Steric effect polymer reaction

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The steric effects in isocyanates are best demonstrated by the formation of flexible foams from TDI. In the 2,4-isomer (4), the initial reaction occurs at the nonhindered isocyanate group in the 4-position. The unsymmetrically substituted ureas formed in the subsequent reaction with water are more soluble in the developing polymer matrix. Low density flexible foams are not readily produced from MDI or PMDI enrichment of PMDI with the 2,4 -isomer of MDI (5) affords a steric environment similar to the one in TDI, which allows the production of low density flexible foams that have good physical properties. The use of high performance polyols based on a copolymer polyol allows production of high resiHency (HR) slabstock foam from either TDI or MDI (2). 

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. 

Assuming that all B groups have the same reactivity, the chemical reaction giving rise to a branched molecule is identical to the reaction resulting in a linear polymer. Statistically this will eventually result in a hyperbranched polymer. However, dependent on the chemical structure of the monomer, steric effects might favor the growth of linear polymers. Computer simulations of of ABX-monomer condensation and AB -monomers co-condensed with B-functional... 

In addition to the coordination and the binding the steric effect caused by the polymer legand influences the chemical reaction in metal-containing polymer catalysts one is the specificity in the coordination and the other is the entropic effect in the active center. The former would control the catalytic activity of the active center. 

Fluoride-ion-induced reactions A similar polymer to that in Figure 7.87 is obtained upon anionic polymerisation of hexafluoro-2-butyne initiated by fluoride ion in a solvent [311-313] (Figure 7.89). This is a clear example of an anionic polymerisation of an unsaturated fluorocarbon, although the growing anion can be trapped by a sufficiently reactive system [291, 314], such as pentafluoropyridine [315] (Figure 7.90). There is little difference between the ultraviolet spectra of 7.90A and 7.90B, confirming that conjugation in the polyene system is inhibited by steric effects. 

Steric influences may retard some radical polymerizations and copolymerizations. Double bonds between substituted carbon atoms are relatively inert (unless the substituents are F atoms) and 1,2-substituted ethylenes do not homopolymerize in normal radical reactions. Where there is some tendency of such monomers to enter into polymers, the trans isomer is more reactive. When consideration is restricted to monomers that are doubly substituted on one carbon atom, it is usually assumed that steric effects can be neglected and that the influence of the two substituents is additive. Thus vinylidene chloride is generally more reactive in copolymerizations than is vinyl chloride. 

In the absence of steric effects the activation energy of the form I — form II transition should approximate the energy barrier to rotation about the peptide linkage (21 kcal/mole) provided that mutarotation involves the proposed cis — froas-isomerization. Downie and Randall (1959) measured the rates of forward mutarotation of poly-L-proline I in acetic acid at various temperatures and obtained an activation energy, AE = 23 kcal/mole. The rate of the reaction was independent of concentration over a sevenfold dilution of the polymer. That is, at any stage of mutarotation (as measured by [ ][,) the velocity constant, fc, was found to be independent of concentration. On the other hand k decreased from 15 X 10 sec to 2.5 X 10 sec during the course of mutarotation. 

The unpaired electron is thought to reside in a pure p-orbital so that RiR2R3C- is a planar molecule. R4R5C=CRgR7 is also planar with a filled jr-orbital so that reaction can occur only on parallel approach of the reactants from above or below the plane of the RjRgRgC radical. Steric effects due to the bulkiness of the substituents R to Ry markedly reduce the rate of reaction. For example 1,2-disubstituted olefin monomers are known to be unreactive in homopolymerization (5). A polymer crystal lattice must have a similar effect if the reactive free radical end of the molecule is fixed in the crystal lattice. 

Best yields from alkylbenzenes were obtained from toluene and t-butylbenzene with all three blsmaleimldes. This probably reflects a balance between Inductive effects, which would favor both 2+2 and 2+4 cycloaddltlons in the order -butyl> i-propyl ethyl> methyl, and steric effects which would hinder the reaction In the same order. Additional steric effects a-rlslng from the ortho methyl groups of N,N -4,4 -(3,3 -dimethylbiphenyl)bismaleimide are evident since no polymer was formed from this monomer with ethylbenzene or 1-propylbenzene (Zbubanov and Akkulova reportediz their best yields were obtained with 1,10-decamethYleneblsmalelmlde and 1-propylbenzene.)... 

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