Polymerisation chain length

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

Fig. 22.1. (a) The ethylene molecule or monomer (b) the monomer in the activated state, ready to polymerise with others (<)-(f) the ethylene polymer ("polyethylene") the chain length is limited by the addition of terminators like —OH. The DP is the number of monomer units in the chain. 

Bulk polymerisation is heterogeneous since the polymer is insoluble in the monomer. The reaction is autocatalysed by the presence of solid polymer whilst the concentration of initiator has little effect on the molecular weight. This is believed to be due to the overriding effect of monomer transfer reactions on the chain length. As in all vinyl chloride polymerisation oxygen has a profound inhibiting effect. 

The cyclic trimer (trioxane) and tetramer are obtained by a trace of sulphuric acid acting on hot formaldehyde vapour (i) Figure 19.1). Linear polymers with degrees of polymerisation of about 50 and a terminal hydroxyl group are obtained by evaporation of aqueous solutions of formaldehyde (ii). In the presence of strong acid the average chain length may be doubled. Evaporation of methanol solution leads to products of type (iii). 

In the absence of any transfer, only the type of termination reaction has to be taken into account to obtain the number average degree of polymerisation (or chain length) Xn. Indeed, if the reaction terminates by addition, two radicals give one chain, while if the reaction terminates by disproportionation, one radical generates one chain. Using 0.5 < x < 1, one obtains... 

Equation (4) clearly shows that the number average degree of polymerisation Xn is inversely proportional to the reaction rate Rp, meaning that, in radical chain polymerisation high reaction rates are linked to low molecular masses and vice versa. One way to avoid this dilemma is to use emulsion polymerisation where the lifetime of a radical (i.e., the "kinetic" chain length) is independent of... 

As there is one functional group of each type per chain, one can write for Xn (number average chain length or degree of polymerisation)... 

Fig. 10.7 RNA synthesis in vesicles. Membrane permeability can be regulated by choosing the correct chain length of the fatty acids in the phospholipids. Short chains (a) make the bilayer so unstable that even large molecules such as proteases can enter the vesicle interior and damage the polymerase. Carbon chains which are too long (b) prevent the entry of substrate molecules such as ADR RNA polymerisation in the vesicle occurs only with C14 fatty acids (c)...

In order to investigate, for a monodisperse distribution of the chain length, the effect of the degree of polymerisation on the fibre strength we write Eq. 89 as... 

For the same values of ea g, r0 and u0 as for the monodisperse distribution, Figs. 45 and 46 show the results for the Flory distribution of chain lengths. The curves start at degrees of polymerisation determined by zn=[(2a)-1+l]. A comparison of Fig. 45 with Fig. 42 shows that, for a diameter equal to the chain di-... 

Fig. 45 Ultimate strength of PpPTA fibres versus the degree of polymerisation applying the Flory distribution of chain lengths for various values of the diameter 2 r calculated with Eq. 93...

The Molecular weight of the polymer is related to its chain length or to the degree of polymerisation (DP). 

Except the method of endgroup analysis all the above methods are based on the determination of properties of polymer solution. The Molecular weight of the polymer is related to its chain length or to the degree of polymerisation (DP) defined as the ratio between the Molecular weight of polymer and that of the monomer. 

Polymerisa- tn chain length or chain growth can take place through the formation of either free radicals or ionic species. However, the free radical governed addition or chain growth polymerisation is the most common mode. 

Van der Waals radii, 8 Vibrational modes, 342 Vinyl ethers, polymerisation, 189 Vinyl polymerisation, 320 branching in, 321 chain length in, 321 coordination, 322 induction period in, 321... 

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