Free radical polymerization narrow molecular weight distribution

The anionic homopolymerization of polystyrene macromonomers was carried out successfully. The methacrylic ester sites at the chain end do not require very strong nucleophiles to be initiated diphenylmethylpotassium was used, and the process was carried out at — 70 °C in THF solution24). The products are comparable with those obtained by free-radical polymerization. The molecular weight distribution should be narrower but this cannot be easily checked because these polymer species are highly branched and compact as already mentioned. 

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

Currently, more SBR is produced by copolymerizing the two monomers with anionic or coordination catalysts. The formed copolymer has better mechanical properties and a narrower molecular weight distribution. A random copolymer with ordered sequence can also be made in solution using butyllithium, provided that the two monomers are charged slowly. Block copolymers of butadiene and styrene may be produced in solution using coordination or anionic catalysts. Butadiene polymerizes first until it is consumed, then styrene starts to polymerize. SBR produced by coordinaton catalysts has better tensile strength than that produced by free radical initiators. 

In 1993, Georges and coworkers [23,202,203] first succeeded in the synthesis of poly(St) with a narrow molecular weight distribution through the free-radical polymerization process of St. The polymerization was carried out in the presence of BPO and 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) ... 

Controlled free-radical polymerization methods, like atom-transfer radical polymerization (ATRP), can yield polymer chains that have a very narrow molecular-weight distribution and allow the synthesis of block copolymers. In a collaboration between Matyjaszewski and DeSimone (Xia et al., 1999), ATRP was performed in C02 for the first time. PFOMA-/)-PMMA, PFOMA-fr-PDMAEMA [DMAEMA = 2-(dimethylamino)ethyl methacrylate], and PMMA-/)-PFOA-/)-PM M A copolymers were synthesized in C02 using Cu(0), CuCl, a functionalized bipyridine ligand, and an alkyl halide initiator. The ATRP method was also conducted as a dispersion polymerization of MMA in C02 with PFOA as the stabilizer, generating a kine-... 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

Initiation is the slow reaction in the initiation-propagalion-termination sequence in free radical reactions whereas selected initiation reactions in anionic systems can be very rapid compared to the subsequent propagation reaction. This facilitates the preparation of anionic polymers with narrow molecular weight distributions, if the polymerization is conducted carefully. 

The absence of termination during a living polymerization leads to a very narrow molecular-weight distribution with polydispersities as low as 1.06. By comparison, polydispersities above 2 and as high as 20 are typical in free radical polymerization. 

Atom transfer free-radical polymerization (ATRP) proceeds by a transient/ stable radical mechanism analagous to nitroxide-mediated free-radical polymerizations (see Section 3). This controlled polymerization concept was first described independently by two research groups in 1995, and exhibits a high degree of control over the molecular weight of the desired polymer and more remarkably, the ability to realize very narrow molecular weight distributions < 1.05). ATRP methodologies involve (see... 

As ionic polymerizations with stringent reaction conditions are more difficult to carry out than normal free-radical processes, the latter are invariably preferred where both free-radical and ionic initiations give a similar product. For example, commercial polystyrenes are all free-radical products, though styrene polymerization can be initiated with free radicals as well as with appropriate anions or cations. However, to make research grade polystyrenes with exceptionally narrow molecular-weight distributions and di-block or multi-block copolymers of styrene and other monomers, ionic processes are necessarily employed. 

The use of stable free-radical polymerization techniques in CO2 represents an emerging new area of research. Odell and Hammer have demonstrated the use of reversibly terminating free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6,-tetramethyl-l-piperidinyloxy free radical (TEMPO) to polymerize styrene at a temperature of 125°C and pressures of 245-280 bar in CO2 [92]. At low monomer concentrations (10% by volume), the polymerization resulted in low conversions of PS with an of about 3000 g/mol and a narrow molecular weight distribution (PDI <1.3). NMR analysis of the resulting polymer confirmed that the precipitated polystyrene chains are predominantly end-capped with TEMPO. Additionally, the polymer could be isolated and later extended by the addition of more monomer under an inert argon blanket. It was also determined that the precipitated PS could be extended while still in the CO2 continuous phase simply by increasing monomer concentration in the reactor. 

Over the past few years there has been a tremendous interest in living radical polymerizations. One type of living radical polymerization is stable free radical polymerization, SFRP, where a stable free radical such as TEMPO (2,2,6,6-tetramethylpiperidinoxyl) is used to reversibly cap the growing polymer chain (L2). SFRP has the advantage over conventional radical polymerization in that the polymers prepared are living and can be used for further polymerization to make blocks or other complex architectures. The polymers prepared by the SFRP process have a narrower molecular weight distribution compared to polymers prepared by conventional radical polymerization in the case of block copolymers this may be a desirable attribute. This article focuses on the use of the SFRP process to prepare random copolymers. 

Chain-transfer polymerizations (telomerizations), which were developed to produce polymers of narrower molecular weight distributions than in conventional free-radical polymerization, of poly-N-isopropylacrylamide (PNIPAM), poly-N,N-dimethylacrylamide (PNDMAM), and poly-N- 3-(dimethylamino)propyl acrylamide... 

The advantage of conducting the precipitation polymerization in supercritical fluids is the ease with which the unreacted monomer can be recovered from the reaction medium and the ease of recovering the produced polymer from the solvent. Free-radical polymerization in SCF hydrocarbon solvents makes use of the relationship between solvent power and SCF density to alter the threshold of precipitation of the polymer chains and also to minimize the swelling of the precipitate. This process produces polymers with controlled molecular weight with a narrow molecular weight distribution. 

In each of these mechanisms, the reverse reaction dominates the equihbriimi and keeps the overall concentration of the propagating radical (P ) low, typically [Pn ]/[Pn— X] < 10-5. If tijis reversible radical trapping process occurs frequently, it minimizes the irreversible termination reactions but also means that the polymer chains all have an equal chance to grow, resulting in polymers with a narrow molecular weight distribution. It also follows that, unlike conventional free-radical polymerizations, the polymer chain length will increase steadily with the reaction time, similar to living anionic polymerizations. 

CRP provides a versatile route for the preparation of (co) polymers with controlled molecular weight, narrow molecular weight distribution (i.e., Mw/Mn, or PDI < 1.5), designed architectures, and useful end-functionalities. Various methods for CRP have been developed however, the most successful techniques include ATRP, stable free radical polymerization, " and reversible addition fragmentation chain transfer (RAFT) polymerization. " " CRP techniques have been explored for the synthesis of gels " " and cross-linked nanoparticles of well-controlled polymers in the presence of cross-linkers. 

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