Carbon-13 spin-lattice relaxation time isotactic

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

Figure 2. The 13C spin-lattice relaxation times at 15 MHz for isotactic poly (propylene) methylene ( ), methine (O), and methyl (A) carbons.

C NMR spin-lattice relaxations in solid PMMAs of different tacticity were studied by CP/MAS NMR spectroscopy.324-328 The relaxation proceeds more slowly in isotactic PMMA than in syndiotactic PMMA even in the solid state this is because of the faster reorientation of the a-CH3 group in the isotactic chain.135 The nC-T times of the a-methyl carbons of PMMAs with different tacticities were measured over a wide range of temperatures by cross polarization, magic-angle spinning, dipole dephasing (CP/DD)... 

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