Chain end, stabilization

Although emulsion polymerization has been carried out for at least 50 years and has enormous economic importance, the detailed quantitative behavior of these reactors is still not well understood. For example, there are many more mechanisms and phenomena reported experimentally than have been incorporated in the existing theories. Considerations such as non-micellar particle formation, non-uniform particle morphologies, polymer chain end stabilization of latex particles, particle coalescence, etc. have been discussed qualitatively, but not quantitatively included in existing reactor models. 

Studies on model compounds also suggest that unsaturated chain-end groups should not have an important influence on the thermal stability of PVC [21]. In conclu-... 

Instability at the chain end could then arise via the benzylic hydrogen atoms due to the possible resonance stabilization of the resulting radical [Eq. (18)] ... 

As with MAH, the extent of grafting varies dramatically vi ith the polyolefin substrate. Some differences have been attributed to variations in the type and amount of stabilizers present in the polyolefins substrate.326 In the case of isotactic PP, the maximum graft levels attained with 27 were found to correspond to only one unit of DEM per PP molecule 5 0 This would support a mechanism whereby grafts appear only at the chain ends. Higher graft levels were obtained with atactic PP. The higher reactivity of the atactic PP (and atactic sequences in... 

The majority of polymers formed by living radical polymerization (NMP, ATRP, RAFT) will possess labile functionality at chain ends. Recent studies have examined the thermal stability of polystyrene produced by NMP with TEMPO (Scheme 8.3),2021 ATRP and RAFT (Scheme 8.4).22 In each case, the end groups... 

Note, however, that chain ends 4 and 5 may give different chemistry to those formed in termination by disproportionation (2, see Scheme 8.5) or the processes under (a) above. Chain scission (3 to the double bond will not lead to a MMA propagating species. It is not established whether the presence of these ends will give impaired thermal stability. 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

All-or-none transitions occur if the chain length is relatively short (n < 15 tripeptide units) and if the cooperativity is high (a < 1) since in this special case, the concentration of intermediates is negligibly low. Besides, in the case of short chains we may conclude that back folding and oligomerization are negligibly small because of the shortness of the chain ends beyond the helical part. A further simplification is the assumption that only one helical sequence exists, which excludes the formation of loops within a helical part, because of reasons of stability. Under these circumstances, only two different products exist in a measurable concentration at equilibrium. 

The description of reactive intermediates, which are short-lived species, is the main field of application of quantum chemical model calculations, due to the fact that the intermediates are difficult to observe and characterize. For example, the influence of structure on the stability of various carbenium ions — which have been used as models of the cationic chain end — and the delocalization of the positive charge were treated on this basis. 

The electrophilicity and therefore the stability of the cationic chain ends are relatively limited, because, on the one hand, the electrophilicity must be large enough to aid a nucleophilic attack by the monomer, but on the other hand, not so large that a chain termination occurs due to recombination with the counterion. For this reason the stability of the cationic chain ends is a function of ... 

The heats of formation are less suited to characterizing the stability and/or reactivity of carbocations as models of cationic chain ends in cationic polymerizations71). Model reactions closely connected to the cationic polymerization mechanism are better suited to this characterization, for example ... 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

The data indicate that the formation of cyclic intermediates creates a stabilization of the cationic chain ends (AH° < 0 and AH s < 0), also expressed by a decrease of both the acceptor strength (Ae(LUMO) > 0) and the donor strength (Ae(HOMO) < 0) of the cations. The positive charge of the cationic centre is distinctly decreased (Aqc+ < 0) as a consequence of the interaction of this centre with the oxygen of the methoxy group. A partially covalent C + —O-bond is formed (pt Q(f) > 0.6 rc+ 0if) 146 pm). 

Optimization of the valence and dihedral angles yields planar cyclic structures for the 3- to 5-ring intermediates in contrast to a chair conformation for that of the 6-ring. In the cases of n = 4, 5, 6 the oxygen atom is placed almost in the plane of the three C-atoms directly bonded to it. Therefore, an intramolecular solvation of the cationic chain end by methoxy groups which are bonded to the polymer backbone is preferred in the gas phase. The calculations show that for a non-polar solvent such as CH2C12 a decrease in stability of the cyclic intermediates exists. But this decrease does not result in a total break of the intramolecular solvation (Table 13). An equilibrium between open chain and cyclic intermediates must only be taken into account in more polar solvents, due to the competition of intra- and intermolecular solvation. 

Thus, the preferred intramolecular stabilization of the cationic chain end by the formation of 5- and 3-membered cycles, which is possible for DME but impossible for vinyl ethers, can explain the characteristics of the cationic polymerization of DME in solvents of low polarity. 

The propagation of the cationic chain end can only occur if the nucleophilicity of the counterion is reduced sufficiently that recombination with the cation is prevented. The counterion Br, which recombines rapidly with the cationic chain end, can be stabilized by the interaction with the Lewis acid, e.g. SbBr5. An increase in stability, resulting from increasing complexation of the counterion, can be seen by means of... 

Izumisawa, S. and Jhon, M. S., "Stability Analysis and Molecular Simulation of Nanoscale Lubricant Films with Chain-End Functional Groups, /. Appl. Phys., 2002, Vol. 91,2002, pp. 7583-7585. 

Polymer structure and formulation. As an example, Woo et al. [7] measured OIT values for series of commercial PVC resins and polyester thermoplastic elastomers (TPEs). The researchers used the ASTM D3895-80 procedure, but substituted air as the oxidising gas instead of pure oxygen. A dependency on thermal processing history of the TPE film samples appeared to influence the measured OIT in the PVC study, chemically different chain ends affected polymer stability and hence OIT values. 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

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