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With appropriate choice of conditions, copolymerization of monosubstituted monomers [CH2&dbd;CHX:  e.g., styrene, butyl acrylate (BA)] in the presence of small amounts of an α-methylvinyl monomer [CH2&dbd;C(CH3)Y:  e.g., α-methylstyrene (AMS), methyl methacrylate (MMA), methacrylonitrile (MAN)] and a cobaloxime as chain transfer catalyst provides a route to macromonomers that are composed largely of the monosubstituted monomer and yet have a chain end derived from the α-methylvinyl monomer (−CH2−C(&dbd;CH2)Y). The various factors (temperature, concentrations, type of cobaloxime, types of monomer) that influence molecular weight and end group purity and the importance of the various side reactions that may complicate the process are described. Macromonomer purity is enhanced by increasing the concentration of the α-methylvinyl monomer and reducing the cobaloxime concentration. It also depends on the structure of the cobaloxime, increasing in the series where the ligands are derived from dimethyl glyoxime < diethyl glyoxime ~ diphenyl glyoxime, and the α-methylvinyl monomer, increasing in the series where Y is CO2R < CN < Ph. For styrene−AMS copolymerization, macromonomer purity (the fraction of AMS-derived ends) was enhanced by increasing the reaction temperature. For BA−AMS copolymerization, macromonomer purity was enhanced by decreasing the reaction temperature. However, it is necessary to use a high reaction temperature to limit the extent of macromonomer copolymerization that occurs as a side reaction at high monomer conversion. High purity, AMS terminal BA macromonomers can be prepared by BA−AMS copolymerization at 125 °C with as little as 2 mol % AMS. We also show how the overall composition, molecular weight, and end group functionality of copolymers formed in the presence of a chain transfer agent can be predicted using classical statistics and point out some problems with previous treatments. Analytical expressions which describe zero conversion binary copolymerization in the presence of a transfer agent are derived and successfully applied to the above-mentioned system. The effective transfer constants of cobaloxime transfer catalysts are reported. Those observed in copolymerizations of vinyl and α-methylvinyl monomers are reduced with respect to values observed in homopolymerization of α-methylvinyl monomers because of reversible consumption of the cobalt complex as an adduct to the vinyl monomer. Transfer constants of macromonomers with AMS end groups in styrene polymerization at 120 °C are in the range 0.11−0.15. At lower temperatures (80 °C), macromonomer copolymerization dominates over chain transfer and the effective transfer constant is ~0.

Radical loss processes in RAFT-controlled emulsion polymerization are investigated using γ-initiated seeded emulsion polymerization of styrene in “relaxation mode”, i.e., following the rate of polymerization after removal from the radical source. This provides direct measurement of radical loss processes. A water-insoluble RAFT agent, 2-phenylprop-2-yl phenyldithioacetate, was transported to preformed seed particles using acetone, the acetone then removed, the particles swollen with monomer, and RAFT-mediated polymerization initiated by γ-irradiation. The systems show good control over the molecular weight of the products and a retardation dependent on the concentration of RAFT agent. Kinetic parameters are obtained from the γ-relaxation experiments, using pseudo-bulk kinetics as an approximation to full compartmentalization/chain-length-dependent kinetics to describe the systems. The rate of radical loss was strongly affected by the presence of RAFT agent and was found to decrease with increasing length of the dormant chain. The interpretation of this observation is as follows. The termination rate coefficient depends on chain length; the dominant mode of termination is the reaction between chains of similar length in RAFT-mediated systems but between short and long chains in conventional systems. Radical entry into particles is assumed to be by chains of degree of polymerization z formed in the aqueous phase. Dormant z-meric chains in the particles are postulated to lead to an increase in the rate of radical exit from the particles, with the reactivation of these species generating a z-meric radical that is able to desorb from the particle surface in a RAFT-induced exit mechanism, leading to the rapid exchange of radicals between particles and hence radical loss. A simple model for the radical loss rate coefficient, expected to be valid for longer chain lengths, both reproduces much more computationally expensive Monte Carlo calculations and (when used with the scaling suggested by reptation theory) gives quantitative accord with the relaxation data.

