Effects of Chain-End Structures

To clarify the effects of chain-end structures of PLA, Lee et al. [34] synthesized C1-, NH2-, and COOH-terminated PLLAs from OH-terminated PLLAs. The thermal stability of OH-terminated PLLAs was poor, whereas NH2- and Cl-terminated PLLAs were more resistant to thermal degradation. The main mechanisms of PLLA thermal degradation are transesterification and backbiting reactions that cause random degradation and unzipping depolymerization, respectively, starting from the carboxyl and/or hydroxyl chain ends [7, 8, 10, 29, 49]. 

End protection of the hydroxyl group has been considered as a way of improving the thermal stability of PLLA and some reports have been published on such end protection by an acetyl group, resulting in an increase in the degradation temperature of PLLA by tens of degrees [31, 36]. However, the acetylation process not only achieves end protection, but also eliminates the residual metals in PLLA, such as Sn, Zn, and Al from the polymerization process inducing the decrease in thermal stability of PLLA [7, 11, 29, 33, 36, 41 3, 50]. 

Abe et al. [21] also showed the same effect for the hydroxyl-end acetylation. Additionally, they investigated effects of ce-carboxylic acid chain end protection. From changes in activation energy and molecular weight of residual polymers together with Py-GC/MS analysis results, they deduced that the residual Zn compounds catalyzed the selective unzipping depolymerization of PLA with 

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Mori, T., Nishida, H., Shirai, Y. and Endo, T. (2004) Effects of chain end structures on pyrolysis of poly(L-lactic acid) containing tin atoms. Polymer Degradation and Stability, 84. 243-251. 

MALDI-TOF MS. To investigate the effect of chain-end structure on this functionalization reaction, PSLi was end-capped with a few units of butadiene to form the butadienyllithium chain end. Functionalization of the poly(styrene)-l -oligo(butadie-nyl)lithium with oxetane was performed under conditions similar to those described above. TLC showed no spots corresponding to unfunctionalized polymer, and isolation by column chromatography yielded over 97 wt.% functionalized polymer. A peak at S 63.5 ppm was observed in the NMR which was assigned to the carbon bonded to oxygen. 

The effect of chain-end structure (stability and steric requirements) has also been investigated. The steric and electronic nature of the anionic chain end can be modified by reaction with DPE as shown in eqn [17]. When the direct... 

In order to optimize specific anionic functionalization reactions such as carbonation with carbon dioxide, the effect of chain end structure (stability and steric requirements) has often been investigated. The steric and electronic nature of the anionic chain end and the chain-end aggregation can be modified by reaction with 1,1-diphenylethylene as shown in Eq. (13). When the direct carbonation is effected in benzene at room temperature with the diphe-nylalkyllithium species formed by addition of poly(styryl)lithium to 1,1-diphenylethylene (Eq. 31), the carboxylated polymer can be isolated in 98% yield compared to only a 47% yield for poly(styryl)lithium without end-capping under the same conditions [141] ... 

Here, we are concerned with amorphous polymers. Hence, amorphous structures have to be generated, by some means, that are low in energy, have an appropriate distribution of torsional angles, a physically acceptable distribution of unoccupied volume, and so on. In order to avoid surface effects, it is customary to employ periodic continuation conditions The primary simulation box is surrounded by periodic images of itself and when an atom exits the box through one face of the box it reenters it through the opposite face. In order to keep the effect of chain ends minimal, we usually employ a single polymer chain per simulation box. For the chain the periodic continuation conditions imply, that it is folded into the box (if it leaves the box on one face, it reenters it... 

The results of Babanalbandi and co-workers (221), in which new aliphatic chains ends formed by cleavage of the main chain at the ester unit are observed, are in support of a mechanism in which chain scission dominates cross-linking. These authors reported G-values for the formation of chain end structures comparable with earlier study. Furthermore, the main volatile products of radiolysis of PLA and poly(glycolic acid) (GPA) are CO2 and CO, consistent with chain scission being the most important reaction. In addition, small amounts of hydrogen and ethane gas were observed on the radiolysis of PLA. Finally, Montanari and co-workers (226) have examined the effects of radiation sterilization on the stability of PLGA microparticles used for drug delivery. 

In the product, there should be a ladder-type blocks linked by segments composed of p-cresyl methacrylate units. This type of structure was confirmed by IR and NMR spectrometry. However, by preparation of such copolymers with labeled end-groups (using radioactive AIBN), and by fractionating and radiometric analysis, it was shown that copolymers obtained are slightly branched. There is slightly more branch points than in the case of copolymers with styrene. It could be an effect of chain transfer reaction. 

Fig. 50. Possible mechanism by which chains with multiple reactive sites can graft to an interface. This example, which would be typical of a maleic anhydride functionalized polymer reacting on a polyamide, shows on one side end-grafted chains and on the other side of the interface, a loop structure. The effect of this loop structure on the mechanical strength of the interface is not fully clear but loops that are too short will weaken the interface...

Because of the structures of the tetrafunctional epoxy and amine, small cycles are formed [131]. These short distance intramolecular cyclisation reactions probably continue while the material is vitrifying. This will decrease the munber of chain ends, without decreasing the absolute number of mechanically effective crosslinks already formed. Thus, it will increase the stiffness of the macromolecules (on a molecular scale) and contribute to a further increase of conversion and Tg. These points again correlate well with the higher increase in conversion and Tg of the amine-cured epoxy in the glassy state. 

All lipids form crystal structures with the hydrocarbon chains arranged in layers. The chains are extended in sAl-trans conformation, so that a planar zig-zag carbon chain is formed. The chains are close-packed with parallel chain axes, except for one crystal form of soaps which pack with crossed chain axes. Such a complex chain packing is due to the dominating influence of the ionic end groups in relation to the effect of chain packing on the lattice energy. 

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