Slow Growth Polymerization

Table 13. Slow growth polymerization of methyl methacrylate in toluene by BuLi at -78 °C for 24 hr . Ref. >...

During step growth polymerization, the rate of reaction slows down as the reaction progresses. What factors are responsible for this observation ... 

The report of Bestian Clauss that methyltitanium trichloride will undergo a slow growth reaction with ethylene in polar inert solvents at low temperatures (6) is considerably important because of its relation to the Ziegler polymerization reaction. 

Dendritic polymers with enhanced amplification and interior functionality were prepared by Tomalia et al. (3) by slow step-growth polymerization techniques including polyimine formation followed by rapid ring-opening reactions. 

Although the growth of polymer chains results from a free-radical reaction, the reaction rate is relatively slow, resembling that of typical step-growth polymerizations. When the number of double bonds equals the number of SH groups, the system described by Eq. (2.100) can be classified as an A2 (ene) + B4 (thiol) reaction. 

That s our quick and dirty look at step-growth polymerization. The crucial feature of this type of reaction is the slow build-up of chains in a step-wise process. Now let s take a closer look at addition polymerization. 

In step-growth polymerizations, overall costs of monomers, solvent recovery, and preparing the polymer for further processing usually dictate a preference for reactions that are slow at room temperature. (The reasons behind this generalization are summarized in Section 5.3.1.) The ratio of rates of macromolecular growth reactions in typical chain and step-growth polymerizations is often of the order of 10 . ... 

Andrews and Feast d have described a further application of the Patemo-Buchi reaction in which furans are used in step-growth polymerization. Irradiation of m-dibenzoylbenzene and furan produced the monomeric 2 1 adduct (182), which was polymerized further with m-dibenzoylbenzene to produce polymers containing isomeric oxetane units. The step-growth polymerization in this case was limited by low monomer solubility, slow oxetane formation, and hydrogen abstraction processes that led to crosslinking. 

Flory [6] estimated lO collisions between functional groups before a reaction in a typical step growth polymerization. Even in the case of rodlike molecules, the local dynamics are fast though reorientation is a very slow process. Clearly, a brute force simulation of the polymerization process starting with reactive monomers and allowing them to react to form polymers seems infeasible... 

In step-growth polymerization, only one kind of reaction is involved in the formation of a polymer and the reaction proceeds step by step. The main feature of this type of reactions is that two monomers, which bear different functionalities, can react with each other, or with a polymer of any size, through the same kind of reactions. In this case, the individual polymer molecules can grow over the course of the whole process each reaction step of a polymer molecule implies that the reactive end of a monomer or polymer encounters another species with which it can form a link. The functional group at the end of a monomer is usually assumed to have the same reactivity as that on a polymer chain of any size. The polymerizations proceed by the stepwise reaction between the functional groups of reactants as in the reactions described in Chapter 1, Section 1.2.2, of this handbook. The size of the polymer molecules increases at a relatively slow pace in polymerizations that proceed from monomer to dimer, trimer, tetramer, pen tamer, and so on. 

Kinetic considerations are of paramount importance in understanding the mechanism of step-growth polymerization. As stated in Chapter 1, chain-growth polymerizations take place in discrete steps. Each step is a reaction between two functional groups for instance, in a polyesterification reaction it is a reaction between -COOH and -OH. The increase in molecular weight is slow. The first step is a condensation between two monomers to form a dimer ... 

The term step-growth polymerization refers to the process in which the polymer molecular weight increases in a slow, step-like manner as reaction time increases. This polymeri-... 

Important polymers that are produced by polyaddition are polyamide 6 (nylon) and all kinds of polyurethanes. In polycondensation one mol of a small molecule (typically H2O) is liberated per step of chain growths, important polymers that are produced by polycondensation are polyamide 6.6, poly(ethylene terephthalate) (PET), polycarbonate, polyarylate, and polysulfide. Step growth polymerization is usually slow, equilibrium limited and isothermal to slightly exothermic. Polyaddition and polycondensation reactions of monomers with three or more reactive end groups lead to three-dimensionally crosslinked resins. 

At this temperature, the ethylene insertion was sufficiently slow, which allowed us to monitor the quasi-living behavior at the start of typical chain-growth polymerizations. The chain length increased linearly with time, but the increase in the amount of polymer was nonlinear. Assuming quasi-living behavior, the calculation of the number of active centers revealed that only 7% of the chromium centers were active after 10 min. This number increased to 14% after 30 min at -30 °C. 

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