A series of tertiary dithiobenzoates differently substituted on the phenyl group (Z) were synthesized in order to investigate the RAFT-mediated polymerization of MMA. The chain-transfer agent 2-cyanoprop-2-yl dithiobenzoate, although an excellent RAFT agent for polymerization of MMA, does not yield a very narrow polydispersed polymer at low conversion. The introduction of electron-withdrawing substituents on the dithiobenzoate aromatic ring improves the activity of the chain-transfer agent narrowing the molecular weight distribution, especially in the first 2 h of the process; conversely, electron-donating substituents deteriorate the performance of the process in the early stages. Substituents on the dithiobenzoate that, for reasons of steric hindrance reduce the conjugation of the phenyl with the C&dbd;S double bond of the dithiogroup, diminish the effectiveness of the chain-transfer agent. The nature of the leaving group (R) also has a strong influence on the process:  the substitution of 2-cyanoprop-2-yl with the bulkier 2-cyano-4-methylpent-2-yl as leaving group improves remarkably the MMA polymerization, especially in the early stages. The solvent effect on MMA RAFT polymerization using benzene, acetonitrile, and DMF was also investigated. The lower propagation rate constant of MMA in benzene gives rise to reduced rates of polymerization but narrower polydispersities of the polymer, especially at low conversions. The RAFT polymerizations of MMA were carried out at 60 and 90 °C to assess the temperature dependence of the process. The higher temperature increases both the rate of polymerization and the transfer constant of the RAFT agent, resulting in lower polydispersities at a given conversion. Ab initio calculations confirmed the observed experimental results.

Poly(N-isopropylacrylamide)-block-poly(acrylic acid), PNIPAAm-b-PAA, with low polydispersity was prepared by reversible addition−fragmentation chain transfer (RAFT) polymerization in methanol. The block copolymers respond to both temperature and pH stimuli. The behavior of the double-responsive block copolymers in solution was investigated by dynamic light scattering, temperature-sweep NMR, cryogenic transmission electron microscopy, and IR spectroscopy. The block copolymers form micelles in aqueous solutions in dependence of pH and temperature. Cloud point measurements indicated the formation of larger aggregates at pH 4.5 and temperatures above the lower critical solution temperature (LCST) of PNIPAAm. The solution behavior is strongly influenced by hydrogen bonding interactions between the NIPAAm and acrylic acid blocks.

Chain transfer constants have been determined for an unsaturated methyl methacrylate trimer, CH2&dbd;C(CO2Me)CH2[C(CO2CH3)(CH3)CH2]2H (MMA3), in polymerizations of methacrylate esters (ethyl, EMA; n-butyl, BMA; tert-butyl, tBMA; 2-ethylhexyl, EHMA) and for the analogous trimers of butyl methacrylate (BMA3) and methacrylic acid (MAA3), a hydroxyethyl methacrylate dimer (HEMA2), a hydroxyethyl methacrylate−methyl methacrylate−hydroxyethyl methacrylate trimer (HEMA−MMA−HEMA), and a HEMA macromonomer in methyl methacrylate (MMA) polymerization. These data have been assessed with reference to our previously reported data on chain transfer of MMA macromonomers in MMA polymerizations. The transfer constants (Ctr) for MMA3 in polymerizations of methyl, ethyl, n-butyl, tert-butyl, and 2-ethylhexyl methacrylate at 60 °C are similar (~0.18). The Ctr of MAA3 (0.28 at 60 °C in 2-butanone solvent) in MMA polymerization is ca. 50% higher than that of MMA3 under similar conditions. Other trimers (BMA3, HEMA−MMA−HEMA) and a low molecular weight HEMA macromonomer (degree of polymerization ca. 7) have Ctr similar to that of MMA3. The transfer constants for the various trimers show no significant temperature dependence over the range 60−100 °C. The Ctr of the dimers (MMA2, HEMA2) in MMA polymerization are an order of magnitude lower than those of the corresponding trimers. The Ctr of HEMA2 (0.018 at 60 °C) in MMA polymerization is ca 50% higher than that of MMA2 (0.013 at 60 °C). The Ctr of both dimers increases with increasing temperature between 60 and 100 °C (Ctr HEMA2 0.031, MMA 0.018 at 60 °C). Issues relating to the synthesis and characterization of trimers (RMA3) by catalytic chain transfer are discussed. Methods for synthesis of di-end-functional trimers MAA−MMA−MAA, HEMA−MMA−HEMA based on selective hydrolysis and reesterification are described. These trimers may find use in telechelic synthesis.

