Backbone Length

The length of the backbone will control the frequency of the hard segments that are present in the polyurethanes. The most obvious of this is the overall hardness of the material. The longer the backbone, the more flexible it will be. Short backbones with a degree of coordinate cross-linking (i.e., some 

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Only about 5% of the /3 bulges are between parallel strands, and most of the antiparallel ones are between a closely spaced (see Section II,B) rather than a widely spaced pair of hydrogen bonds. The additional backbone length of the extra residue on the longer side is accommodated partly by bulging that strand to the right and toward... 

So far, a great number of well-defined macromonomers as branch candidates have been prepared as will be described in Sect. 3. Then a problem is how to control their polymerization and copolymerization, that is how to design the backbone length, the backbone/branch composition, and their distribution. This will be discussed in Sect. 4. In brief, radical homopolymerization and copolymerization of macromonomers to poly(macromonomers) and statistical graft copolymers, respectively, have been fairly well understood in comparison with those of conventional monomers. However, a more precise control over the backbone length and distribution by, e.g., a living (co)polymerization is still an unsolved challenge. 

Narrow distribution in the backbone length as well as in the chemical composition or the branch frequency may be expected from a living-type copolymerization between a macromonomer and a comonomer provided the reactivity ratios are close to unity. This appears to have been accomplished to some extent with anionic copolymerizations with MMA of methacrylate-ended PMMA, 29, and poly(dimethylsiloxane) macromonomers, 30, which were prepared by living GTP and anionic polymerization, respectively [50,51]. Recent application [8] of nitroxide (TEMPO)-mediated living free radical process to copolymerizations of styrene with some macromonomers such as PE-acrylate, la, PEO-methacr-ylate, 27b, polylactide-methacrylate, 28, and poly(e-caprolactone)-methacrylate, 31, may be a promising approach to this end. 

Figure 6. Time dependence of =Si-H group concentration (%) for poly addi- tion of a,ro-dihydridedimethylsiloxane to divinylorganotricyclodecasiloxane at 90°C for dimethylsiloxane backbone lengths 1) n = 12 2) n = 6 3) n = 4...

Thermomechanical studies of synthesized copolymers indicate that the glass transition temperature of copolymers is decreased with an increase linear dimethylsiloxane backbone length, n (Figure 8). Since n=12 carbotricyclodecasiloxane fragments in copolymers cause no effect on the dimethylsiloxane backbone and Tg of copolymer 6 (Table 6) remains equal -123°C. 

Contrary to previous considerations [25], dimethylsiloxane backbone length increase ( =21) does not cause formation of two-phase systems in cyclolinear copolymers with rigid decaphenyltricyclodecasiloxane fragments and flexible dimethylsiloxane units (n = 25). 

Diffraction patterns of copolymers with cyclic carbosiloxane fragments in the backbone are cha-racterized by the presence of two diffraction maximums typical of amorphous polymers. Data in Ta-ble 15 show that interchain distances, d, decrease with the increase of dimethylsiloxane backbone length and at n = 37 reach the value typical of PDMS. Simultaneously, the increase of cyclic frag-ment volume in copolymers induces a considerable increase of d. ... 

Such deviations from ideal behavior can be prohibited by varying the chelate backbone lengths. Thus, an Lj—C2—L—C3—L—C2— system or an Lf—C3—L—Cg—L—C3—Lj system might be expected to relieve strains a-c, and it has been found that altering chelate backbone can have quite startling effects on the resulting complex stereochemistry... 

Control of Poly(Acrylic Acid) Backbone Length by a Chain Transfer Reagent... 

The possibility of controlling backbone length in the free radical copolymerization of TBA monomer with PS-MA macromonomer has been studied by radical polymerization of TBA in the presence of various amounts of benzyl mercaptan (BnSH) as chain-transfer agent. Under the assumption that each... 

Table 1 Graft copolymer constitution as determined by NMR and SEC experiments Comonomer composition and molecular weight of the PS-grafts (Mn> graft)> the PAA backbone length (Mn> Bb). and the graft density (GD)... 

Length of the i th type of bond along the shortest path across the chain backbone. Length of a statistical chain segment (Kuhn segment). 

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