Monomer addition, sequential

Sequential addition of monomer works well in anionic polymerization for producing well-defined block copolymers [Morton, 1983 Morton and Fetters, 1977 Quirk, 1998 Rempp et al., 1988]. An AB diblock copolymer is produced by polymerization of monomer A to completion using an initiator such as butyllithium. Monomer B is then added to the living polyA carbanions. When B has reacted completely a terminating agent such as water or 

the synthesis of a styrene-methyl methacrylate block polymer requires that styrene be the first monomer. Further, it is useful to decrease the nucleophilicity of polystyryl carbanions by adding a small amount of 1,1-diphenylethene to minimize attack at the ester function of MMA [Quirk et al., 2000]. Block copolymers of styrene with isoprene or 1,3-butadiene require no specific sequencing since crossover occurs either way. Block copolymers of MMA with isoprene or 1,3-butadiene require that the diene be the first monomer. The length of each segment in a block copolymer is controlled by the ratio of each monomer to initiator. The properties of the block copolymer vary with the block lengths of the different monomers. 

Difunctional initiators such as sodium naphthalene are useful for producing ABA, BABAB, CAB AC, and other symmetric block copolymers more efficiently by using fewer cycles of monomer additions. Difunctional initiators can also be prepared by reacting a diene such as /n-diisoprope ny I benzene or l,3-bis(l-phenylethenyl)benzene with 2 equiv of butyl-lithium. Monomer B is polymerized by a difunctional initiator followed by monomer A. A polymerizes at both ends of the B block to form an ABA triblock. BABAB or CABAC block copolymers are syntehsized by the addition of monomer B or C to the ABA living polymer. The use of a difunctional initiator is the only way to synthesize a MMA-styrene-MMA triblock polymer since MMA carbanion does not initiate styrene polymerization (except by using a coupling reaction—Sec. 5-4c). 

In addition to the triblock thermoplastic elastomers, other useful copolymers of styrene with a diene are produced commerically by living anionic polymerization. These include di-and multiblock copolymers, random copolymers, and tapered block copolymers. A tapered (gradient) copolymer has a variation in composition along the polymer chain. For example, S-S/D-D is a tapered block polymer that tapers from a polystyrene block to a styrene-diene random copolymer to polydiene block. (Tapered polymers need not have pure blocks at their ends. One can have a continuously tapered composition from styrene to diene by 

Clear impact-resistant polystyrene is a commercial plastic with the desirable combination of toughness and exceptional clarity. It is a styrene-1,3-butadiene multiblock copolymer containing more than 60% styrene. Most of these products are mixtures of block copolymers formed by incremental additions of initiator and monomers followed by coupling (Sec. 5-4c). The products generally have a tapered and multiblock composition with branching (due to the coupling agent). 

The living polymer technique is particularly suitable for preparing block copolymers by sequential addition of monomers to a living anionic polymerization system. Depending on whether monofunctional or difunctional initiators are used, one or both chain ends remain active after monomer A has completely reacted. Monomer B is then added, and its polymerization is initiated by the living polymeric carbanion of polymer A. This method of sequential monomer addition can be used to produce block copolymers of several different types, as classified above. 

Monofunctional Initiators Using monofunctional initiators (e.g., n-butyllith-ium), AB, ABA, and multiblock copolymers can be formed. For example, the synthesis of an AB block. copolymer can be shown schematically as 

The monomers are to be added in proper order. For example, to prepare an AB type block copolymer of styrene and methyl methacrylate, styrene is polymerized first using a monofiinctional initiator and when styrene is fully consumed, the other monomer MMA is added. The copolymer cannot be made by polymerizing MMA first because living poly(methyl methacrylate) is not basic enough to add to styrene. The length of each block in the copolymer is determined by the amount 

Bifunctional Initiators Bifunctional initiators like alkali metal complexes of polycyclic aromatic compounds (e.g., naphthalene and biphenyl) can be used to produce ABA triblock copolymers. In 

Thermoplastic elastomers of this type usually have molecular weights in the range 50,000 to 70,000 for the polybutadiene blocks and 10,000 to 15,000 for the polystyrene blocks. 

