Single-stage polymerization

In the single-stage production of polyurethanes, di-, tri-, or polyisocyanates are mixed directly with molecules containing two or more reactive hydrogen atoms, such as diols, diamines, and dicarboxylic acids. We typically increase the reaction rate by adding amine or organotin based catalysts. The reaction normally occurs rapidly, evolving much heat. In order to ensure 

The feedstreams can consist of either neat reactants or their solutions. When the feedstreams consist of solutions, the reaction mixture is pumped into a polymerization vessel where the reaction that started in the mixing head proceeds to its conclusion. The polymer is subsequently precipitated from solution, separated, dried, and pelletized. Solvent-free mixtures of reactants are pumped directly to a mold where polymerization proceeds. In this case, other additives, such as, fillers or fire retardants, are co-mixed wth the reactants in the mixing head. These additives are permanently incorporated into the finished molding. 

---

Single stage polymerization generates much more heat than the polymerization of a prepolymer in the chain extension stage of the two stage polymerization process. Why is this so, and what benefit is there in having a lower heat of polymerization ... 

The second method is a high-temperature, single-stage polymerization (Figure 5.24) that involves a polycondensation of bis(o-aminophenol)s with aromatic dicarboxylic acids or dicarboxylic acids... 

With some limitations. Single-stage soapless emulsion polymerization can be used for the direct copolymerization of a relatively apolar monomer with a polar... 

PS/PHEM A particles in micron-size range were also obtained by applying the single-stage soapless emulsion copolymerization method [124]. But, this method provided copolymer particles with an anomalous shape with an uneven surface. PS or PHEMA particles prepared by emulsifier-free emulsion polymerization were also used as seed particles with the respective comonomer to achieve uniform PS/PHEMA or PHEMA/PS composite particles. PS/PHEMA and PHEMA/PS particles in the form of excellent spheres were successfully produced 1 iLitm in size in the same study. 

In this application bimodal pressure-sensitive adhesives consisting of C i C i x polymethacrylates were prepared that exhibited high adhesion to low solubility threshold (LSE) substrates at ambient temperature and at elevated temperature. These materials were free radically prepared in a two-stage single-step polymerization process. 

Leth-Olsen et al. (1) prepared single-stage seed polymerization to form polymer particles having a size distribution between 10 and 100 pm with a narrow size... 

Single stage Multistage Polymerization reactions Convenient to operate Ease of variation or parameters Internal temperature gradients can be high... 

Several fundamental problems are still to be solved, such as the exact knowledge of the structure of the active centers and the mechanism involving the single elementary polymerization stages. Only an in-depth knowledge of these phenomena may bring about further scientific progress. 

---