Linear Combs with Polystyrene Branches

3 Linear Combs with Polystyrene Branches (Deffieux and Schappacher, 1998 Deffieux and Schappacher, 1999  

The PS comb synthesis was finally achieved by adding dropwise the living PS solution onto a known amount of PCEVE in THF. Discoloration of the living PS was used as an indicator of the 

PS ends grafting. Typically, a slight excess of PSLi chains with respect to the PCEVE units (1.2 lCl)was used to insure a high grafting density and the formation of densely grafted comb polystyrenes (PCEVE-g-PS) (I-l). 

PS branch lengths on the molar mass and on the grafting efficiency  

Thanks to their very high molar masses (often several millions g/mol) and high chain compactness (PCEVE-g-PS) combs, can be readily observed as individual molecules by atomic force microscopy (AEM) in the tapping mode. This technique appears to be a powerful tool to observe the shape, dimensions, absolute molar mass, molar-mass distribution, chain conformation and internal organization of individual macromolecules (Dziezok et ah, 1997 Gallyamov etal, 2004 Matyjaszewski etal, 2003 Percec etal, 1998 Potemkin etal, 2004 Qin et al, 2003 Sheiko et al, 2003 Sheiko and Moller, 2001 Spatz et al, 1997). 

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The synthesis of comb-like polymers with regular branching (in contrast to random branching) has been performed in the following way 91) A linear polystyrene precursor fitted with carbanionic sites at both ends is reacted first with 1,1-diphenylethylene (to decrease the nucleophilicity of the sites) and then with a calculated amount of triallyloxytriazine to get chain extension. Each triazine residue still carries one allyloxy... 

In several cases the melt viscosity of a series of lightly-branched polymers has been determined as a function of MW, and compared with that of linear polymers, and it has been found or may be deduced from the published data that there is a cross-over molecular weight, below which the branched polymer is less viscous, but above which it more viscous, than the linear polymer of equal MW. This behaviour is observed with some comb-shaped polystyrenes (35) and poly(vinyl acetate)s (59, 89), star polybutadienes (57, 58, 123), and randomly-branched polyethylenes (56,61). Jackson has found (141) that if the ratio ZJZC of the number of chain atoms at the cross-over point, Zx, to the number at the kink in the log 0 — logM curve, Zc, [as given in Ref. (52)], is plotted against nb, the number of branches, a reasonable straight line is obtained, as in Fig. 5.1. 

Graessley s theory, though satisfactory for linear polymers, has not yet been shown to apply to branched polymers. Fujimoto and co-workers (65) attempted to apply it to comb-shaped polystyrenes, but obtained only poor agreement with experiment. They attributed this to the failure of the assumption that the state of entanglement is the same in branched polymers as in linear ones. It is not surprising that this theory fails, for (in common with earlier theories) it predicts that the zero shear-rate viscosity of all branched polymers will be lower than that of linear ones, contrary to experiment. 

The main molecular characteristics of the 4-arm comb star polystyrene are collected in Table 21.2. The Mw Ri(app)ls ratio, which is related to the volume contraction due to branching is 0.19 for the star comb versus 0.29 for the linear comb of same structure. These very low values agree with the highly branched chain architecture of the comb polymers on which is superposed the volume contraction related to the comb star architecture. The radius of gyration (/ g) and hydrodynamic radius (/ h) of the star comb on the one hand are very close to each other as are and of the corresponding one-branch linear comb polymer. They are equal to 45 nm, in THE at 25 °C for the star comb M = 9 000 000 g/mol) and 25 nm for the similar linear comb (M = 2 700 000 g/mol) in the same conditions. 

Figure 3.4 Intrinsic viscosity-molecularweight relation in toluene at 35°Cfor O regular H-polystyrene 12-arm star polystyrene A comb polystyrene with f= 30, Afbb = 2.75x 105 and increasing branch MW. Top line linear polystyrene...

Pannell (38) has studied a range of polystyrenes with comb-like branching, but with relatively long branches. He has correlated the low-shear melt viscosities with calculated values of , finding i/o°c(so)4 8, whereas the exponent for linear polymers is about 3.4. Fujimoto s results can be correlated in a similar way, but with a rather higher exponent, 5.1, though rather better correlations would be obtained if separate lines were used for each branching frequency. 

Since a branched polymer has more than two chain ends, the determination of both the thickness of the adsorbed polymer and the adsorbance is of particular interest. Kawaguchi and Takahashi106) investigated well-characterized comb-branched polystyrene adsorbed on a chromium plate from cyclohexane solution at the theta point by ellip-sometry and compared the results with those for the corresponding linear polystyrene. 

Hyperbranched and comb polymers have also been used as surface active additive. Ariura et al. synthesized by combination of anionic and cationic polymerization a monodispersed hyperbranched polystyrene [73]. The authors proved by combination of DSIMS and neutron reflectivity the preferential surface enrichment of the branched protonated macromolecules when blended with its deuterated linear polystyrene counterparts with the same molar mass. Other systems involving the segregation of the branched macromolecules in binary blends were demonstrated such as in polyamide [74] or poly (methylmethacrylate) [75]. 

Fig. 3.26 Universal calibration curve for crosslinked polystyrene gels with tetrahydrofuran as solvent %y linear polystyrene 0 branched polystyrene (comb type) +, branched polystyrene (star type) A, branched block copolymer of styrene methyl methacrylate x, poly (methyl methacrylate) poly (vinyl chloride) V, graft copolymer of styrene methyl methacrylate , polybutadiene (reprinted with permission from Comprehensive Polymer Science, copyright 1989, Pergammon Press pic).

Star polymers may be considered to be highly branched polymers that have linear chains radiating out from a central area. This area may be one atom, a small molecule, or a "core". The "core" is a quasi-spherical structure as opposed to a linear structure that would be present in a conventional comb or branched polymer. An early example of stars made from small molecules is the star polymer of Schaefgren and Flory (1) who polymerized E-caprolactam in the presence of a tetrafunctional or octafunctional carboxylic acid to produce polymers that have 4 or 8 arms radiating out from a central molecule. Other examples use the coupling of "living" anionically polymerized polystyrene with silicone tetrachloride (2) or chloromethyl-benzene (3). Recent work in this area includes that of Fetters (4) who has made 12 and 18 arm stars with this general technique. 

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