Free-radical polymerization in the presence of suitable addition−fragmentation chain transfer agents [S&dbd;C(Z)S−R] (RAFT agents) possess the characteristics of a living polymerization (i.e., polymer products can be reactivated for chain extension and/or block synthesis, molecular weights are predetermined by RAFT agent concentration and conversion, narrow polydispersities are possible). Styrene polymerizations (110 °C, thermal initiation) were performed for two series of RAFT agents [S&dbd;C(Z)S−CH2Ph and S&dbd;C(Z)S−C(Me)2CN]. The chain transfer coefficients decrease in the series where Z is Ph > SCH2Ph ~ SMe ~ Me ~ N-pyrrolo ≫ OC6F5 > N-lactam > OC6H5 > O(alkyl) ≫ N(alkyl)2 (only the first five in this series provide narrow polydispersity polystyrene (< 1.2) in batch polymerization). More generally, chain transfer coefficients decrease in the series dithiobenzoates > trithiocarbonates ~ dithioalkanoates > dithiocarbonates (xanthates) > dithiocarbamates. However, electron-withdrawing substituents on Z can enhance the activity of RAFT agents to modify the above order. Thus, substituents that render the oxygen or nitrogen lone pair less available for delocalization with the C&dbd;S can substantially enhance the effectiveness of xanthates or dithiocarbamates, respectively. The trend in relative effectiveness of the RAFT agents is rationalized in terms of interaction of Z with the C&dbd;S double bond to activate or deactivate that group toward free radical addition. Molecular orbital calculations and the estimated LUMO energies of the RAFT agents can be used in a qualitative manner to predict the effect of the Z substituent on the activity of RAFT agents.