---

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

Though living anionic polymerization is the most widely used technique for synthesizing many commercially available TPEs based on styrenic block copolymers, living carbocationic polymerization has also been developed in recent years for such purposes [10,11], Polyisobutylene (PlB)-based TPEs, one of the most recently developed classes, are synthesized by living carbocationic polymerization with sequential monomer addition and consists of two basic steps [10] as follows ... 

Tsunogae Y. and Kennedy J.P., Thermoplastic elastomers by sequential monomer addition. VI. Poly(p-methylstyrene-b-isobutylene-b-/7-methylstyrene), Polym. Bull., 31, 1436, 1993. 

Multiblock copolymeric structures containing PCHD blocks were also synthesized using s-BuLi as the initiator and either TMEDA or DABCO as the additive. Sequential monomer addition was performed with CHD being the last monomer added in all cases [35]. The structures prepared are PS-b-PCHD, PI-fc-PCHD and PBd-b-PCHD block copolymers, PS-fo-PBd-fo-PCHD, PBd-fr-PS-b-PCHD and PBd-fo-PI-fr-PCHD triblock terpolymers, and PS-fc-... 

The direct synthesis of poly(3-sulfopropyl methacrylate)-fr-PMMA, PSP-MA-fr-PMMA (Scheme 27) without the use of protecting chemistry, by sequential monomer addition and ATRP techniques was achieved [77]. A water/DMF 40/60 mixture was used to ensure the homogeneous polymerization of both monomers. CuCl/bipy was the catalytic system used, leading to quantitative conversion and narrow molecular weight distribution. In another approach the PSPMA macroinitiator was isolated by stopping the polymerization at a conversion of 83%. Then using a 40/60 water/DMF mixture MMA was polymerized to give the desired block copolymer. In this case no residual SPMA monomer was present before the polymerization of MMA. The micellar properties of these amphiphilic copolymers were examined. 

ABC, ACB, and BAC triblock terpolymers, where A is PMMA, B is PDMAEMA, and C is poly[hexa(ethylene glycol)methacrylate], PHEGMA, were synthesized via GTP and sequential monomer addition [89]. The polymerizations were conducted in THF using MTS and TBABB as the initiator... 

A series of bis(phenoxide) aluminum alkoxides have also been reported as lactone ROP initiators. Complexes (264)-(266) all initiate the well-controlled ROP of CL, NVL.806,807 and L-LA.808 Block copolymers have been prepared by sequential monomer addition, and resumption experiments (addition of a second aliquot of monomer to a living chain) support a living mechanism. The polymerizations are characterized by narrow polydispersities (1.20) and molecular weights close to calculated values. However, other researchers using closely related (267) have reported Mw/Mn values of 1.50 and proposed that an equilibrium between dimeric and monomeric initiator molecules was responsible for an efficiency of 0.36.809 In addition, the polymerization of LA using (268) only achieved a conversion of 15% after 5 days at 80 °C (Mn = 21,070, Mn calc 2,010, Mw/Mn = 1.46).810... 

This was the first report using ATRP and sequential monomer addition. Hydrolysis of these diblock copolymer brushes yielded poly(styrene-fc-acrylic acid) brushes. 

ATRP is a very potent method for preparing block copolymers by sequential monomer addition as well as star polymers using multifunctional initators. Furthermore, it can be applied also in heterogenous polymerization systems, e.g., emulsion or dispersion polymerization. In Example 3-15 the ATRP of MMA in miniemulsion (see also Sect. 2.2A.2) is described. 

This observation is corroborated with what has been found in Figures 8-10. There is more of an inversion phenomenon occurance at 20°C. However, the difference between 30°C and 40°C is small and apparently similar, within experimental error. Nevertheless, the new established reactivity ratios of butadiene and isoprene at all three temperatures differ by a smaller factor than what were reported by the work of Korotkov (8) (e.g. rj - 3.38 and 2 = 0.47). Moreover, butadiene is more reactive and initial copolymer contains a larger proportion of butadiene randomly placed along with some incorporation of isoprene units. The randomness of the copolymer via direct copolymerization has been confirmed by the comparison with pure diblock copolymer produced by sequential monomer addition. Both copolymers have similar chemical composition (50/50) and molecular weight. Their... 

