Molecular Weight Control in Linear Polymerization

MOLECULAR WEIGHT CONTROL IN LINEAR POLYMERIZATION 2-6a Need for Stoichiometric Control... 

Anionic polymerizations carried out in aprotic solvents with an efficient initiator may lead to molecular weight control (Mn is determined by the monomer to initiator mole ratio) and low polydispersity indices. The chains are linear and the monomer units are placed head-to-tail. Such polymers are commonly used as calibration samples and for investigation of structure-properties relationships. 

In an elegant recent work, Loos et al. [51, 52] reported the synthesis of polydialanine) via lipase-catalyzed ring-opening of 2-azetidinone (Scheme 5.4) (see Chapter 14 for details on the polymerization mechanism). After removal of cyclic side products and low-molecular-weight species, pure linear poly(P-alanine) was obtained. The average DP of the polymer obtained was limited to 8 because of the solubility of the polymer in the reaction medium. Control experiments with P-alanine as substrate confirmed that the ring structure of the 2-azetidinone was necessary to obtain the polymer. 

The polymerization takes place fairly effectively and is complete within a residence time of 0.5 s. Even if the residence time is much smaller (0.05 s), the polymer can be obtained quantitatively. The molecular weight can be controlled by changing the molar ratio of the initiator to the monomer. The molecular weight (M ) increases linearly with the amount of NBVE, indicating that transfer reactions does not play significant roles in this system (Figure 14.4). 

As a first comprehensive 2D experiment, Kilz et al. [36] described the characterization of complex styrene-butadiene star polymers. The four-arm star polymers were prepared by anionic polymerization to give samples with perfect composition and molecular weight control. The polymerization was designed to yield a mixture of linear (of molar mass M), two-arm (2M), three-arm (3M), and four-arm (4M) species. Four samples with varying butadiene content (about 20,40, 60, and 80%) were prepared. The 2D verification sample was a mixture of four samples with different chemical composition and molar masses. Therefore, this verification sample consisted of 16 components representing the wide range in chemical composition and molar mass. 

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