Initial polymerisation degree

In case of rigid polyether polyols, where the polymerisation degree is low (maximum 6-16 PO units/mol of polyol), it is not possible to neglect the initiation step. 

Therefore, initial polymerisation rates of these monomers (as in the case of AMA [60]) grows with length of fluoroalkyl radical, and degree of the gel effect display decreases. 

The biotransformation of pulp causes, in comparison to the initial pulp, several changes as follows decrease in the average polymerisation degree and crystalline degree, reduction of hydrogen bonds, increase in intrinsic surface and increase in the solubility in aqueous sodium hydroxide. 

R = I Qq - number of chain scissions per unit volume normalised by - initial volume degree of crystalhnity rn - average degree of polymerisation of short chains a,p - empirical parameters for ohgomer production... 

N q - initial average degree of polymerisation of amorphous polymers... 

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

In many technical polymerisations transfer reactions to modifier, solvent, monomer and even initiator may occur. In these cases whereas the overall propagation rate is unaffected the additional ways of terminating a growing chain will cause a reduction in the degree of polymerisation. 

The formation of crosslinks in silk fibroin increases the tenacity and resistance to deformation of the fibres, as reflected in the initial modulus and the yield point. This protective effect conferred by fixation of the bifunctional dye Cl Reactive Red 194 was not shown by the monofunctional Orange 16, which is unable to form crosslinks. The loss in tenacity of undyed silk that is observed on treatment at 90 °C and pH 7 for 2 hours is attributable to lowering of the degree of polymerisation (DP) by hydrolysis of peptide bonds. The crosslinking action of bifunctional dyes tends to compensate for this loss in DP and provides an intermolecular network that helps to maintain the physical integrity of the fibre structure [124] ... 

The average degree of polymerisation, DP 5 (DP = n> for cationic polymerisation) is found to be independent of monomer and initiator concentrations. 

The mechanism of initiation of cationic polymerisations by metal halides was clarified and systematized to some extent by the discovery of the phenomenon of co-catalysis or co-initiation. But, whereas there was, by the mid-1960s, good evidence that at any rate in many systems the halides of boron, titanium, and tin required a co-initiator, the position with regard to the best-known and most popular initiator, and the one which was of greatest economic significance, aluminium chloride, remained obscure. Of the vast number of published experiments on the system, aluminium chloride + isobutylene, hardly any could provide evidence concerning the initiation reaction, because they were almost exclusively concerned with measurements of yields and degree of polymerisation (DP). 

In the calorimetric studies, the kinetic acceleration only became apparent when the calorimeter was stabilised to a constant temperature, rather than to a constant pre-cooling rate as had been the practice in the earlier work this improvement in technique had revealed the acceleration. However, the acceleration and the corresponding increase in conductivity were also observed in the isothermal dilatometric studies so that they cannot have been caused simply by the increase in temperature during the adiabatic reactions in the calorimeter. As is well-known [la] with this system, the degree of polymerisation of the polymer increases slightly as the concentration of the initiator is lowered (Table 1). 

The glucose tetra-acetate was used as an initiator and results in each araban chain (average degree of polymerisation 23.6) terminating in a glucosyl reducing unit. 

When the rate of a polymerisation is followed dilatometrically, i.e. by the change in density of the reaction mixture, one makes use of the fact that most monomers are less dense than the corresponding polymers so that the polymerising mixture contracts as the reaction proceeds. Provided that for any one initial monomer concentration there is a unique relation between the degree of conversion Y = and the density p of the solution,... 

The range of monomers which can be employed is largely dictated by the physical chemistry of the emulsion system. For instance, monomers must be sufficiently hydrophobic to allow the formation of stable w/o HIPEs. In addition, most systems which have been studied have used polymerisation methods which require either an initiation step, or addition of a catalyst. This is due to the fact that the first step in the preparation of the polymer is the preparation of HIPE this can only proceed satisfactorily in the absence of any significant degree of polymerisation. Thus, it can be seen that radical addition polymerisation is suitable for the synthesis of PolyHIPE polymers, whereas condensation polymerisation can be more problematical. Also, the latter reactions often generate water as the by-product, hence the aqueous component of the HIPE is inhibiting to the polycondensation. 

While the latter measurements, strictly speaking, evaluate the state of dissociation of the initiator ions only, they also provide at least a guide as to the degree of dissociation of the propagating polymeric ion pairs. In the case of vinyl polymerisations, where no living cations have been observed to date, direct evaluation of the dissociation constant Kd, of the growing ion pair is not possible. However, in a number of cyclic monomer polymerisations living characteristics are observed, and direct measurements have been possible (27). 

The early work of Wessely suggested the occurrence of some side reactions which led to the formation of diketopiperazines (77) or hydan-toins (27). These reactions may put an upper limit to the degree of polymerisation which may be obtained in poly-additions of NCA s but it is more probable that these products arise from an unsuccessful initiation (see p. 40) rather than from a termination. Their participation in the process decreases the yield of C02 evolved in the polymerisation. 

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