Radical polymerization with reversible addition−fragmentation chain transfer (RAFT polymerization) can be used to synthesize a wide range of polymers of controlled architecture and narrow molecular weight distribution. The polymerizations use addition−fragmentation chain transfer agents (RAFT agents) that possess high transfer coefficients in free radical polymerization and confer living character on the polymerization. This paper explores the effect of the substituents R of dithiobenzoate RAFT agents [S&dbd;C(Ph)S−R] on the outcome of polymerizations of styrene, methyl methacrylate (MMA) and butyl (BA) or methyl acrylate (MA). In MMA polymerization at 60 °C, effectiveness depends strongly on R decreasing in the order where R is:  −C(Alkyl)2CN ~ −C(Me)2Ar > −C(Me)2C(&dbd;O)O(alkyl) > −C(Me)2C(&dbd;O)NH(alkyl) > −C(Me)2CH2C(Me)3 ≥ −C(Me)HPh > −C(Me)3 ~ −CH2Ph. Of these, only the compounds with R = −C(Me)2Ph and −C(Me)2CN provided polymers with substantially narrowed polydispersities in batch polymerization and gave molecular weight control of the form expected for a living polymerization. These compounds have high transfer coefficients in MMA polymerization (transfer coefficient >20 for 0.003 M RAFT agent in bulk MMA). Reagents with R = −C(Me)2C(&dbd;O)O(alkyl), C(Me)2C(&dbd;O)NH(alkyl), −C(Me)2CH2C(Me)3, −C(Me)HPh, −C(Me)3, and −CH2Ph appear relatively ineffective in MMA polymerization because they have much lower transfer coefficients (ca. 1.7, 0.7, 0.4, 0.15, 0.03, and 0.03 respectively). In polymerizations of styrene or acrylates at 60 °C, transfer coefficients of RAFT agents are several orders of magnitude higher and all compounds in the above series yield narrow polydispersity polymers. The transfer coefficients of benzyl dithiobenzoate in styrene and methyl acrylate polymerization at 60 °C are ca. 50 (0.0093 M RAFT agent in bulk monomer) and 105 (with 0.003−0.006 M RAFT agent in 4.43 M monomer), respectively. The rate constant for addition to the thiocarbonyl group does not depend strongly on R. Differences in activity depend on the properties of R and the propagating chain as free radical leaving groups and on the partitioning of R^• between adding to monomer (to reinitiate) and adding to polymeric RAFT agent. When the reaction of R^• with the polymeric RAFT agent to reform the initial RAFT agent is significant, the transfer coefficient may reduce with concentration of RAFT agent and with conversion. In these circumstances, the activity of RAFT agents is best characterized by two transfer constants associated with the forward and reverse reactions. Steric factors, polar factors, and the stability of R^• all appear to play an important role in determining the transfer coefficient and the effectiveness of RAFT agents.

The synthesis of multiarm star polymers by radical polymerization with reversible addition−fragmentation chain transfer (RAFT) is described. When the precursor RAFT agents to star polymers are trithiocarbonate derived, fragmentation of the radical intermediate can lead to different products depending on the leaving abilities of the two groups attached to sulfur. To demonstrate this, two types of RAFT agents, ones that allow growth of arms away from the core and the other attached to the core during propagation, were designed, and an example of each was synthesized. While both star RAFT agents gave excellent molecular weight control and low polydispersities (typically <1.2), the one that grows its arms away from the core offers polymers free from star−star coupled products. Thus, the direction of fragmentation should be considered when designing efficient star RAFT agents. The living nature of the arms of these polymers was demonstrated by extending the arms with a different monomer to afford a star block copolymer. The RAFT agents described are easily synthesized from commercially available reagents by a simple experimental procedure.

Styrene was polymerized under a source of ultraviolet radiation in the presence of certain thiocarbonylthio compounds. Use of 1-phenylethyl phenyldithioacetate (1-PEPDTA) produced well-defined polymers with molecular weights close to those predicted from theory, up to conversions of 30%. The mechanism of polymerization was examined and shown to proceed via reversible addition−fragmentation chain transfer, as opposed to reversible termination with a thiocarbonylthiyl radical. UV-induced decomposition of the dithioester moiety in 1-PEPDTA and 1-phenylethyl dithiobenzoate (1-PEDB) mediated RAFT polymerizations was followed using UV/vis spectrophotometry; 1-PEPDTA decomposed much slower than 1-PEDB. Analysis of the decomposition products of 1-PEPDTA with gas chromatography/mass spectrometry was used to elucidate a possible mechanism for its degradation, suggesting that both 1-phenylethyl and benzyl radicals are decomposition products.

Reversible addition−fragmentation chain transfer (RAFT) polymerization techniques are successfully used to control molecular weight and polydispersity in the seeded emulsion polymerization of styrene. A novel technique was used to assist the transport into the seed particles of a RAFT agent that is of very low water solubility, ensuring the RAFT agent was localized in the particle phase. The polymer produced in these experiments was seen to have low molecular weight polydispersity (1.1 < &Mmacr;w/&Mmacr;n < 1.4), and the molecular weight could be controlled by the amount of RAFT agent used. The procedure permitted living polymerization to be performed in an emulsion polymerization system. Importantly, the RAFT agent had no adverse effect on latex stability, and unusual amounts of coagulum were not observed. Reductions in the rate of polymerization (measured by dilatometry) were observed along with significant inhibition periods, for which possible explanations and remedies are discussed. This appears to be the first instance where good living character has been demonstrated in a true emulsion polymerization of styrene while maintaining good colloidal stability.