This finding is a significant improvement over aqueous ROMP systems using aqueous ROMP catalysts. The propagating species in these reactions is stable. The synthesis of water-soluble block copolymers can be achieved via sequential monomer addition. The polymerization is not of living type in the absence of acid. In addition to eliminating hydroxide ions, which would cause catalyst decomposition, the catalyst activity is also enhanced by the protonation of the phosphine ligands. Remarkably, the acids do not react with the ruthenium alkylidene bond. 

When the reactivity of the two monomers is similar and steric factors are absent Rcr=Rv. For instance IB and styrene (St) possess similar reactivity, therefore, diblock copolymers poly(IB-b-St) [7] as well as the reverse order poly(St-fo-IB) [8, 9] could be readily prepared via sequential monomer addition (Scheme 2). Moreover, identical reaction conditions (-80 °C and TiCl4 as Lewis acid) could be employed for the living cationic polymerization of both monomers. However, whereas the living PIB chain ends are sufficiently... 

Scheme 2 Synthesis of poly(IB-b-St) and poly(St-b-IB) diblock copolymers via sequential monomer addition...

Scheme 3 Synthesis of poly(aMeSt-b-IB) copolymer by modifying the chain end of living PaMeSt, followed by sequential monomer addition...

Scheme 4 Synthesis of block copolymers via capping reaction of living PIB with DPE, followed by Lewis acidity tuning and sequential monomer addition...

Since soluble multifunctional initiators are more readily available in cationic polymerization than in the anionic counterpart, ABA type linear triblock copolymers have been almost exclusively prepared using difunctional initiation followed by sequential monomer addition. The preparation and properties of ABA type block copolymer thermoplastic elastomers (TPEs), where the middle segment is PIB, have been reviewed recently [47]. 

Polyethylene-Wock-poly(clhylcnc-co-norbornene) (PE-fo-P(E-co-NBl ) block copolymer was successfully synthesized by a titanium complex with two non-symmetric bidentate /J-cnaminokclonalo ligands [136,137]. Bis(pyrrolide-imine)titanium complex also has the ability to produce the PE-fo-P(E-co-NBE) block copolymer. PE-fo-PS was synthesized via sequential monomer addition during homogeneous polymerization with bis(phenoxy-imine)metal catalysts [138]. 

This technique is based on the use of well-defined soluble multifunctional initiators, which, in contrast to anionic multifunctional initiators, are readily available. From these multiple initiating sites a predetermined number of arms can grow simultaneously when the initiating functions are highly efficient independently of whether the other functions have reacted or not. Under these conditions the number of arms equals the number of initiating functions and living polymerization leads to well defined star polymers with controlled MW and narrow MWD. Subsequent end-functionalization and/or sequential monomer addition can also be performed leading to a variety of end-functionalized An or (AB)n star-shaped structures. 

Increase of molar mass on sequential monomer addition ... 

Increase of Molar Mass on Sequential Monomer Addition In an ideally living polymerization, molar mass increases on the sequential addition of two or more monomer batches. In detail, Mn should proportionally increase with increasing ratios of mm/mncI as pointed out for criterion No. 3 Linear dependence of Mn on monomer/catalyst-ratio at constant monomer conversion . Usually the storage temperature and the length of shelf life prior to the addition of the second monomer batch are not further defined. 

Shen et al. reported on the first experiment with sequential polymerization of two batches of IP monomer with a Ln-based catalyst system. They demonstrated that the monomer added in the second batch was completely polymerized and that solution viscosity increased by a factor of about 2 [92]. Jun et al. studied sequential monomer addition with the catalyst system NdN/TIBA/DIBAH/EASC. These authors found that a second batch of diene monomer is polymerized even 10 days after the polymerization of the first batch has gone to full completion [657]. In a similar experimental set-up two charges of BD were sequentially polymerized with the catalyst system NdV/DIBAH/EASC (Fig. 13) [205]. It was found that Mn increased linearly upon addition of the two monomer batches. The slopes of the polymerizations of the two batches are not the same due to different monomer/catalyst ratios in the two stages of the experiment. 

The results obtained from these experiments with sequential monomer addition performed with the two Nd-carboxylate-based catalyst systems (1)... 

---