End-groups in polymers

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

Thus the absence of unfired and once-fired end groups in polymers obtained in semicontinuous experiments with trinifer is indicated by the close to theoretical M /Mn values obtained and independently by 1H NMR spectroscopy. 

The latter method has allowed the quantitative determination of the concentration of ethoxy end groups in polymers with DP as high as 10. Small and precisely known proportions of the non-deuterated mers serve as internal standard. This work also indicates that, with the properly chosen conditions (when the j oportion of macrocycles is minimized) and with well purified components, one molecule of initiator produces one macromolecule. 

Analytical pyrolysis can be used successfully for the analysis of end groups in polymers. For example, the pyrolysis of poly(methyl methacrylate) obtained from polymerization with benzoyl peroxide as an initiator shows the presence of characteristic aromatic products in the pyrolysate. Peak intensities in the pyrograms of these characteristic compounds allow the evaluation of initiator levels and the understanding of polymerization mechanism [22],... 

With every non-carbene catalyst system there is the question as to the nature of the initiating metal carbene complex and how it is formed from the catalyst/cocatalyst/ (substrate olefin). When the cocatalyst contains an alkyl group, there is usually an obvious path whereby this becomes the source of the alkylidene group attached to the metal and there is much evidence from the initial products of reaction or from end groups in polymers formed by ROMP that can be brought to bear on this question. 

Neglecting end groups in polymer chains, molecular weight M = xMj. ... 

The presence of n-butyl end groups in polymer chains formed during propene polymerization in the presence of hydrogen supports this hypothesis [85, 91-93]. The possibility of formation of the inactive centers Mt-CH(CH3>2 by the reaction of chain transfer with the monomer in a secondary 2,1-orientation or by a secondary insertion in the Mt-H bond, formed as a result of chain transfer to hydrogen, has been considered [76] ... 

Analogous chemistry (i.e., end functionalization of polymeric organolithiums with l,l-bis(4-tert-butyldimethylsi-loxyphenyl)ethylene) has been utilized for preparation of dendrimacs, a new type of dendritic polymer wherein a key step in the iterative process utilizes the deprotected phenol end groups in polymers as nucleophiles for Williamson ether-type coupling reactions. Hirao and co-workers " have utilized di-monosaccharide-substituted DPEs to prepare well-defined polyisoprenes and polystyrenes labeled at the chain end with monosaccharide residues. 

Much use has been made of isotopically enriched AIBN in studies of end-groups in polymers made by radical polymerization. The 1-cyano-l-methyl-... 

An application of NMR for study of end-groups in polymers produced by anionic polymerization involved use of the spin-echo technique [23]. It was applied for detection of tert-hutyl end-groups in polyMMA made using unenriched di-t rt-butyl magnesium as initiator. 

A method for the estimation of -COOH end groups in polymers is proposed,... 

We, however, find that the chain length of the polymer and the character of the solvent affect the dye extraction process. Polarity of the polymer plays only a minor role. Extraction of pinacyanol dye from alkaline solution by polymeric carboxylic acids of varying chain lengths (including polystyrene carboxylic acid) into a variety of low dielectric constant organic solvents is discussed in this paper. Based on this study a method for the estimation of carboxyl end groups in polymers is proposed. 

From the foregoing discussion it is clear that the success of the dye partition method for the analysis of ionic end groups in polymers is controlled principally by the character of the organic solvent used in extraction and not by the polarity of the polymer. Solvents with some pennanent dipole moment capable of solvating... 

As might be expected, groups at either end of a polymer chain differ from those in the main polymer chain. Various methods are available for determining low concentrations of such end-groups in polymers. 

A first-shell substitution effect (ESSE) is the simplest kind of departure from ideal behavior. It is the situation where reactivity is affected only by the reaction of the functional groups attached to the same monomer unit. FSSEs are encountered in many polycondensations, as reactivities of functional groups in monomers are often different from the reactivities of end groups in polymers because of mutual steric, resonance, or electrostatic interactions. 

This is a relatively new technique developed amongst other things, for the determination of repeat groups and end groups in polymers and copolymers, also molecular weight distribution. 

The appearance of sulfonium salts as end groups in polymers produced using these photoinitiators supports this conclusion Among those monomers which undergo facile polymerization with dialkylphenacylsulfonium and dialkyl-4-hydroxyphenyl-sulfonium salts include oxiranes, oxetanes, thiiranes, vinyl ethers and s-trioxane. Polymerizations of tetrahydrofuran, e-caprolactone, a-methylstyrene, and N-vinyl-2-pyrrolidone are very sluggish and often fail using these photoinitiators. 

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