Two cyclic allylic sulfide monomers, 6-methylene-2-methyl-1,4-dithiacycloheptane (MDTP) and 7-methylene-2-methyl-1,5-dithiacyclooctane (MDTO), were copolymerized with methyl methacrylate (MMA) using the pulsed laser polymerization technique. MDTO was also copolymerized with styrene (STY) under the same conditions. Reactivity ratios in bulk monomer at 25 °C, based on the terminal model, were determined from ¹H NMR analyses of the copolymers as rMDTP = 0.85, rMMA = 11 (MDTP−MMA); rMDTO = 1.22, rMMA = 3.08 (MDTO−MMA); and rMDTO = 0.3, rSTY = 8.6 (MDTO−STY). The average propagation rate coefficients, 〈kp〉, were determined for the copolymerizations in the monomer composition ranges 0 ≤ fMDTP ≤ 0.6 (MDTP−MMA), 0 ≤ fMDTO ≤ 0.4 (MDTO−MMA) and 0 ≤ fMDTO ≤ 0.3 (MDTO−STY). It was not possible to obtain values for 〈kp〉 from mixtures containing higher mole fractions of MDTP or MDTO or for either homopolymer. The failure of pulsed laser polymerization in MDTP- and MDTO-rich solutions is presumably due to high rates of transfer to the cyclic allylic sulfide monomer. The MMA copolymerization data show clear deviation from the terminal model, similar to that observed by this group for 2-methylene-1,3-dioxepane. Significant solvent effects on the kp of MMA were observed in the presence of 1,5-dithiacyclooctane, used as a nonpolymerizing model compound for MDTP and MDTO.

The copolymerization behavior of 7-methylene-2-methyl-1,5-dithiacyclooctane (MDTO) with a range of common comonomers has been investigated and found to be highly unusual. The resulting copolymers contained blocks of homo-poly(MDTO), even at low MDTO feed ratios. Changes in temperature and monomer concentration have a significant effect on the copolymer composition of methyl methacrylate−MDTO and styrene−MDTO mixtures and therefore on the apparent reactivity ratios. The magnitude of the concentration effect is independent of the solvent. A new copolymerization mechanism is proposed in which the addition of an MDTO radical to the comonomer is reversible while all other propagation reactions are effectively irreversible. To our knowledge, this is the first observation of such a mechanism. Parameters for MDTO−M2 (M2 = STY, MMA) according to this model are reported as (rMDTO, r2, k-12/k22) = (0.06, 7, 17; M2 = STY) and (0.38, 3.2, 2.5; M2 = MMA) at 60 °C.

The bulk polymerizations of styrene and methyl methacrylate in the presence of model polymer−dithiocarbonate adducts as mediators and benzoyl peroxide (BPO) as a conventional initiator were kinetically studied. The polymerization rate, and hence the concentration of polymer radical P^•, was proportional to [BPO]^1/2. The pseudo-first-order activation rate constants kact were determined by the GPC peak-resolution and the polydispersity-analysis methods. The results showed that kact is directly proportional to [P^•], indicating that reversible addition−fragmentation chain transfer (RAFT) is the only important mechanism of activation. The magnitude of the exchange rate constant kex (= kact/[P^•]) was strongly dependent on both the structures of the dithiocarbonate group and the polymer. The kex values for the three RAFT systems examined in this work were all very large, which explains why these systems can provide low-polydispersity polymers from an early stage of polymerization. The activation energy of kex for a polystyryl dithioacetate (PSt−SCSCH3) was 21.0 kJ mol^-1, which is reasonable for a fast addition reaction.

