MWD—See Molecular weight

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

Traditionally, Sephadex has been used for molecular weight analyses of proteins (Determan and Brewer, 1975) and clinical dextran (Granath and Kvist, 1967). Today, Sepharose, Sephacryl, or Superose is used for the assay of broad MWDs. Superdex is the premiere choice for attaining the highest resolution of components of similar size (see Fig. 2.1). 

Figure 1 shows a typical set of GPC traces for poly(IBVE) s obtained at -90°C. The MWD s are narrow (M Mn = 1.4 - 1.7 see Figure 2) and shifting towards higher molecular weights with time no significant broadening can be observed. 

In a very similar way, hydroxy functionalized ATRP initiators such as 2,2,2-tribromoethanol can be used for the simultaneous polymerization of eCL and MMA (Scheme 25) [83]. Purposely, the ROP of eCL is promoted by Al(OfPr)3 added in catalytic amount so that the rapid alcohol-alkoxide exchange reaction (see Sect. 2.4) activates all the hydroxyl functions. In order to avoid initiation by the isopropoxy groups of Al(0/Pr)3. The in-situ formed zPrOH is removed by distillation of the zPrOH/toluene azeotrope. On the other hand, the ATRP of MMA is catalyzed by NiBr2(PPh3)3. The two aforementioned one-step methods provide block copolymers with controlled composition and molecular weights, but with a slightly broad MWD (PDI=1.5-2). 

Under same reaction conditions, multi-site catalysts such as Ziegler catalysts provide a MWD that can be considered as a superposition of a number of Flory-Schulz distributions (see ref. 10). Current results show that even metallocene catalysts can be interpreted as "two-site" catalysts [13-15]. Often, it is not clear whether a certain shift of the molecular weight over the reaction time (at constant reaction conditions) can be attributed to small changes of the hydrogen concentration. 

Mechanical Properties. Mechanical characteristics of ethylene copolymers are functions of their structural characteristics, such as content and type of a-olefin, branching uniformity, molecular weight and width of molecular weight distribution (MWD), and orientation (see Table 5 for properties of films made from three grades of LLDPE). 

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