Chains end groups

End Groups and Branching. Both saturated and unsaturated end groups can be formed during polymerization by chain transfer to monomer or polymer and by disproportionation. Some of the possible chain end groups are... 

At the end of the 1930s, the only generally available method for determining mean MWs of polymers was by chemical analysis of the concentration of chain end-groups this was not very accurate and not applicable to all polymers. The difficulty of applying well tried physical chemical methods to this problem has been well put in a reminiscence of early days in polymer science by Stockmayer and Zimm (1984). The determination of MWs of a solute in dilute solution depends on the ideal, Raoult s Law term (which diminishes as the reciprocal of the MW), but to eliminate the non-ideal terms which can be substantial for polymers and which are independent of MW, one has to go to ever lower concentrations, and eventually one runs out of measurement accuracy . The methods which were introduced in the 1940s and 1950s are analysed in Chapter 11 of Morawetz s book. 

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-... 

Hjertberg et al. [72] have studied various PVC samples using Shimizu and Ohtsu s method [69]. They concluded that this method mainly gave a measure of the content of saturated 1,2-dichloroethyl chain end groups, the presence of which has been conclusively demonstrated [73]. With caution they contend there are 0-0.2 head-to-head structures per 1,000 monomer units. 

In some cases it is possible to differentiate between the various alkyl substituents. Primary, secondary and tertiary nitrates and nitrites all show clearly different infrared absorptions. The spectra of acid fluorides can be used to differentiate chain-end groups from pendant acid groups. Furthermore, the loss of all -OH species upon sulfur tetrafluoride exposure allows the reliable estimation of ketones, esters and lactones without the complication of hydrogen-bonding induced shifts in the spectra. Preliminary results from the use of these reactions to characterize y-ray oxidized polyethylene and polypropylene are used to illustrate the scope of the methods. 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

The nature of the chain end groups have also been shown to have a dramatic effect on the glass transition temperature of dendritic macromolecules. A variety of workers have investigated this question by the preparation of chain end... 

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has contributed remarkably to unravelling the termination and initiation steps of the styrene/CO copolymerisation catalysed by the highly active bis-chelated complex [Pd(bipy)2](Pp5)2 in TFE [40]. Chain-end group analysis of the material produced in the absence of BQ showed that the termination by P-H elimination is accompanied by three different initiators two palladium alkyls from Pd-H formed by reaction of the precursor with CO and water (a and b) and a palladium carboalkoxy species formed by reaction of the precursor with the fluorinated alcohol and CO (c) (Chart 7.4). The suppression of the chain-transfer by alcoholysis was proposed to be responsible for the enhanced stability of the palladium acyl intermediates and hence for the high molecular weight of the copolymers produced. 

Incorporation of F-chain end-groups into biodegradable polymers can help modulate their biodegradation (e.g., the hydrolysis rate of polyesters) and drug release [68], Surface treatment of polymer for example, poly(lactic-co-glycolic acid)... 

A study of the intrinsic viscosities of the polymers and of the chain-end groups enabled us to establish that there are processes affecting the molecular weight which are independent of the catalyst concentration. They are now discussed on the basis of experimental data obtained by different tests. 

The termination mechanism involved provides one vinylic chain end group per polymer chain, which can be functionalized. 

The third step is to determine the polypeptide chain end groups. If the polypeptide chains are pure, then only one N-terminal and one C-terminal group should be detected. The amino-terminal amino acid can be identified by reaction with fluorodinitrobenzene (FDNB) (fig. 3.18). Subsequent acid hydrolysis releases a colored dinitrophenol (DNP)-labeled amino-terminal amino acid, which can be identified by its characteristic migration rate on thin-layer chromatography or paper electrophoresis. A more sensitive method of end-group determination involves the use of dan-syl chloride (see Methods of Biochemical Analysis 3B). 

Polypeptide chain end-group analysis, (a) Amino-terminal group identification. A more sensitive method, the dansyl chloride method, is described in Methods of Biochemical Analysis 3B. (b) Carboxyl-terminal group identification. Identification of this amino acid is considerably more difficult. 

The dendrimer type with a stilbene scaffold and long-chain end groups mentioned in Section 4.1.5.3 was shown to have a disc-like flattened molecular shape in solution by SAXS and SANS studies performed by Ballauff et al. [51]. 

Introduced as a chain-end group, the BBN group was converted to an alkoxide group, that is, -0 K+, an efficient initiator for anionic polymerization for ethylene oxide. The ring opening living anionic polymerizations of ethylene oxide were carried out with potassium alkoxide polyolefins to prepare polycthylcnc-block-poly(cthylcnc oxide) (PE-fc-PEO), poly(ethylene-co-styrene)-foZock-poly(ethylene oxide), and poly(ethylene-co-octene)-Mock-poly(ethylene oxide) (EOR-fo-PEO) [37]. 

Polystyrene-b/ock-polyisoprene-idock-polystyrene (SIS) with different compositions were synthesized and cyclized by the coupling reaction between two chain end groups on the same molecules as depicted in Fig. 14 [62], From... 

Figure 11.23 Presumed structure of chain-end groups of polybutadiene and polyisoprene initiated with ethyllithium...

In this case, the occurrence of two types of stereoisomeric form of disyndiotactic polymers, erythro and threo, should be emphasised let us recall that there were no differences in the stereostructure (erythro or threo) of disyndiotactic deuterated poly (a-olefins), e.g. polymers of [> - mo n od e u t e r a t e d 7-olefins, — -CH(R)-CH(2H)-]n—, when omitting different chain end groups. ... 

---