3-Methylene-6-methyl-1,5-dithiacyclooctane (MDTO, 1) was polymerized in both the presence and absence of a number of chain transfer agents, viz. 1-butanethiol (BuSH), dibutyl disulfide (BuSSBu), and dibutyl sulfide (BuSBu). Mark−Houwink−Sakurada (MHS) constants for poly(MDTO) were found to be K = 23 × 10^-5 dL g^-1 and α = 0.67. Using these parameters, chain transfer constants were obtained at 60 °C and are reported as 0.13 (BuSH), 0.19 (BuSSBu), and 0.0025 (BuSBu). These values are compared to the corresponding chain transfer constants obtained for styrene (STY) and methyl methacrylate (MMA). Subsequently, the chain transfer activity was measured over a range of temperatures (40−80 °C), and Arrhenius parameters were determined. Significant differences in chain transfer activity between carbon- and sulfur-centered radicals were observed. In addition, the extent of transfer to monomer was determined across the range of temperatures 40−100 °C, yielding a value for CM of 55 × 10^-4 at 60 °C. The likely extent of transfer to polymer in MDTO polymerizations was estimated using a model compound, 3-butylthio-2-butylthiomethyl-1-propylene (BTMP, 2), yielding an estimated value for CP of 0.35 at 60 °C.

A series of relatively monodisperse samples of poly(N-isopropylacrylamide) were synthesized by reversible addition−fragmentation chain transfer (RAFT) over the molecular weight range 2 × 10³−3 × 10⁵. For molecular weights below 4 × 10⁴, conditions were found so that polydispersity remained below 1.4 up to high conversion (72%). Molecular weight distributions of polymer obtained using GPC in THF and MALDI were in accord for the low molecular weight range (typically 10⁵). Particular care is required in GPC sample preparation:  it is necessary to ensure that trace amounts of water are initially present when drying a polymer sample prior to dissolution in THF, to avoid irreversible chain aggregation. The log/log plot of intrinsic viscosity against molecular weight for polyNIPAM was found to be linear for molecular weights <10⁵, after which the hydrodynamic volume seems to be independent of molecular weight. The Mark−Houwink parameters obtained from the lower molecular weight data are K = 10^{-(4.24±0.42)} dL g^-1 and a = 0.78 ± 0.09.

The effect of substituents on the free-radical ring-opening polymerization behavior and polymerization volume shrinkage of the cyclic allylic sulfides 6-methylene-1,4-dithiacycloheptane (2a) and 3-methylene-1,5-dithiacyclooctane (2b) has been investigated. The monomers were 2-(hydroxymethyl)-6-methylene-1,4-dithiacycloheptane (3a), 6-methylene-1,4-dithiacycloheptan-2-ylmethyl acetate (3b), 6-methylene-1,4-dithiacycloheptan-2-ylmethyl methacrylate (3c), 7-methylene-1,5-dithiacyclooctan-3-ol (4a), 7-methylene-1,5-dithiacyclooctan-3-yl acetate (4b), 7-methylene-1,5-dithiacyclooctan-3-yl benzoate (4c), and the bicyclic monomer bis(6-methylene-1,4-dithiacycloheptan-2-ylmethyl) diglycolate (5). The monomers were polymerized in bulk with thermal (AIBN, VAZO88) and photochemical initiators (Darocur 1173) with selected solution polymerizations also performed. The presence of a substituent on the monomers gave clear amorphous polymers unlike the crystalline polymers obtained from the unsubstituted parent monomers (2) previously reported. The substituted, monofunctional seven-membered ring monomers (3a,b) polymerize to moderate conversions (ca. 60%) and gave amorphous, soluble sticky gumlike polymers. The difunctional monomer 3c in contrast gave an insoluble rigid material. Eight-membered ring monomers 4a, 4b, and 4c were made to undergo complete conversions. The dual cyclic monomer 5 was polymerized to give an insoluble material containing only traces of extractable monomer. Those liquid monomers 3c, 4b, and 5 that polymerized to complete conversion (3c, 4b) or near complete (5) were further examined and gave polymerization shrinkages of 7.3, 2.0, and 2.1% (respectively) and glass transition points of 78, −0.2, and 28 °C, respectively.

