Polymers sterically hindered

It should be noted that when an R group, e.g. a methyl group, is present at the internal unsaturated carbon atom in a terminal diene molecule, as in the CH2=C(Me)—(CH2)x-C(Me)=CH2 monomer, the cyclic diene metathesis ceases [7,8]. The steric effect exerted by the R substituent can even be important at the a position to the double bond in the monomer. Sterically encumbering this position hinders polymer formation [9]. 

Coagulation of the polymer particles is hindered by steric stabilization. The nature of the soluble segment of the stabilizer polymer is not important as long as it is miscible with the reaction medium, while the counterpart of the stabilizer... 

Similarly, ring structures, while they might flex a little bit, inhibit rotation. In some cases, such as shown in the bottom of Figure 1.6, large atoms or bulky side groups might bump into each other as the molecule rotates around single bonds. This is called sterically hindered or steric hindrance. Hindered or inhibited rotation stiffens the polymer molecule and dramatically affects its physical properties. 

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

First the protected oligopeptide is coupled with polymer-bound nitrophenol by DCC. N"-Deblocking leads then to simultaneous cycliiation and detachment of the product from the polymer (M. Fridkin, 1965). Recent work indicates that high dilution in liquid-phase cycli-zation is only necessary, if the cyclization reaction is sterically hindered. Working at low temperatures and moderate dilution with moderately activated acid derivatives is the method of choice for the formation of macrocyclic lactams (R.F. Nutt, 1980). 

These polymers could not be crystallized, despite their apparent stereoregularity, probably because of the sterically-hindered character of the chains. It was proposed by Farina and Bressan62 that the chain growth was stereoregulated by the optically active anion of the ion-paired chain carrier. Further studies63 showed that the first portion of the polymer produced in a given reaction always possessed a less regular structure than later portions, unless the reaction was started in the presence of previously prepared polymer. This observation was interpreted as evidence for the pre-... 

As expected, HTMAB made a respectable showing in these experiments. Trioctylmethylammonium chloride (TOMAC) and trioctylmetliylammonium bromide (TOMAB) outperformed all other catalysts. It was postulated that the three octyl groups were the proper length for solvation of the polymer while at the same time small enough to avoid sterically hindering the reaction. In order to determine if TOMAB could be used to catalyze PET depolymerization for more than one treatment cycle, the catalyst was recovered upon completion of one treatment and added to a second run for 60 min. Tetraethylammonium hydroxide (TEAOH) was studied as a catalyst in order to demonstrate the effect of hydroxide ion as a counterion. The percent PET conversion for the second cycle was 85.7% compared to a conversion of 90.4% for the first treatment cycle. 

Nevertheless, the potential uses of these polymers are intriguing, especially since preliminary evidence has been obtained that hydrolysis to glucose, phosphate, and ammonia takes place slowly in aqueous media. Mixed-substituent glucosyl polymers appear to have the greatest biomedical potential since the use of a second, less hindered, side group allows the steric hindrance and deprotection problems to be minimized (37). 

No mention has been made of diene polymers in this discussion. Among 1,4 units the substituents are well separated from one another, and the steric repulsions between them should therefore be negligible. A succession of 1,2 units of isoprene (VIII), on the other hand, would form a chain structure of the sterically hindered type XI. 

Sterically hindered phenols and other additives containing thioesters, phosphites, phosphonites and hindered amine moieties were analysed by FAB-MS and LD-FTMS. The laser desorption technique was preferred for analysis of polymer additives because of undesirable fragmentation from FAB experiments [93]. 

Three factors determine the effectiveness of antioxidants in polymers, namely (i) intrinsic molar activity (ii) substantivity in the polymer and (iii) solubility in the polymer. Multifunctional AOs combine multiple functions in one molecule. Sterically hindered amine stabilisers (HAS), such as Chimassorb 944, Tinuvin 622 and Tinuvin 783 are prime examples. 

The changes with steric hindrance and temperature must be due to conformational effects along the polymer chains. It is now generally agreed that the thermochromism results from an increase in the proportion of trans- to gauche conformations in the polymer chain as the temperature is decreased. Similarly, the introduction of sterically hindering substituents could increase the amount of trans junctions. Evidence in favor of this model is presented in several recent papers, (28i) as well as the chapter by Michl in this volume. 

Polymer formation is still observed with some sterically hindered thiolates, 2,4,6-Ao-propyl thiophenolate, and 2,4,6-trimethyl thiophenolate complexes have been studied. They can be synthesized by the reaction of Zn[N(SiMe3)2]2 with the appropriate arene chalcogenol. With the least steric bulk, the trimethyl derivative forms coordination polymers that can only be dissolved in strongly coordinating solvents. The iso-propyl derivative is more soluble forming oligomers in solution. 1... 

A further effect of sterically hindered amines that could contribute to polymer stabilization has been described by Allan / McKellar31i-lft-41. They reported a conversion of a,3 to 3,Y unsaturated ketones in polyolefins and assumed that this played a role in the photooxidation of the polymers. The conversion... 

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