with fine control over the placement of the different chromophores. 

The mixed aggregate, which was identified in the NMR spectra, reacted faster with additional aldehyde k = 0.66 s" ) than the homoaggregate k = 0.29 s )... 

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