Living radical polymerization with reversible addition−fragmentation chain transfer (RAFT polymerization) can be achieved with the use of dithiocarbamate derivatives that have the nonbonded electron pair of the nitrogen included as part of an aromatic system. These compounds have been shown to be highly effective in RAFT polymerization of styrene and (meth)acrylate esters to produce polymers of predetermined molecular weight and narrow polydispersity (usually <1.2). By contrast, simple N,N-dialkyl dithiocarbamates (those compounds previously described as “photoiniferters”) are ineffective as RAFT agents. The reasons for the different behavior are discussed.

Derivatives of 2,2,5,5-tetraalkylimidazolidin-4-one-1-oxyl have been found to offer significant advantages over many of the nitroxides previously employed in nitroxide-mediated polymerization:  homopolymers [e.g., polystyrene and poly(alkyl acrylate)], statistical copolymers [e.g., poly(styrene-co-acrylonitrile)], and block copolymers [e.g., poly(4-methylstyrene)-block-polystyrene and poly(n-butyl acrylate)-block-polystyrene] can be synthesized with controlled molecular weight, narrow molecular weight distribution, and defined end group functionality. The effect of substituents α to the nitroxide nitrogen and at the transannular nitrogen on the outcome of polymerization is explored. Appropriate selection of these substituents provides polymers of lower polydispersities than, for example, 2,2,6,6-tetramethylpiperidin-2-oxyl and derivatives. The nitroxides are synthesized from readily available precursors by a simple experimental route. Moreover, they are subject to fewer side reactions. In particular, disproportionation between the propagating radical with nitroxide appears to be of lesser significance.

The two heterocyclic compounds produced on irradiation of 3β-acetoxy-6- nitrocholest-5-ene (1) have been shown by chemical degradation to be 3β-acetoxycholest-4-eno[6,5,4-cd]isoxazole (3) and 3β-acetoxy-A- norcholestano[6,5-c]-2?-isoxazoin-5?-one (4), while those formed from 6-nitrocholest-5-ene (2) are the analogous compounds, cholest-4- eno[6,5,4-cd]isoxazole (5) and A-norcholestano[6,5-c]-2?-isoxazolin-5?- one (6).

The photochemistry of 3β-acetoxy-6-nitrocholest-5-ene (1) and 6- nitrocholest-5-ene (11) has been investigated in a number of solvents. It is suggested that for both compounds the major primary photo-product is the corresponding aci-nitro tautomer, which then undergoes further reactions depending mainly on the conditions employed. The aci-nitro compounds are believed to be formed by a 1,5-hydrogen shift from C4 to oxygen, and their intermediacy is supported by the formation of a mixture of 6α-deutero-6β-nitrocholest-4-ene and 6β-deutero-6α- nitrocholest-4-ene on irradiation of 6-nitrocholest-5-ene in Me3COD. Several other reactions of 3β-acetoxy-6-nitrocholest-5-ene and 6- nitrocholest-5-ene are described.

The reactions of tert-pentyl peroxypivalate (1b) and tert-hexyl peroxypivalate (1c) with methyl methacrylate (MMA) in the presence of the free radical scavenger 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (2) have been studied at 60 °C. tert-Pentyloxyl radicals (3b) and tert-hexyloxyl radicals (3c) were formed in the thermolysis of 1b and 1c. The major reaction pathways of the alkoxyl radicals were β-scission and rearrangement (1,5-H shift) to generate alkyl radicals, which occurred to the extent of 83% (3b) and 90% (3c), respectively. The resulting alkyl radicals underwent selective addition to MMA. The overall ratios of addition to H-abstraction in the reactions of 3b and of 3c with MMA were 13:1 and 24:1, respectively. From the competitive addition/trap reactions of alkyl radicals, relative reactivities of alkyl radicals toward addition to MMA were in the ratio:  Et^•:n-Pr^•:4-hydroxy-4-methylpentyl radical:Bu^t• = 0.39:0.48:0.51:1.00.

