Linear chains, growth

Attempts to channel the polymerization toward linear chain growth have concentrated on the observed kinetic preference for addition of one monomer unit at a time to the growing chain (Eq 3). 

Note The ligand architecture ensures linear chain growth. Because the two NHC centres are at opposite ends of the same vector, a chelating coordination is de facto impossible. 

Note If need be, metal coordination can be activated in two steps (i) NHC and (ii) phe-noxide ensuring a linear chain growth. 

In Fig. 2, the relative fractal density is plotted as a function of increasing radius for selected values of fractal dimension. A fractal dimension of 1.0 represents linear chain growth, whereas a fractal dimension of 3.0 represents three-dimensional nonfractal growth. True fractal growth is bounded by these two extremes in fractal dimension. As can be seen in Fig. 2, the decrease in density can be severe. For example, a fractal growing with a dimension of 2.0 has a relative density of 0.25 its core density at only 4 times its core radius. Fractal dimensions of approximately 2.0 have been observed for silica gels evolved under both mildly acid and mildly basic conditions [5,12]. 

Vinyl, Acrylic and Other Monomers for Linear Chain-growth Polymerisation... 

When Si alkoxides are used, a stoichiometric amount of water and a catalyst are normally required to ensure the hydrolysis of Si(OR)4 in Si(OH)4 and high yield of Si02 in the final material. Acid catalysis results in fast hydrolysis and slow condensation, which leads to linear chain growth of the organic phase. In contrast, slower hydrolysis and faster polycondensation were observed in the case of base catalysis, leading to spherical colloidal particles. 

Recent studies by Rowan and coworkers have confirmed that even end-groups with extremely low association constants can have a dramatic effect on the sohd-state properties of polymers. As part of a seminal study, " bis-(3-aminopropyl)-terminated pTHF (Mn 1400 g mol ) was functionalized with thymine (T), adenine (A), or cytosine (C) residues (5, Figure 8). The association constants for these end-groups in CHCI3 solution were very low (K AA = 1.5M", K CC = 5M", KfTT = 3.5M- ), far below the values needed to form supramolecular polymers by linear chain growth in solution (1). 

Tilley TD (1993) The coordination polymerization of silanes to poiysiianes by a o-bond metathesis mechanism. Implications for linear chain growth. Acc Chem Res 26(l) 22-29... 

T. D. Tilley, Acc. Chem. Res., 26, 22 (1993). The Coordination Polymerization of Silanes to Polysilanes by a cr-Bond Metathesis Mechanism. Implications for Linear Chain Growth. 

The first example of a copolymerization of polar monomers in LCCP was described in our group, copolymerizing the styrene monomer (13) with isobutylene in amounts of 1-5 mol% of comonomer (Figure 3.7) (Hackethal etal, 2010). Incorporation of the polar monomer (13) can be achieved in amounts up to 2.5 mol%, as proven by NMR spectroscopy and MALDI methods. As followed by in-situ kinetic analysis, the polymerization follows a linear chain growth, together with a linear consumption of monomer when plotted as ln[Mo]/[Mt] vs. time t). 

Scheme 23.1.5 Overview of the Proposed Mechanism for the Preparation of Polysilanes via Borane Catalysis A — linear chain-growth, B — ligand scrambling, C — branching, D — termination.

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