Intramolecular reaction at gelation

Results for thepolyurethane systems of Figure 6 and Table II are shown in Figure 9, where Mc/M is plotted versus pr,c the extent of intramolecular reaction at gelation(29). Here... 

Figure 1, Ratio of molar mass between elastically effective junctions to front factor (M(-/A) relative to molar mass between junctions of the perfect network (M ) versus extent of intramolecular reaction at gelation (pj- (.) Polyurethane networks from hexamethylene diisocyanate (HDI) reacted with polyoxpropylene (POP) triols at 80°C in bulk and in nitrobenzene solution(5-7,12). Systems 1 and 2 HDI/POP triols >i= 33, V2= 61. Systems 3-6 ...

The results are shown in Figure 4, where Mc/Mc° is plotted versus Pp The molar mass between junction points of the perfect network(Mc°) is calculable from the molar mass and structure of the reactants (1, ) and was evaluated from the measured modulus using Equation 6 with A=l. pr Q is the extent of intramolecular reaction at gelation, given by the... 

Figure 8. Predicted correlations(24)betvi/een reduction in shear modulus at complete reactionTMc/Mc°) extent of intramolecular reaction at gelation(pi- q) for an RA3 polymerisation.

Equation(3) was derived from a generalisation of Kilb s linear sequence(20) for the gelation condition in an RA2 + RB3 polymerisations, as depicted in Figure 4. In equation(5), (f-2) represents the number of opportunities for intramolecular reaction at each branch unit along the linear sequence i = 1, 2, 3,. The sequence grows from Bi through Ai to B and... 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

An expression has been deriued(4)for the extent of reaction at gelation in RA2 + RBf random(13) or condensation polymerisation which accounts more completely than earlier expressions(14-1 )for intramolecular reaction. It may be rearranged to give... 

Wasted Loop Effect An intramolecular reaction involving gelation that does not occur at the predicted value because not all of the branching points on the growing chain are used effectively many multifunctional units in the chain are wasted, and one theory is that the branch point leading out from the multifunctional repeating unit loops back to form a ring with another branch point on the same polymer molecule. (Odian GC... 

The paper first considers the factors affecting intramolecular reaction, the importance of intramolecular reaction in non-linear random polymerisations, and the effects of intramolecular reaction on the gel point. The correlation of gel points through approximate theories of gelation is discussed, and reference is made to the determination of effective functionalities from gel-point data. Results are then presented showing that a close correlation exists between the amount of pre-gel intramolecular reaction that has occurred and the shear modulus of the network formed at complete reaction. Similarly, the Tg of a network is shown to be related to amount of pre-gel intramolecular reaction. In addition, materials formed from bulk reaction systems are compared to illustrate the inherent influences of molar masses, functionalities and chain structures of reactants on network properties. Finally, the non-Gaussian behaviour of networks in compression is discussed. 

Flory [1,2] and also Stockmayer [3,4] used a statistical approach to derive an expression for predicting the extent of reaction at the time where gelation will occur by calculating when Xw approaches infinite size. This statistical approach in its simplest form assumes that the reactivity of all functional groups of the same type is the same and independent of molecular size and shape. It is further assumed that there are no intramolecular reactions between functional groups on the same molecule such as cyclization reactions. 

We have investigated the free-radical polymerization of a variety of diallyl dicarboxylates extensively, especially DAP, DAI, and DAT, in terms of allyl polymerization, cyclopolymerization, and gelation [38]. The cyclopolymerization resulted in cyclization constants Kc of 7.5, 1.6, and 0.6 mol/L at 60 °C for DAP, DAI, and DAT, respectively. In this coimection, it should be noted that in the polymerizations of DAP, DAI, and DAT, two different modes of intramolecular cyclization should be considered. The cyclization reaction of DAP... 

In experimental studies, where the loss of fluidity is taken as marking the gel point, the conversion at the observed gel point is almost always found to be higher than that (calculated) at the theoretical gel point. This can be explained by the model proposed by Bobalek et al (1964) for the gelation process, as shown in Fig. 5.8. According to this model, at the theoretical gel point, a number of macroscopic three-dimensional networks (gel particles) form and undergo phase separation. The gel particles so formed remain suspended in the medium and increase in number as reaction continues. At the experimentally observed gel point, the concentration of gel particles reaches a critical value and causes phase inversion as well as a steep rise in viscosity. The lower value of pc predicted by the statistical approach is also attributed to the occurrence of some wasteful intramolecular cy-clization reactions not taken into account in the derivation and also in some cases to the limited applicability of the assumption of equal reactivity of all functional groups of the same type, irrespective of molecular size. 

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