Initiator residue

Free-radical polymerization is the preferred iadustrial route both because monomer purification is not required (109) and because initiator residues need not be removed from polymer for they have minimal effect on polymer properties. 

In earlier investigations chain ends were suggested to be important initiation sites for dehydrochlorination. Provided there are no transfer reactions during polymerization, at least half the polymer chain ends will carry initiator fragments. In practice, transfer reactions swamp the normal termination processes and <30% of the chain ends carry initiator residues [59]. 

The low concentration of initiator residues in polymers formed by radical polymerization means that they can usually only be observed directly in exceptional circumstances or in very low molecular weight polymers (Section 3.5.3). Thus, the study of the reactions of initiator-derived radicals with monomers has seen the development of some novel techniques. Three basic approaches have been employed. These involve ... 

Selective labeling of the initiator with 13C allow s substantial enhancement of the signals of the initiator residues relative to signals due to the backbone in l3C NMR spectra. Initiators labeled with or containing NMR active nuclei such as 19F or J P can also be applied. These methods are described in Section 3.5.4.2,... 

NMR methods can be applied to give quantitative determination of initiator-derived and other end groups and provide a wealth of information on the polymerization process. They provide a chemical probe of the detailed initiation mechanism and a greater understanding of polymer properties. The main advantage of NMR methods over alternative techniques for initiator residue detection is that NMR signals (in particular nC NMR) are extremely sensitive to the structural environment of the initiator residue. This means that functionality formed by tail addition, head addition, transfer to initiator or primary radical termination, and various initiator-derived byproducts can be distinguished. 

On plant surfaces, as in soils, numerous studies have demonstrated that endosulfan is oxidized to endosulfan sulfate. Initial residues of endosulfan on treated vegetables generally range from 1 to 100 mg/kg. However, residue levels typically decrease to less than 20% of initial levels within 1 week after treatment (NRCC 1975). Residues of endosulfan isomers are generally negligible after 2-3 weeks the a-isomer is much less persistent than the P-isomer. In most plant residue studies, endosulfan sulfate residue levels tend to increase relative to the parent isomers and other metabolites and appear to be very persistent (Coleman and Dolinger 1982). 

On Delicious apples the initial residues were 0.9 and 0.4 p.p.m. in plots 7 and 10, respectively, both of which were sprayed with 1.25 ounces of parathion. The difference between the two plots was consistent throughout the individual trees sampled. The spray mixture used on plot 7 also contained DDT, while that used on plot 10 contained only parathion. These plots showed the same relative magnitude of residues 18 days after spraying, and at harvest, 74 days after the spraying. Plot 11, sprayed with 0.6 ounce of parathion. showed an initial residue of 0.3 p.p.m. 

Initial residues on the Winesap apples were somewhat higher than those on Delicious. Six days later approximately one third of the residue had been lost, and 12 days after the spraying the deposits were down to 0.10 and 0.03 p.p.m. for the respective strengths. At harvest little more than a trace of parathion was found on the fruit. 

The quantitative estimation of available residue data from the published literature may be investigated in a further step, and these data may then be used to calculate the initial residue on the day of application (after drying of the spray mixture). An assessment of literature data by Schrader (1994) has revealed an initial dislodgeable foliar residue (DFR) below or around 1 ug/cm2/kg active substance (a.s.)/ha, if the residue values are corrected for a standard rate of 1 kg a.s./ha. 

Because half-lives may vary substantially, the values of the actual DFR available for transfer to the worker will diverge with prolonged re-entry time. For example, the difference between the actual DFRs will increase approximately 3% per day for both pesticides in our study. These pesticides have long half-lives which differ by a factor of 2, assuming a similar initial residue deposit therefore, the half-life of a pesticide must be considered a critical factor for the level of re-entry exposure. Increase of re-entry time will decrease the transfer factor however, the decrease observed in this study is small and negligible compared to all other factors and variances that influence re-entry exposure. 

The majority of commercial polystyrene molecules consist of a backbone of carbon atoms with phenyl groups attached to half of the carbon atoms, as shown in Fig. 21.1. Free radical initiator residues terminate each end of the chain. Minor variants include chains terminated by anionic or cationic initiator residues. All commercial polystyrene products are atactic that is, the placement of the phenyl groups on either side of the chain is essentially random, as illustrated in Fig. 21.2. 

Spinach, Spinacea oleracea, initial residue of 9.5 mg/kg FW Initial residue degraded to 2.8 mg/kg in 15 days, and ND in 30 days 4... 

Sugarcane, Saccharum officianarum, initial residue immediately after application was 18.8—28.2 mg/kg dry weight leaf Residues after 7 days were 2.1-5.4 mg/kg DW Tb 1/2 of 2.2-2.4 days 5... 

The initial residual, i.e., the initial vector of the functions we are trying to cancel is... 

Thus initiator residues get incorporated into the polymer at one or both the ends of the Polymer chain. These residues do not have any appreciable effect on properties of polymers. 

Stjemdahl A, Wistrand AF, Albertsson A-C (2007) Industrial utilization of tin-initiated resorbable polymers synthesis on a large scale with a low amount of initiator residue. Biomacromolecules 8 937-940... 

To compare the catalytic effectiveness of our polymer with that reported for other substances that accentuate nitrophenyl ester cleavage, we26 have carried out a series of experiments (at pH 7.3) in which the residue molar concentration of polymer imidazole groups was substantially in excess of the concentration of substrate, p-nitrophenol acylate. Pseudo-first-order rate constants k[ were determined at each of a number of polymer concentrations. Under these conditions k[ was found to be linear with [P-Im]0, the initial residue concentration of methylene-imidazole groups ... 

More recently, the chemo-enzymatic synthesis of inulin-containing hydrogels was reported [54]. The key point was the solubility of inulin [a mixture of oligomers and polymers containing 2-60 (or more) 5-2,1 linked D-fructose molecules having a glucose unit as the initial residue] in dimethylformamide (DMF), a fact that allowed its esterification by action of a protease from Bacillus subtilis. 

Unlike spiked penicillin G, incurred residues in bovine plasma showed a slower rate of degradation. The half-life was estimated at 276 days for storage at 20 C (24). Further experimentation showed that incurred residues in tissues stored at 20 C decreased more rapidly than in plasma. Losses of up to 20% were observed in kidney and liver samples stored at 20 C after as little as 10 days of storage. On the other hand, gluteal muscle samples stored at the same temperature showed within 10 days of storage losses of nearly 50% of the initial residue present. However, penicillin G residues in tissue samples stored at 76 C remained stable. 

Methionine Has Only One Codon Methionine is one of two amino acids with only one codon. How does the single codon for methionine specify both the initiating residue and interior Met residues of polypeptides synthesized by E. coli ... 

The use of alkyllithium initiators which contain functional groups provides a versatile method for the preparation of end functionalized polymers and macromonomers. For a living anionic polymerization, each functionalized initiator molecule produces one macromolecule with the functional group from the initiator residue at one chain end and the active carbanionic propagating species at the other chain end. 

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