The photochemistry of cyclohexylidenenitromethane (6), 3-ethyl-2-nitropent-2-ene (7), (Z)-α-methyl-α- nitrostilbene (12), (E)-3-methyl- 1-nitro-1-phenylbut-1-ene (15) and 4,4-dimethyl-3-nitro-5α-cholest-2- ene (27) has been examined to determine the structural features required for deconjugation to the β, γ-unsaturated isomer. The mechanism of the light-induced formation of the corresponding α, β- unsaturated ketone , which may accompany deconjugation , has also been examined. Support for a scheme involving γ-hydrogen abstraction to give an unsaturated nitronic acid, followed by N- hydroxyoxaziridine formation and collapse to the ketone, is provided in the photochemistry of the stable nitronic acids, 4-t-butyl-aci-nitrocyclohexane (17) and 9-aci-nitrofluorene (18). Syntheses of the nitro olefins (6), (7), (12), (15) and (27), and the nitronic acid (17) are also reported.

The chain transfer activities of a series of ω-unsaturated methyl methacrylate oligomers [(dimer (1), trimer (2), tetramer (3), and a methyl methacrylate macromonomer with average chain length of 24 units (4)] have been evaluated in methyl methacrylate polymerizations over the temperature range 45−100 °C. Transfer constants were determined by analysis of the ln chain length distributions. The dimer (1) was found to be substantially less effective as a chain transfer agent than the trimer (2) or the higher macromonomers (3, 4) [CT(60 °C) 1, 0.013; 2, 0.19; 3, 0.31; 4, 0.21]. The transfer constants show only a small temperature dependence and no variation with conversion. No discernible retardation was observed in these polymerizations. A reduced yield of polymer observed at conversions >10% in bulk polymerizations of MMA carried out in the presence of 1−4 can be attributed to the absence of the gel (Trommsdorff) effect. The results are interpreted in terms of the addition−fragmentation mechanism for chain transfer.

A new type of ring-opening monomer has been investigated and found to undergo facile and efficient free-radical polymerization. The monomers are cyclic allylic sulfides with 6-methylene-1,4-dithiepane (3) and 3-methylene-1,5-dithiacyclooctane (4) synthesized and investigated as two examples. 6-Methylene-1,4-dithiepane (3) is a clear oil and 3-methylene-1,5-dithiacyclooctane (4) has a low, near ambient temperature melting point, and in contrast to many ring-opening monomers, they readily survive exposure to moisture and aqueous acids and bases. These compounds were polymerized in bulk with complete ring opening to give highly insoluble crystalline homopolymers of high molecular weight (Mw ca. 500 000−700 000) with glass transition temperatures of ca. −30 to −50 °C and melting points of ca. 100−130 °C. The homopolymers and their 1:1 copolymer were characterized by FT-IR and high-temperature ¹H and ¹³C NMR spectroscopies, high-temperature GPC, and DSC. They displayed volume shrinkages in the range expected of ring-opening monomers (4.5−8.5%) of their molecular weights in spite of the crystallinity present in the final polymers.

The reactions of three electrophilic radicals, benzoyloxy, phenyl, and t-butoxy, have been examined with a series of α, β unsaturated esters; methyl acrylate, methyl methacrylate, and methyl crotonate. For the three monomers the ratio of hydrogen abstraction to double bond addition was found to increase and the ratio of head against tail addition to decrease in the series: benzoyloxy; phenyl; t-butoxy. Relative rates for these reactions have been determined and the factors influencing the mode of reaction are discussed.