Linear scission

Nyrkova, I.A., and Semenov, A.N. "Non-linear scission-recombination kinetics of living polymerization". Eur. Phys. ]. 24,167-183 (2007). 

Finally, in Figure 9.11 we show MWDs for two different scission models. The linear scission case assumes scission of unbranched chains. The topological scission case employs the fragment length function of Eq. (79). A marked difference is observed. 

Fig. 9.11. Radical polymerization of ethylene in a CSTR. Linear and topological scission with the same polydispersity of 26 as experimental MWD from SEC-MALLS [35], Solid line, experimental MWD dash-dot line, topological scission result dash-dot-dot line, linear scission result.

As pointed out by Flory [16], the principle of equal reactivity, according to which the opportunity for reaction (fusion or scission) is independent of the size of the participating polymers, implies an exponential decay of the number of polymers of size / as a function of /. Indeed, at the level of mean-field approximation in the absence of closed rings, one can write the free energy for a system of linear chains [11] as... 

Fig. 50. Yield for chain scission as a function of strain rate for different fractions of polycarbonate (PC) in benzyl alcohol/dioxan (90 10 v.v) at 20 °C. A normal PC with Mp = 417000 B normal PC with Mp = 321000 C normal PC with Mp = 256000 D PC with weak bonds, Mp = 217000 Mp molecular weight at peak maximum sc critical strain rate for chain scission (extrapolated from the linear portion of the degradation curve)...

The recombination of fragments stemming from one macromolecule, at times shorter than the diffusion time, prevents the linear increase in RD with the absorbed dose per pulse, as not all main-chain scissions result in the formation of fragments. The effect of molecular oxygen on RD in the case of PBS can be interpreted by formation of peroxyl radicals, e.g. 

An alternative explanation suggested by the authors for the non-linearity of R° with dose is the formation of reactive solvent species capable of intercepting the scission reaction, with a yield which becomes greater the higher the absorbed dose per pulse. However, this mechanism does not explain the effect of oxygen. 

Because of the absence of chain limiter, the catalyst itself may initially act as the chain limiter (Fig. 8.22). The catalyst reacts with the olefinic regions of the polymer backbone and causes chain scission to occur, forming two new chains. The reactive carbene which is produced then moves from chain to chain, forming two new chains with each scission until the most probable molecular weight distribution is reached (Mw/Mn = 2), producing linear chains end capped with [Ru] catalyst residues. 

Other more complex linear block co-, ter- and quarterpolymers, such as ABC, ABCD, ABABA can be prepared using the previously mentioned methods. An important tool in the synthesis of block copolymers involves the use of post-polymerization chemical modification reactions. These reactions must be performed under mild conditions to avoid chain scission, crosslinking, or degradation, but facile enough to give quantitative conversions. Hydrogenation, hydrolysis, hydrosilylation and quaternization reactions are among the most important post-polymerization reactions used for the preparation of block copolymers. 

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

The two polymer substrates investigated as part of the study of DBDPO mixtures were polypropylene (PP) and linear high density polyethylene (HDPE). while both PP and HDPE decompose by similar random chain scission, radical mechanisms, chain transfer occurs much more teadily during the pyrolysis of PP because of the presence of the tertiary hydrogens. In addition, only primary chain end radicals are formed when the HDPE chain cleaves homolytically. Therefore, a comparison of the PP/DBDPO and the HDPE/DBDPO mixtures volatile product distributions was undertaken. 

Hydrocarbons with up to 16 carbon atoms are detected in a typical alkylate (82). With the liquid acids, it was found that the oligomerization rate is higher for isoalkenes than for linear alkenes (49). The same is true for solid acids (14,83). Because of their tertiary carbon atoms, isobutylene and isopentene obviously react more easily with carbenium ions. This point can be inferred from the reverse reaction, (3-scission (see below), which is fastest for reactions of tertiary cations to give tertiary cations. In oligomerization experiments, the following pattern of... 

Equations have been derived which relate G(scission) and G(crosslinking) to changes in Mn, Mw and Mz. Crosslinking produces branched molecules and the relative hydrodynamic volume (per mass unit) decreases compared with linear molecules. Therefore, molecular weights derived from viscometry and gel permeation chromatography will be subject to error. 

Some polymer materials, particularly biomedical materials and step-growth polymers, comprise crosslinked networks. The effect of irradiation on networks, compared with linear polymers, will depend on whether scission or crosslinking predominates. Crosslinking will cause embrittlement at lower doses, whereas scission will lead progressively to breakdown of the network and formation of small, linear molecules. The rigidity of the network, i.e. whether in the glassy or rubbery state (networks are not normally crystalline), will affect the ease of crosslinking and scission.. ... 

The calibration standards included sodium form polystyrene sulfonates obtained from Pressure Chemical Co., Pittsburgh, Pa., and sodium toluene sulfonate. Measurements were taken at 0.5 to I.Oml/mln flow rates. The logarithm of the molecular weight of the standards was linear it suggests a framework for approaching an interpretion of the structure of the scission products. This application of size exclusion chromatography measurements must be viewed as a first approximation because of the unmeasured differences between the chromatographic behavior of the linear standards and the expected branched structure of the scission products. 

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