Free radical polymerization polydispersity index

The minimum polydispersity index from a free-radical polymerization is 1.5 if termination is by combination, or 2.0 if chains ate terminated by disproportionation and/or transfer. Changes in concentrations and temperature during the reaction can lead to much greater polydispersities, however. These concepts of polymerization reaction engineering have been introduced in more detail elsewhere (6). 

This unusual behavior was detected for measurements of polydispersity as well. While polydispersity is close to 2 for DP > 20 as expected for free-radical polymerizations in which there is a high level of chain transfer,40-42 the polydispersity index decreases to values approaching 1.1 —1.2 for DP < 10. To explain this unusual behavior for Cc, low MMA oligomers were separated into fractions by HPLC43 and Cc was calculated for each individual radical, Cc(n)- The resulting dependence of Cc(n) on DP is shown in Figure 5. 

In the case of polycarbonate and other polymers which possess a broad MMD, MALDI fails to give reliable Mn and Myf values. MALDI underestimates The Mn value falls invariably too close to so that MALDI underestimates the polydispersity index. Generally, MALDI cannot be applied to polymers obtained by free-radical polymerization, since free-radical processes yield polymers which possess a broad MMD. For instance, we recorded [62] the MALDI spectrum of a PMMA sample with a polydispersity index of around 2.5 (the averages were % = 13 kDa and = 33 kDa)... 

To prepare degradable polymers, graft copolymers of PLA macromonomer and tert-huXy acrylate were prepared by free radical polymerization. An increase in lactic acid units resulted in an increase in degradation rate [84]. ATRP of MMA (96.5%) and (meth)acrylate-terminated PLA macromonomer (Mn 2800g/mol, 3.5%) yielded a homogeneously branched PMMA-g-PLA of low polydispersity index (PDI = 1.15) [85]. The reactivity ratio of MMA for conventional radical polymerization is 1.09 while with ATRP is 0.57. This accounts for the lower PDI of ATRP synthesized PMMA-g-PLA. 

Vicente et al. [30] used the heat of reaction and the open-loop observers developed in Section 7.2.5.3 to determine the concentration of monomer and CTA and hence to infer the instantaneous number-average molar masses during emulsion homo- and copolymerization reactions. In addition, the authors used the inferred values for online control of the molar mass distributions of copolymers with predefined distributions. They demonstrated that polymer latexes with unimodal MMD with the minimum achievable polydispersity index in free-radical polymerization (PI = 2) and bimodal distributions could be easily produced in linear polymer systems [15, 30]. 

PVA is produced by free radical polymerization and subsequent hydrolysis, resulting in a fairly wide molecular weight distribution. A polydispersity index of 2 to 2.5 is common for most commercial grades. However, polydispersity indices of 5 are not uncommon. The molecular weight distribution is an important characteristic of PVA because it affects many of its properties including crystallizability, adhesion, mechanical strength, and diffhsivity. 

The stable free radical polymerization technique is characterized by the growing polymer chains that are reversibly capped by a stable free radical [e.g., 2,2-tetramethyl-l-piperidynyloxy nitroxide (TEMPO)]. For example, stable polystyrene dispersions were prepared by the stable free radical polymerization of styrene conducted in miniemulsion polymerization at 135 C [62]. Sodium dodecylbenzene sulfonate, hexadecane, and potassium persulfate/ TEMPO were used as the surfactant, costabihzer, and initiator system, respectively. Prodpran et al. [63] studied the styrene miniemulsion polymerization stabilized by Dowfax 8390 and hexadecane and initiated by benzoyl peroxide at 125 °C. A molar ratio of TEMPO to benzoyl peroxide equal to 3 to 1 resulted in polystyrene with the lowest polydispersity index (1.3) of polymer molecular weight distribution. 

Living free radical polymerizations were also carried out in miniemulsion systems via the reversible addition-fragmentation chain transfer mechanism [66]. The colloidal stability of miniemulsions is the key issue, and nonionic surfactants result in the best results. The polydispersity index of molecular weight distribution for the resultant miniemulsion polymer is generally smaller than 1.2. 

The first reported controlled polymerization based on the OMRP-RT principle appears to have been presented by Minoura in a series of articles starting in 1978, where the redox initiating system BPO/Cr was used for the polymerization of vinyl monomers.Not only were the kinetics different than in free-radical polymerization (very low reaction orders in Cif and BPO), but also the polymerization was observed to continue after all Cr had been converted by the peroxide to Cr and the degree of polymerization was found to increase with monomer conversion at low temperatures (<30 0). These studies included the report of a block copolymer (PMMA-b-PAN). Polydispersity indexes were not reported for these studies. Minoura formulated the mechanistic hypothesis of the formation of a metal complex with the free radical and stated that "the recombination of free radicals formed by the dissociation of the complexed radicals competes with a disproportionation of free radicals". However, these studies did not have a great impact in the polymer community, being cited only a handful of times before 1994. A few subsequent contributions reported the application of similar conditions to other metals but well-controlled polymerizations were not found."- " ... 

Figure 1. Polydispersity index of the polymer produced in Interval II of an emulsion polymerization terminated solely by combination as a function of the average number of free radicals per particle...

FIGURE 9.6 Instantaneous and cumulative number- and weight-average chain lengths and the cumulative polydispersity index for an isothermal, free-radical, batch polymerization (Example 9.12). 

---